Chapter 2 - 0 Genetic continuity between cells and between...

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Unformatted text preview: 0 Genetic continuity between cells and between organisms of sexually reproducing species is maintained through the processes of mitosis and meiosis, respectively 0 Diploid eukaryotic cells contain their genetic information in pairs of homologous chromosomes, with one membe:r of each pair being derived from the maternal parent and one from the paternal parent. - Mitosis provides a mechanism to distribute chromosomes that have duplicated into progeny cells during cell reproduction. - Mitosis converts a diploid somatic cell into two diploid daughter cells. Chromosomes m if}: ,Urcunefatm'mase stage ofnz-ifosfs, derived from a cis- i' in the flower of Haemaniiius The process of meiosis distributes one member of each homologous pair of chromosomes into each gamete or spore, thus reducing the diploid chromosome number to the haploid chromosome number. Meiosis generates genetic variability by distributing various combinations of maternal and paternal members of each homologous pair of chromosomes into gametes or Spores. it is during the stages of mitosis and meiosis that the genetic material has been condensed into discrete structures called chromosomes. Piants alternate between a diploid sporophyte stage and a haploid gametophyte stage. 17 n I > 12 .1 m :0 ._.| E O 18 Chapter 2 Mitosis and Meiosis it every living thing there exists a substance referred to as the genetic material. Except in certain viruses. this mater: ial is composed of the nucleic acid DNA. A molecule of DNA is organized into units called genes. the products of which direct the metabolic activities of cells. DNA, with its array of genes. is organized into structures called chromosomes, which serve as vehicles for transmitting genetic information. The mane net in which chromosomes are transmitted from one generation of cells to the next. and from organisms to their descendants, must be exceedingly precise. in this chapter we consider exactly how genetic continuity is maintained between cells and organisms. Two major processes are involved in eukaryotes: mitosis and meiosis. Although the mechanisms of the two processes are similar in many ways, the outcomes are quite different. Mito- sis leads to the production of two cells, each with the same number of chromosomes as the parent cell. Meiosis. on the other hand. reduces the. genetic content and the number of chro- mosomes by precisely half. This reduction is essential if sex- ual reproduction is to occur without doubling the amount of genetic material at each generation. Strictly speaking, mitosis is that portion of the cell cycle during which the duplicated chromosomes are precisely and equally divided into daughter cells. Meiosis is part of a special type of cell division that leads to the production of sex cells: gametes or spores. This process is an essential step in the transmission of genetic information from an organism to its offspring. Normally. chromosomes are visible during a cell’s life only during mitosis and meiosis. When cells are not undergoing di- vision. the genetic material making up chromosomes unfolds and uncoils into a diffuse network within the nucleus, generally referred to as chromatin. We will briefly review the structure of cells. emphasizing the components that are of particular sig- nificance to genetic function. Then We will devote the. remain- der of the chapter to the behavior of chromosomes during cell division. Cell Structure ls Closely Tied to Genetic Function Before describing mitosis and meiosis, a brief review of the structure of cells will be helpful. As we shall see. many cell components. such as the nucleolus, ribosome. and centriole, are involved directly or indirectly with genetic processes. Other components, the mitochondria and chloroplasts, contain their own unique genetic information. it is also useful to compare the structural differences between the prolcaryotic bacterial cell and the eukaryotic cell. Variation in the structure and function of cells is dependent on specific genetic expression by each cell type. Before 1940. our knowledge of cell structure was limited to what we could see with the light microscope. Around l940. the transmission electron microscope was in its early stages of development. and by 1950, many details of cell ultrastnrcture had emerged. Under the electron microscope. cells were seen as highly organized, precise structures. A new world of whor- ling membranes, organelles, microtubules, granules, and filae ments was revealed. These discoveries revolutionized thinking in the entire field of biology. We will be concerned with the aspects of cell structure that relate to genetic. study. The typie cal animal cell shown in Figure 24 illustrates most of the struc- tures we will discuss. Cell Boundaries All cells are surrounded by a plasma membrane, an outer cov- ering that defines the cell boundary and delimits the cell from its immediate external environment. This membrane is not pasw sive; instead, it actively controls the movement of materials into and out of the cell. In addition to this membrane, plant cells have an outer covering called the cell wall. One major component of this rigid structure is a polysaccharide called cellulose. Bacterial cells also have a cell wall, but its chemical compo- siti on is quite different from that of the plant cell wall, the major component being a complex macromolecule called a peptide- glycan. As its name suggests. the molecule consists of peptide and sugar units. Long polysaccharide chains are cross-linked with short peptides, which impart great strength and rigidity to the bacterial cell. Some bacterial cells have still another cov— ering, a capsule. This mucuslilce material protects these bacteria from phagocytic activity by the host during their pathogenic invasion of eukaryotic organisms. The presence of the capsule is under genetic control. in fact, as we will see in Chapter 9, its loss due to mutation in the pneumonia—causing bacterium Dipiococcus pneumontae provided the underlying basis for a critical experiment, proving that DNA is the genetic material. Many, if not most, animal cells have a covering over the plasma membrane, referred to as the cell coat. Consisting of glycopro- teins and polysaccharides, its chemical composition differs from comparable structures in either plants or bacteria. The cell coat. among other functions, provides biochemical identity at the sun face of cells. These forms of cellular identity at the cell surface are under genetic control. For example. various antigenic de- terminants, such as the AB and MN antigens. are found on the surface of red blood cells. In other cells, histocompatihility antigens. which elicit an immune response during tissue and organ transplants. are present. A variety of receptor molecules are also important components at the surface of cells. These constitute recognition sites that transfer specific chemical sig nals across the cell membrane into the cell. The Nucleus The presence of a nucleus and other membranous organelles characterizes eukaryotic cells. The nucleus houses the genetic material, DNA, which is complexed with an array of acidic and basic proteins into thin fibers. During nondivisional phases of the cell cycle. these fibers are uncoiled and dispersed into chromatin. As we will soon discuss. during mitosis and meio— sis, chromatin fibers coil and condense into structures called chromosomes. Also present in the nucleus is the nucleolus. an amorphous component where ribosomal RNA is synthe- sized and where the initial stages of ribosomal assembly occur. The areas of DNA encoding rRNA are collectively referred to as the nucleolus organizer region. or the NOR. Nucleus Nuclear envelope Nucleolus Chromatin Nuclear pore Lysosome Smooth endoplasmic reticulum Free ribosome Centriole 2.1 Cell Structure l5 Closely Tied to Genetic Function 19 Ribosome Rough endoplasmic reticulum Plasma membrane Cell coat Cytoplasm Golgi body Mitochondrion FIGURE 2—1 A generalized animal cell. The cellular components discussed in the text are emphasized here. The lack of a nuclear envelope and membraneous organelles is characteristic of prokaryotes. In bacteria such as Escherichia coli, the genetic material is present as a long, circular DNA mol- ecule that is compacted into an area referred to as the nucleoid area. Part of the DNA may be attached to the cell membrane. but in general the nucleoid constitutes a large area throughout the cell. Although the DNA is compacted, it does not undergo the extensive coiling characteristic of the stages of mitosis where, in eukaryotes, chromosomes become visible. Nor is the DNA in these organisms associated as extensively with proteins as is eukaryotic DNA. Figure 2—2, which shows two bacteria form- ing during cell division, illustrates the nucleoid regions that house the bacterial chromosome. Prokaryotic cells do not have a distinct nucleolus, but do contain genes that specify rRNA molecules. The Cytoplasm and Cellular Organelles The remainder of the eukaryotic cell enclosed by the plasma membrane, excluding the nucleus, is composed of cytoplasm and all associated cellular organelles. Cytoplasm consists of a nonparticulate, colloidal material referred to as the cytosol. which surrounds and encompasses the cellular organelles. Beyond these components, an extensive system of tubules and filaments comprising the cytoskeleton provides a lattice of sup— port structures within the cytoplasm. Consisting primarily of tubulin-derivetl microtubules and actin-derived microfila- merits, this structural framework maintains cell shape. facili- tates cell mobility. and anchors the various organelles. One organelle, the membranous endoplasmic reticulum (ER), compartmentalizes the cytoplasm. greatly increasing the surface area available for biochemical synthesis. The ER may appear smooth. in which case it serves as the site for synthe- sizing fatty acids and phospholipids, or it may appear rough because it is studded with ribosomes. Ribosomes serve as sites where genetic information contained in messenger RNA (mRNA) is translated into proteins. Three other cytoplasmic structures are very important in the eu- karyotic cell’s activities: mitochondria, chloroplasts, and centrioles. Mitochondria are found in both animal and plant cells and are the sites of the oxidative phases of cell respiration. These chemical re actions generate large amounts of adenosine triphosphate (ATP), an energy—rich molecule. Chloroplasts are found in plants, algae, and some protozoans. These organelles are associated with pho- tosynthesis. the major energy-trapping process on Earth. Both mi- tochondria and chloroplasts contain a type of DNA that is distinct from that found in the nucleus. Furthermore. these organelles can 20 Chapter 2 Mitosis and Meiosis - Nucieoid regions FIGURE 2—2 Color—enhanced electron micrograph of E. coli undergoing cell division. Particularly prominent are the two chromosomal areas (shown in red), called nucleoids, that have been partitioned into the daughter cells. duplicate themselves and transcribe and translate their genetic information. it is interesting to note that the. genetic machinery of mitochondria and chloroplasts closely resembles that of prokaryu otic cells. This and other observations have led to the proposal that these organelles were once primitive frceeliving organisms that established a symbiotic relationship with a primitive eukary- otic cell. This theory. which describes the evolutionary origin of these organelles. is called the endosymbiont hypothesis. Animal cells and some plant cells also contain a pair of com— plex structures called the centrioles. These cytoplasmic bodies. located in a specialized region called the centrosome. are as sociated with the organization of spindle fibers that function in mitosis and meiosis. in some organisms. the centrioic is de- rivcd from another structure. the basal body. which is associ— ated with the formation of cilia and flagella. Over the years. research has suggested that Centrioles and basal bodies contain DNA. which could be involved in the replication of these strucr titres. Currently. this is thought not to be the case. The organization of spindle fibers by the. centriolcs occurs during the early phases of mitosis and meiosis. Composed of arrays of microtubulcs. these fibers play an important role in the movement ofchromosomes as they separate during cell divi- sion. The microtuhules consist of polymers of polypeptide sub— units of the protein ttthulin. In Diploid Organisms, Chromosomes Exist in Homologous Pairs As we discuss the pl‘tiCCSSES of mitosis and meiosis. it is im- portant that you understand the concept of homologous Citro- mosomes. Such an understanding will also be of critical importance in our future disease sions of Mendelian generics. Chromosomes are most easily \‘i- sualized durng mitosis. When they are examined carefully. they are seen to take on distinctive lengths and shapes. Each contains a condensed or constricted region called the centromere. which es- tablishes the general appearance of each chromosrnne. Figure 2—3 shows chromosomes with cen- tromere placements at different points along their lengths. Ex- tending from either side of the centromerc are the arms of the Chromosome. Depending on the position ol‘ the centromcrc. different arm ratios are produced. As Figure 273 illustrates. chro~ mosomes are classified as metacentric. submetacentric. acrocentric. or telocentric on the basis of the ccntromere location. The shorter arm. by convention. is shown above the ccntromcrc and is called the 1] arm ip stands for "pctite"). The longer arm is shown below the centrornere and is called the (1 arm to being the next letter in the alphabet). When studying mitosis. several other observations are of par— ticular relevance. First. all somatic cells derived from menv bers of the same species contain an identical number of chromosomes. in most cases. this represents the diploid num— ber (2n). When the lengths and cctttromerc placements ol‘ all such chromosomes are examined. a second general feature is apparent. Nearly all ol‘ the chromosomes exist in pairs with re— gard to these two criteria. The members of each pair are called homologous chromosomes. For each chromosome exhibiting a specific length and centromcre placement. another exists with identical features. There are exceptions to the rule ofchroino— somcs in pairs. Bacteria and viruses have htit one chromosome. and organisms such as yeasts and molds. and certain plants such as hryophytcs (mosses) spend the predominant phase of the life cycle in the haploid stage. That is. they contain only one member of each homologous pair of chromosomes during most of their lives. Figure ’34 illustrates the physical appearance of different pairs of homologous chromosome-s. There. the human mitotic chromosomes have been photographed. cut out of the print. and matched up. creating a karyotype. As yoti can see. hu- mans have a 2n number of 46 and. on close examination. the chromosomes exhibit a diversity of sizes and centromere place— ments. Note also that each of the 46 chromosomes is clearly a double structure consisting of two parallel sister chromatids connected by a common centromere. Had these chromosomes been allowed to continue dividing. the sister chromatids. which are replicas of one another. would have separated into the two new cells as division continued. 2.2 In Diploid Organisms, Chromosomes Exist in Homologous Pairs 21 Middle Metacentric Between middle and end Submetacentric Close to end Acrocentric Teiocentric FIGURE 2-3 Centrornere locations and designations of chromosomes based on centromere location. Note that the shape of the chromosome during anaphase is determined by the position of the centromere. 13.“. 2;};- iii ii ii ill I” ‘l 5 W a it ii?» ii M iii 66 Mi 6 7 9 10 11 12 v nnnnan 3:53;“: 8 9% fi 13 14 15 16 17 1a" a t: a n A 4 e A n I I' 19 20 21 22 x 1/ FIGURE 2-4 A metaphase preparation of chromosomes derived from a dividing cell of a human male (left), and the karyotype derived from the metaphase preparation (right). All but the X and Y chromosomes are present in homologous pairs. Each chromosome is clearly a double structure, constituting a pair of sister chromatids joined by a common centromere. 22 Chapter 2 Mitosis and Meiosis TABLE 2.1 THE HAPLOID NUMBER or CHROMOSOMES FOR A VARIETY or ORGANISMS Common Scientific Hapioid Name Name Number Black bread mold Aspergillus nio‘ulans 8 Broad bean Vicia faba 6 Cat Fells domesticus 19 Cattle Bos teams 30 Chicken Gallus domesticus 39 Chimpanzee Pan troglodytes 24 Corn Zea mays 10 Cotton Gossypium hirsutum 26 Dog Canis familiaris 39 Evening primrose Oenothera blenni’s 7 Frog Rana pipiens 13 Fruit fiy Drosophila melanogaster 4 Garden onion Alli'um cepa 8 Garden pea Pisum sativum 7 Grasshopper Melanoplus differentialis 12 Green alga Chlamydomonas reinhardi' 18 Horse Equus caballus 32 House 'lly Musca domestica 6 The haploid number {a} ofchromosomes is equal to one-half the diploid number. Collectively. the total set of genes con- tained in a haploid set of chromosomes constitutes the genome of the species. The examples listed in Table 2.1 demonstrate the wide range of n values found in plants and animals. Homologous pairs of chromosomes have important genetic similarities. They contain identical gene sites along their lengths, each called a locus (p1. loci). Thus, they are identical in their genetic potential. in sexually reproducing organisms. one member of each pair is derived from the maternal parent {through the ovum) and one is derived from the paternal par- ent (through the sperm). Therefore. each diploid organism con- tains two copies of each gene as a consequence of biparental inheritance. As we shall see in the chapters on transmission genetics. the members of each pair of genes. while influencing the same characteristic or trait, need not be identical. In a pop— ulation of members of the same species. many different alter- native forms of the same gene. called alleles. can exist. The conceptual issues of haploid number, diploid number, and homologous chromosomes are. important in understanding the process of meiosis. During the formation of gametes or spores, meiosis converts the diploid number of chromosomes to the haploid number. As a result. haploid gametes or spores contain precisely one member of each homologous pair of chromosomesithat is. one complete haploid set. Following fusion of two gametes in fertilization, the diploid number is reestablished: that is. the zygote contains two complete haploid sets oi‘chromosomes. The constancy of genetic material is thus maintained from generation to generation. 2 Common Scientific Haploid Name Name Number House mouse Mus musculus 20 Human Homo sapiens 23 Jimson weed Datura stramoni'um 12 Mosquito Culex pipiens 3 Mustard plant Arabidopsis thaliana 5 Pink bread mold Neurospora crassa 7 Potato Solanum tuberosum 24 Rhesus monkey Macaca mulatta 21 Roundworm Caenorhabdi'tls elegans 6 Silkworm Bombyx mori 28 Slime mold Dictyostelium discoidium '1‘ Snapdragon Antirrhlnum majus 3 Tobacco Nicotiana tabacum 24 Tomato Lycopersr'con esculentum 12 Water fly Nymphaea alba 80 Wheat Triticum aestivum 21 Yeast Saccharomyces rerevisiae 16 Zebrafish Danio rerio 25 How [)9 WE Know? With the initial appearance oj'ihisfiiamie, a briefin- trorluctlou is in order: Throughout the text. set-12ml tunes in each chapter; we trill ideittifv important research ques— tions that have been torso-"cred as u result rif'geuerr‘c erv perimeumtiori. You are asked to relate each question to the previous discussion and review how We acquired the lt'l- forimuion elucidating the scientz'ficfinrllng. it is our hope that these simple arci‘cises will stimulate andfinetirne your analytical thinking Skills. How do we know that chromosomes exist in homolo— gous pairs? There is one important exception to the concept of homol- ogous pairs of chromosomes. In many species. one pair. the sex-determining chromosomes, is often not homologous in size. centromere placement. arm ratio. or genetic content. For example. in humans. females carry two homologous X chromosomes, while males carry one Y chromosome in ad- dition to one X chromosome (Figure 2—4). The X and Y chromosomes are not strictly homologous. TheY is consid erably smaller and lacks most of the gene sites contained on the X. Nevertheless, in meiosis they behave as homologs so that gametes produced by males receive either one X or one Y chromosome. Mitosis Partitions Chromosomes into Dividing Cells The process of mitosis is critical to all eukaryotic organisms. In some. single-celled organisms. such as protozoans and some fungi and algae. mitosis (as a part of cell division) provides the basis for asexual reproduction. Multicellular diploid organisms begin life as single-celled fertilized eggs called zygotes. The mitotic activity of the zygote and the subsequent daughter cells is the foundation [or the development and growth of the or- ganism. In adult organisms, mitotic activity is prominent in wound healing and other forms- of cell replacement in certain tissues. For example. the epidermal skin cells of humans are continuously sloughed off and replaced. Cell division also re- sults in the continuous production of reticulocytes that even- tually shed their nuclei and replenish the supply of red blood cells in vertebrates. in abnormal situations. somatic cells may lose control of cell division. forming a tumor. The genetic material is partitioned into daughter cells during nuclear division or karyokinesis. This process is quite com- plex and requires great precision. The chromosomes must first be exactly replicated and then accurately partitioned. The end result is the production of two daughter nuclei. each with a chromosome composition identical to that of the parent cell. Karyokinesis is followed by cytoplasmic division. or cytokinesis. The less complex division of the cytoplasm re- quires a mechanism that partitions the volume into two parts. then encloses both new cells in a distinct plasma membrane. Cytoplasmic organelles either replicate themselves. arise from existing membrane structures. or are synthesized rite now (anew) in each cell. The subsequent proliferation of these struc— tures is a reasonable and adequate mechanism for reconstitut- ing the. cytoplasm in daughter cells. Following cell division, the initial size of each new daughter cell is approximately one-half the size of the parent cell. How- ever. the nucleus of each new cell is not appreciably smaller than the nucleus of the original cell. Quantitative measurements of DNA confirm that there is an amount of genetic material in the daughter nuclei equivalent to that in the parent cell. lnterphase and the Cell Cycle Many cells undergo a continuous alternation between division and nondivision. The events that occur from the completion of one division until the beginning of the next division constitute the cell cycle (Figure 2—6]. We will consider the initial stage of the cycle. called interphase. as the interval between divisions. It was once thought that the biochemical activity during inter— phase was devoted solely to the cell‘s growth and its normal function. However. we now know that another biochemical step critical to the ensuing mitosis occurs during interphasc: the replication fifths DNA (ifeach chromosome. Occurring bel‘orc the cell enters mitosis, this period during which DNA is syn‘ thesized is called the S phase. The initiation and completion of synthesis can be detected by monitoring the incorporation oi‘ra- dioactive precursors into DNA. 2.3 Mitosis Partitions Chromosomes into Dividing Cells 23 Nondividing cells (31 Telophase Prophase Ana phase Metaphase FIGURE 2—5 The intervals comprising an arbitrary cell cycle. Following mitosis, cells enter the (31 stage of interphase. initiating a new cycle. Cells may become nondividing (GO) or continue through Gt, where they become committed to begin DNA synthesis (5) and complete the cycle (GZ and mitosis). Following mitosis, two daughter cells are produced and the cycle begins anew for both cells. Investigations of this nature show two periods during inter— phasc when no DNA synthesis occurs. one before and one after 5 phase. These are designated G1 {gap 1) and G2 {gap ll). re- spectively. During both of these intervals. as well as during 5. intensive metabolic activity. cell growth. and cell dilicerentiation occur. By the end of G2, the volume of the cell has roughly doubled. DNA has been replicated. and mitosis (M) is initie ated. Following mitosis, continuously dividing cells then re— peat this cycle {Gl. S. G2. M') over and over. shown in Figure 2—5. How Do WE Know? What experimental approach was used to demonstrate when DNA is duplicated during interphase‘.’ Much is known about the cell cycle based on in vim; (Latin for “in glass.” meaning in a test tube} studies. While the total length of the cell cycle varies among cells in viva (in living or- ganisms}. when grown in vitro (in culture). many cells traverse the complete cycle in about 16 hours. The actual process of mitosis occupies only a small part of the overall cycle. often less than an hour. The lengths of the S and G2 phase of interphase are l‘airly consistent among different cell types. Most variation is seen in the length of time spent in the Cl stage. Figure 2—6 shows the relative length of these intervals in a human cell in culture. GI is of great interest in the study of cell proliferation and its control. At a point late in Gl, all cells follow one 01‘ two paths. They either withdraw from the cycle. become quiescent and enter the G0 stage (see Figure 2—5 ). or they become com— mitted to initiating DNA synthesis and completing the cycle. Cells that enter G0 remain viable and metabolically active but 24 Chapter 2 Mitosis and Meiosis do not proliferate. Cancer cells apparently avoid entering G0 or pass through it very quickly. Other cells enter GO and never reenter the cell cycle. Still others remain in GO. but they can be stimulated to return to G] , and thereby reenter the cell cycle. Cytologically. interphase is characterized by the absence of visible chromosomes. Instead. the nucleus is filled with chro- matin fibers that have formed as the chromosomes have Lin- coiled and dispersed following the previous mitosis. This is diagrammed in Figure 2—7(a). Once G] . S. and G2 are completed. mitosis is initiated. Mitosis is a dynamic period of vigorous and Minutes continual activity. For discussion purposes, the entire process is FIGURE 2 6 Th t. t ' h h f I t subdivided into discrete stages, and specific events are assigned ' 9 'me Spe” m eac p ase 0 one comp e e to each one. These stages, in order of occurrence, are prophase, cell cycle of a human cell in culture. Times vary according to CE” types and conditions prometaphase, metaphase, anaphase, and telephase. Like interphase. these stages are also diagrammed in Figure 277. A photograph of each stage is shown along with each diagram. (d) Metaphase \ 1 \ .1 Microtubules a. (a) lnterphase ~. (b) Prophase 1.: t Kinetochore (a) lnterphase (b) Prophase (c) Prometaphase (d) Metaphase Chromosomes are Chromosomes coil up Chromosomes are clearly Centromeres align extended and uncalled, and shorten; centrioles double structures; centrioles on metaphase plate forming chromatin. divide and move apart reach the opposite poles; spindle fibers form FIGURE 2-7 Mitosis in an animal cell with a diploid number of 4. The events occurring in each stage are described in the text. Of the two homologous pairs of chromosomes, one contains longer, metacentric members and the other shorter, submetacentric members. The maternal chromosome and the paternal chromosome of each pair are shown in different colors. In if), the late telophase stage in a plant cell illustrates the formation of the cell plate and lack of centrioles. The light micrographs illustrating the stages of mitosis are derived from the flower of Haemanthus. Mitosis and the Cell Cycle Prophase Often. over half of mitosis is spent in prophase [Figure 2—7( 13)]. a stage characterized by several significant activities. One of the early events in prophase of all animal cells involves the mi- gration of two pairs of centrioles to opposite ends of the cell. These structures are found just outside the nuclear envelope in an area of differentiated cytoplasm called the centrosome. It is thought that each pair of centrioies consists of one mature unit and a smaller. newly formed centriole. The direction of migration of the centrioles is such that two poles are established at opposite ends of the cell. Following their migration, the centrioles are responsible for organizing cytoplasmic microtubules into a series of spindle fibers that are formed and run between these poles. This creates an axis along which chromosomal separation occurs. Interestingly. cells of most plants (with a few exceptions), fungi. and certain algae seem to lack centrioles. Spindle fibers are nevertheless appar- (e) Anaphase (e) Anaphase Centromeres split and daughter chromosomes migrate to opposite poles 2.3 Mitosis Partitions Chromosomes into Dividing Cells 25 ent during mitosis. Therefore, centrioles are not universally re- sponsible for the organization of spindle fibers. As the centrioles migrate. the nuclear envelope begins to break down and gradually disappears. In a similar fashion, the nucleo- lus disintegrates within the nucleus. While these events are tak- ing place, the diffuse chromatin fibers begin to conden se. continuing until distinct threadlike structures, or chromosomes. become visible. It becomes apparent near the end of prophase that each chromosome is actually a double structure split longi- tudinally except at a single point of constriction, the centromere. The two parts of each chromosome are called chromatids. Be» cause the DNA contained in each pair of chromatids represents the duplication of a single chromosome. these chromatids are ge— netically identical. Therefore. they are called sister chromatids. In humans. with a diploid number of 46. a cytological preparation of late prophase will reveal 46 chromosomes randomly distributed in the meat formerly occupied by the nucleus. / Cell plate Plant cell telophase (l) Telophase Daughter chromosomes arrive at the poles; Cytokenesis commences 26 Chapter 2 Mitosis and Meiosis Prometaphase and Metaphase The distinguishing event of the ensuing stages is the migration of each chromosome. led by the centromeric region, to the equatorial plane. In some descriptions, the term prometaphase refers to the period of chromosome movement. and metaphase is applied strictly to the chromosome configuration following migration. as depicted in Figures Zw't'tc'). The equatorial plane. also referred to as the metaphase plate. is the midline region of the cell. a plane that lies perpendicular to the axis established by the spindle. fibers. Migration is made possible by the binding of spindle fibers to a structure associated with the centromere ol‘ each chro- mosome called the kinetochore. This structure. consisting of multilayered plates of proteins. forms on opposite sides of each ccntromere. intitnately associating with the two sister chromatids of each chromosome. Once attached to micro- tubules making up the spindle fibers, the sister chromatids are now ready to be pulled to opposite poles during the en- suing anaphase stage. We know a great deal about spindle fibers. They consist of micro‘rubules. which themselves consist of molecular subunits of the protein tubulin. Microtubules seem to originate and “grow” out ofthe two centresome regions {containing the cenu-ioles) at opposite. poles of the cell. They are dynamic structures that lengthen and shorten as a result of the addition or loss 01‘ polar- ized tttbulin subunits. The microtubules most directly responsible for chromosome migration make contact with. and adhere to, kinctochores as they grow from the centrosome region. They are referred to as kinetochore microtubules and have one end near the centrosome region [at one of the poles of the cell} and the other anchored to the kinetochorc. Note that the nttmber of mi- crotubules that bind to the kinetochore varies greatly between or- ganisms. Yeast (Snot'hrnmnyces] have only a single microtttbttle bound to each platelikc structure of the kinetochore. Mitotic cells of mammals. at the other extreme. reveal 30 to 40 mict'otttbules bound to each portion 0]" the kinetochore. At the completion of metaphase. each centromere is aligned at the plate with the chromosome arms extending outward in a random array. This configuration is shown in Figures 277(d]. Anaphase Events critical to chromosome distribution during mitosis occur during the shortest stage of mitosis, anaphase. During this phase. sister chromatids of each chromosome disjoin (sepa— rate) from each other and migrate to opposite. ends of the cell. For complete disjunction to occur, each centromeric region must be split in two. This event signals the initiation of anaphase. Once it occurs. each chromatid is referred to as a daughter chromosome. Movement of daughter chromosomes to the opposite poles of the cell is dependent upon the centrmnere—spindle fiber at— tachment. Recent investigations reveal that chromosome mi- gration results from the activity of a series of specific proteins. generally called motor proteins. These proteins use the energy generated by the hydrolysis of ATP. and their activity is said to constitute molecular motors in the cell. These motors act at several positions Within the dividing cell. but all are involved in the activity of microlubules and ultimately serve to propel the chromosomes to opposite ends of the cell. The centromeres of each chromosome appear to lead the way during migration. with the chromosome arms trailing behind. The location of the centroinere determines the shape of the chromosome during separation. (See Figure 2—3.) The steps occurring during anaphase are critical in providing each subsequent daughter cell with an identical set of chro- mosomes. In human cells. there would now be 46 chromosomes at each pole. one front each original sister pair. Figure Z-th) shows ttiiaphrtse prior to its completion. With the initial appearance of the feature we {all "Now Solve This” a short introduction is in order. Occurring SEV- eral times in this and all ensuing chapters, each entry identifies a problem from the Problems and Discussion Questions section at the end of the chapten Each selec- tion is related to the discussion just presented. A com- ment is made about the problem and then a Hint is offered. Each hint provides you with analytical insight that will be useful as you soive the problem. Problem 2.5 on page 37 involves an understanding of what happens to each pair of homologous chromosomes during mitosis. Hint: The major issue in solving this problem is to un‘ derstand that throughout mitosis, members of each homologous pair do not pair up. but instead behave individually. Telophase Telophase is the final stage of mitosis and is depicted in Figure 2—76). At its beginning, there are two complete. sets of chromosomes, one at each pole. The most significant event is cytokinesis. the division or partitioning of the cytoplasm. Cytokinesis is essential it" two new cells are to be produced from one. The mechanism differs greatly in plant and animal cells. in plant cells. a cell plate is synthesized and laid down across the region of the metaphase plate. Animal cells. hows ever. undergo a constriction of the cytoplasm in much the same way a loop of string might be tightened around the midi die of a halloon. The end result is the same: Two distinct cells are formed. It is not surprising that the process oi" cytokinesis varies among cells of different organisms. Plant cells. which are more regularly shaped and structurally rigid, require a mechanism for depositing new cell wall material around the plasma mem— brane. The cell plate. laid down during telophase, becomes the middle lamella. Subsequently, the primary and secondary layers of the cell wall are deposited between the cell mem- brane and middle lamella on both sides of the boundary be- tween the two daughter cells. In animals. complete constriction of the cell membrane produces the cell furrow characteristic of newly divided cells. Other events necessary for the transition from mitosis to in; terphase are initiated during late telophase. They represent a 2.4 Meiosis Reduces the Chromosome Number from Diploid to Haploid in Germ Cells and Spores 27 general reversal of events that occurred during prophase. In each new cell. the chromosomes begin to uncoil and become diffuse chromatin once again, while the nuclear envelope re— forms around them. The nucleolus gradually reforms and be— comes visible in the nucleus during early interphase. The spindle fibers also disappear. At the completion of telophase, the cell enters interphase. How Do WE Know? How did we learn about the various stages making, up mitosis? ' saga; seats; the chmasssnae Number from Diploid to Haploid in Germ Cells and Spores The process of meiosis, unlike mitosis, reduces the amount of genetic material by one half. Whereas in diploids mitosis pro- duces daughter cells with a full diploid complement meiosis produces gametes or spores with only one haploid set ol‘chro— mosomes. During sexual reproduction, gametes then combine in fertilization to reconstitute the diploid complement found in parental cells. Figure 2—8 compares the two processes by fol- lowing two pairs of homologous chromosomes. Mitosis Prometaphase is Sister ., chromatids Metaphase (four chromosomes, each consisting of a pair of sister chromatids) Daughter cell (2n) (2n) Daughter cell Diploid cell (2n = 4) Meiosis I Prophase l (synapsis) /, Tetrad Metaphase (two tetrads) Reductional division Equational division FIGURE 2—8 Overview of the major events and outcomes of mitosis and meiosis. As in Figure 2W7, two pairs of homologous chromosomes are followed. Ir: ’5 .9 cu .2 “5 d-l' > 0 28 Chapter 2 Mitosis and Meiosis Meiosis must be highly specific because, by definition. haploid gametes or spores contain precisely one member of each homol- ogous pair of chromosomes. Successfully completed, meiosis en- sures genetic. continuity from generation to generation. The process of sexual reproduction also ensures genetic variety among mem- bers of a species. As you study meiosis. you will see that this process results in gametes with many unique combinations of ma— ternally and paternally derive-d chromosomes among the haploid complement. With such a tremendous genetic variation among the gametes a large number of chromosome combinations are possi- ble at fertilization. Furthermore, we shall see that the meiotic event referred to as crossing over results in genetic exchange between members of each homologous pair of chromosomes. This creates intact chromosomes that are mosaics of the maternal and paternal homologs from which they are derived. further enhancing the po- tential genetic variation in gametes turd the offspring derived from them. Sexual reproduction therefore reshuffles the genetic mate- rial. producing offspring that often differ greatly from either par- cut. This process constitutes the major form of genetic recombination within species. An Overview of Meiosis In the preceding discussion. we established what might be considered the goals of meiosis. Before we consider the phases of this process systematically, we will briefly exam- ine how diploid cells give rise to haploid gametes or spores. You should refer to the meiotic portion ofFigure 2—8 during the following discussion. You have seen that in mitosis each paternally and maternally derived member of any given homologous pair of chromosomes behaves autonomously during division. By contrast. early in meiosis, homologous chromosomes form pairs: that is. they synapse. Each synapsed structure is initially called a bivalent, which eventually gives rise to a unit, the tetrad, consisting of four chromatids. The presence of four chromatids demonstrates that both homologs (making up the bivalent) have. in fact. du— plicated. Therefore, in order to achieve haploidy. two divisions are necessary. in meiosis I, described as a reductional divi- sion (because the number of ccntromeres, each representing one chromosome. is reduced by one half following this divi- sion), components of each toned—representing the two honiologs—separate. yielding two dyads. Each dyad is com- posed ol'two sister chromatids joined at a common centromere. During meiosis ll. described as an equational division (because the number of centromeres remains equal following this division). each dyad splits into two mounds of one chromosome each. Thus. the two divisions potentially produce four haploid cells. The First Meiotic Division: Prophase | We turn now to a detailed account of meiosis. As in mitosis, meio- sis is a continuous process. We name the parts of each stage of di- vision only to facilitate discussion. From a genetic standpoint, three events characterize the initial stage. prophase I (Figure 2—9). First, as in mitosis, chromatin present in inteiphase thickens and coils into visible chromosomes. Second. unlike mitosis, mem— Meiotic prophase I Leptonerna Chromomeres l Zygonema Bivalents Pachynema Tetrad Diplonema -Chtasma Dia kinesis Terminalization FIGURE 2—9 The substages of meiotic prophase I for the chromosomes depicted in Figure 2—8. bers of each homologous pair of chromosomes undergo synapsis. Third, crossing over= an exchange process, occurs between synapsed homologs. Because of the complexity of these genetic events. this stage of meiosis has been further divided into live substages: leptonemaf‘ zygonemafi‘ pachynernaf?‘ diplonemafi‘ *Tlicse are the noun forms of these substages. The. adjective forms (leplotcne, zygolenc. pachytene. and diplotenei are also used in the text. 2.4 Meiosis Reduces the Chromosome Number from Diploid to Haploid in Germ Cells and Spores 29 and diakinesis. As we discuss these substagcs. be aware that. even though it is not immediately apparent in the earliest phases of meiosis. the DNA of chromosomes has been replicated during the prior inter-phase. Leptonema During the leptotene stage. the interphase cbro~ matin material begins to condense. and the chromosomes. al- though still extended. become visible. Along each chromosome are ch romomeres. localized condensations that resemble beads on a string. Recent evidence suggests that a process called homology search. which precedes and is essential to the irri- tial pairing of homologs. begins during leptonema. Zygonema The chromosomes continue to shorten and thicken during the zygotene stage. During the process of homology search. homologous chromosomes undergo a loose alignment with one another. which is complete by the end of zygonema. As meiosis proceeds. an ultrastructural component lone visible only under the electron microsopel called the synaptonemal complex is formed between the homologs. We discuss this meiotic component later in the chapter. At the completion oi'zygonema. the paired homologs are re- ferred to as bivalents. Although both members oi’each bivalent have already replicated their DNA. it is not yet visually appar~ ent that each member is a double structure. The number of bi~ valents in each species is equal to the haploid tn} number. Pachynema in the transition from the zygotene to the pachytene stage. coiling and shortening of chromosomes cort- tinues. and further development of the synaptonemal complex occurs between the two members of each bivalent. This leads to a more intimate pairing referred to as synapsis. Compared to the rough-pairing characteristic of zygonema. hoinologs are now separated by only 100 nm. During pachynema. each homolog is first evident as a dou- ble structure. providing visual evidence of the earlier replin cation of the DNA of each chromosome. Thus. each bivalent contains four member chromatids. As in mitosis, replicates are called sister chromatids, while chromatids from maternal and paternal members of a homologous pair are called noti- sister chromatids. The t'ouremembered structure is also re- ferred to as a tetrad. and each tetrad contains two pairs of sister chromatitis. Diplonema During the ensuing diplotene stage. it is even more apparent that each tetrad consists oi" two pairs of sister chromatids. Within each tetrad. each pair of sister chromatids begins to separate. However. one or more areas remain in con- tact where chromatids are intertwined. Each such area. called a chiasma (pl. chiasmata’). is thought to represent a point where nonsister chromatids have undergone genetic exchange through the process referred to above as crossing over. Although the physical exchange between chromosome areas occurred during the previous pachytene stage. the result of crossing over is visible only when the duplicated chromosomes begin to sep- arate. Crossing over is an important sotrrce of genetic varis ability. indicated earlier. new combinations oi" genetic material are formed during this process. Diakinesis The linal stage of prophase l is diakinesis. The chromost‘rmes pull farther apart. but nonsister chromatids re- main loosely associated via the chiasmata. As separation pro- ceeds. the chiasmata move toward the ends of the tetrad. This process. called terminalization. begins in late diplonema. and is completed during diakinesis. During this substage period of prophase I. the nucleolus and nuclear envelope break down, and the two centrorneres oi" each tetrad attach to the recently l'ormed spindle fibers. By the completion of prophase [. the centromeres of each tetrad structure are present on the equato- rial plate of the cell. Metaphase, Anaphase. and Telophase i The. remainder of the meiotic process is depicted in Figure 2—10. Following the first meiotic prophase. steps simr ilar to those of mitosis occur. In the metaphasc of the first di— vision (metaphase I). the chromosomes have maximally shortened and thickened. The terminal chiasmata of each tetrad are visible and appear to be the only factor holding the nonsister chromatids together. Each tetrad interacts with spindle fibers. facilitating movement to the nietaphuse plate. The alignment of each tetrad prior to this first anaphase is random. One ball” of each tetrad is pulled to one or the other pole at random. and the other half then moves to the oppo- sitc pole. During the stages of meiosis l. a single centromere holds each pair of sister chromatids together. It does not tlivide. At anaphase I. one half of each tetrad {one pair of sister chromatitlsicalled a dyad} is pulled toward each pole of the dividing cell. This separation process is the physical basis of what we refer to as disjunction. the separation of chromosomes from one another. Occasionally. errors in meiosis occur and separation is not achieved. as we will see later in this chapter. The term nondisjunction describes such an error. At the completion of the normal anaphase [. a series of dyads equal to the haploid nttmber is present at each pole. If crossing over had not occurred in the first meiotic prophase. each dyad at each pole would consist solely of either paternal or maternal chromatids. However. the exchanges produced by crossing over create mosaic cliromatids of paternal and mater- nal origin. In many organisms. telophase I reveals a nuclear ment— hrane forming around the dyads. Next. the nucleus enters into a short. interphase period. In other cases. the cells go directly from the first anaphase into the second meiotic division. It" interphase occurs. the chromosomes do not replicate because they already consist of two chromatids. in general. meiotic telophase is mtrch shorter than the corresponding stage in mitosis. The Second Meiotic Division A second division, referred to as meiosis II, is essential if each gamete or spore is to receive only one chrornatid from each original tetrad. The stages characterizing meiosis II are shown in the bottom half of Figure 2—10. During pr'ophase [1. each dyad is composed of one pair of sister chromatids 30 Chapter 2 Mitosis and Meiosis attached by a common centromere. During metaphase II, the centromeres are positioned on the equatorial plate. When they divide. anaphase II is initiated, and the sister chro- matids of each dyad are pulled to opposite poles. Because the number of dyads is equal to the haploid number. telophase II reveals one. member of each pair of homologous chro- mosomes present at each pole. Each chromo- some is referred to as a monad. Following cytokinesis in telophase 11, four haploid ga— metes may result from a single meiotic event. At the conclusion of meiosis, not only has the haploid state been achieved, but it" crossing over has occurred, each monad is a combina- tion of maternal and paternal genetic infor- mation. As a result. the offspring produced. by any gamete will receive a mixture of genetic information originally present in his or her grandparents. Meiosis thus significantly in- creases the level of genetic variation in each ensuing generation. Metaphase | n. H: a. "a ‘53 i. . Anaphase l Telophase I roblem 2,-9 on page 37 involves an un- derstanding of what happens to the meter- nal and paternal members of each pair of homologous chromosomes during meiosis. Hint: The major issue in solving this prob- lem is to understand that maternal and paternal homologs synapse during meio- sis. Once it is evident that each chromatid has duplicated, creating a tetrad in the early phases of meiosis, each original pair behaves as a unit and leads to two dyads during anaphase |. Prophase ll Metaphase II . How Do WE Know? How do we know that meiosis has a dif- ferent final outcome than mitosis? Anaphase || Telophase || FIGURE 2—10 The major events in meiosis in an animal with a diploid number of 4, beginning with metaphase I. Note that the combination of chromosomes in the cells produced following telophase II is dependent on the random alignment of each tetrad and dyad on the equatorial plate during metaphase l and metaphase II. Several other combinations. which are not shown, can also be formed. The events depicted here are described in the text. Haploid gametes 2.5 The Development of Gametes Varies during Spermatogenesis and Oogenesis 31 The Development of Gametes Varies during Spermatogenesis and Oogenesis Although events that occur during the meiotic divisions are similar in all cells participating in gametogenesis in most anie mai species, there are certain differences between the produc- tion of a male gamete [sperrnatogenesis) and a female gamete (oogenesis). Figure 2—11 summarizes these processes. Spermatogenesis takes place in the testes, the male re- productive organs. The process begins with the expanded growth of an undifferentiated diploid germ cell called a spermatogonium. This cell enlarges to become a primary Spermatogonium Primary spermatocyte Secondary spermatocytes Spermatozoa l GrowthIMaturation Meiosis | Secondary { l ‘ Spermatids Differentiation spermatocyte, which undergoes the first meiotic division. The products of this division, called secondary spermato- cytes. contain a haploid number of dyads. The secondary spermatocytes then undergo the second meiotic division, and each of these cells produces two haploid spermatids. Spermatids go through a series of developmental changes, spermiogenesis, and become highly specialized, motile spermatozoa, or sperm. Ali sperm cells produced during spermatogenesis receive equal amounts of genetic material and cytoplasm. Spermatogenesis may be continuous or may occur periodi- cally in mature male animals, with its onset determined by the nature of the species’ reproductive cycle. Animals that repro- duce year-round produce sperm continuously, whereas those Oogonium Primary oocyte First polar body Ootid Second polar body Ovum FiGURE 2—11 Spermatogenesis and oogenesis in animal cells. 32 Chapter 2 Mitosis and Meiosis whose breeding period is confined to a particular season pro- duce sperm oniy during that time. In animal oogenesis. the formation of ova (sing. ovum), or eggs. occurs in the ovaries, the female reproductive organs. The daughter cells resulting from the two meiotic divisions receive equal amounts of genetic material. but they do not receive equal amounts of cytoplasm. Instead. during each division. almost all the cytoplasm ofthe primary oocyte. itself derived from the oogonium. is concentrated in one of the two daughter cells. The concentration of cytoplasm is necessary because a major function of the mature ovum is to nourish the developing emv bryo following fertilization. During the first meiotic anaphase in oogenesis. the tetrads of the primary oocyte separate. and the dyads move toward op- posite poles. During the first telophase, the dyads present at one pole are pinched off with very little surrounding cytoplasm to fortn the first polar body. The other daughter cell produced by this first meiotic division contains most of the cytoplasm and is called the secondary oocyte. The first polar body may or may not divide again to produce two small haploid cells. The mature ovum will be produced from the secondary oocyte during the second meiotic division. During this division. the cytoplasm of the secondary oocyte again divides unequally. producing an ootid and a second polar body. The ootid then difi‘erentiates into the mature ovum. Unlike the divisions of spermatogenesis. the two meiotic divisions of oogenesis may not he continuous. In some ani- mal species. the two divisions may directly follow each other. In others. including humans. the first division of all oocytes begins in the embryonic ovary. but arrests in prophase [. Many years later, meiosis resumes in each oocyte just prior to its ovulation. The second division is completed only after fertilization. 'Problem 2.14 on page 38 involves an understanding of meiosis during oogenesis. Hint: To answer this question, you must take into ac— count that crossing over occurred during meiosis I be— tween each pair at homologs. Meiosis Is Critical to ii Successful Sexual Reproduction of All Diploid Organisms The process ofmeiosis is critical to the successful sexual re- production of all dipioid organisms. It is the mechanism by which the diploid amount of genetic information is reduced to the haploid amount. In animals. meiosis leads to the for- mation of gametes. whereas in plants haploid spores are pro- duced. which in turn lead to the formation of haploid gametes. Funhennore. the mechanism of meiosis is the basis for the pro duction of extensive genetic variation among members of a pop- ulation. As we have learned. each diploid organism contains its genetic information in the form of homologous pairs of chromo- somes. one member ofeach pair derived from the maternal par— ent and one member from the paternal parent. Following the reduction to haploidy. gametes or spores contain either the pa- ternal or the maternal representative ol‘every homologous pair of chromosomes. During sexual reproduction. this process has the potential of producing huge quantities of genetically dissimilar ga- metes. As the number of homologous chromosomes (the haploid number) increases. the possibilities of different combinations of maternai and paternal chromosomes in any given gamete increase. An organism can produce 2n nutnber of combinations. where n represents the haploid number. For example. an organism with a haploid number of It} will produce 210, or 1024. combinations. Now calculate the number of different combinations of sperln or eggs in our own species: 233. When you arrive at the answer. you cannot help but be impressed with the potential for genetic vari- ation restiiting from meiosis. The process of crossing over during meiotic prophase I fur- ther resliuffies the genetic information between the maternal and paternal members of each homologous pair. As a t'esuit. endless varieties of each homolog may occur in gametes. rang- ing from either intact maternal or paternal chromosomes. where no exchange occurred. to any mixture of maternal and paternal components, depending on where one or more exchanges oc- curred dun'ng crossing over. In sum. the two tnost significant points about meiosis are that the process is responsible for 1. the maintenance ofequivalent genetic information between generations, and 2. extensive genetic variation within populations. It is important to touch briefly on the significant role that meiosis plays in the life cycles of fungi and plants. In many fungi. the predominant stage of the life cycle consists of hap— loid vegetative cells. They arise through meiosis and prolifet“ ate by mitotic cell division. In multiceliular plants. the life cycle alternates between the diploid sporophyte stage and the hope loid gametophyte stage. While one or the other predominates in different plant groups during this “alternation of genera- tions." the processes of meiosis alid fertilization constitute the "bridge" between the sporophyte and gametophyte generations (Figure 2—12). Therefore, meiosis is an essential component of the life cycle of plants. Finally. we can ask what happens when meiosis fails to achieve the normal outcome. In rare cases during meiosis l or meiosis ll. separation. or disjunction. of the chromatids of a tetrad or dyad fails to occur. Instead. both members move to the same pole during anaphase. Such an event is called nondisjunction. because the two members fail to disjoin. Nondisjunction during meiosis l or meiosis ll leads to gametes with abnormal numbers of chromosomes compared to the hap— loid number. If such a gamete participates in fertilization. abnormal offspring often result. This shall be a major topic discussed in Chapter 8. 2.7 Electron Microscopy Has Revealed the Cytological Nature of Mitotic and Meiotic Chromosomes 33 Chromatin and Microsporangium Chromosomes Duringr interphase. only dispersed chromatin fibers are present in the nu— cleus [Figure 2713mm Once mitosis begins. however. the fibers coil and Fold, condensing into typical mitotic Megasporangium chromosomes [Figure 2—13tbil. lfthe fibers comprising the mitotic chromo- __ some are loosened, areas of greatest Sr‘ophyte ‘ spreading reveal individual fibers sim- ilar to those seen in interphase chro- matin [Figure 2713(ci]. Very few fiber ends seem to be present. and in some cases. none can be seen. Instead. indi- vidual fibers always seem to loop back into the interior. Such fibers are obvi- ously twisted and coiled around one another. forming the regular pattern of the mitotic chromosome. Starting in late telophase of mitosis and continue ing during G] of interphase. chromo— / Zygote Diploid (2n) Fertilization -- . . ‘ Meiosis Haploid (n) ' Megaspore (n) Female Sperm gametophy’ce (embryo sac) Microspore (n) Male . ‘ gametophyte somes then unwind to torm the long (pollen grain) fibers characteristic of chromatin. which consist of DNA and associated FIGURE 2—12 Alternation of generations between the diploid sporophyte (2n) and the proteins‘ partlcumrly protems called haploid gametophyte in) in a multicellular plant. The processes of meiosis and hmtones' h 15 1” “m PhYS'Calfnlange‘ fertilization bridge the two phases of the life cycle. This is an angiospermr where the "mm mm DNA ‘33” mm“. Gimme-“Ely sporophy'te stage is the predominant phage, iunction dtll‘ll‘tg transcription and replication. Electron micrOscopic observations of mitotic chromosomes in varying states of coiling ietl Ernest DuPraw to postulate the folded-fiber model‘ shown in Figure 2—] 3(d). During metaphase. each chromosome consists of two sister chromatids joined at the centromeric region. Each arm of the chromatid appears to consist of a single fiber wound much like a skein of yarn. The fiber is composed of tightly Hint: YOU mUSt apply the iU'es 0f PFObabilitY here coiled double-stranded DNA and protein. An orderly for mUItiple avian?“ Occurring independently 0‘: coiling—twisting—condensing process appears to be involved in one anothfirj Th's mvcwes the prod?“ Ialw Where the transition of the interphase chromatin to the more con) the prgbabmty Of many events occumng i'm‘i'iane‘ densed. mitotic chromosomes. it is estimated that during the ously '5 equal to the pmduct Of the '"d'v'dua' tr'insitionl‘rominterjha‘ ~ i4 ‘ stinn—i‘ 1d - t - ' probabilimi t ' i use to plop iase, a . 0- con raction occurs in the length oi DNA Within the chromatin l'ibei'! This process must be extremely precise. given the highly ordered . ._ .. .. N .. nature and consistent appearance of mitotic chromosomes in all Electron Microscopy Has Revea|ed eukaryotes. Note particularly in the-micrographs the clear dis- the Cytological Nature of Mitotic tinction between the sister Chromatids constituting each chroe . . mosome. They are joined only by the common centromei‘e that and Me'ouc Chromosomes they share prior to anaphase. i § “Problem 2.20 on page 38 concerns the probability that any particular mixture of maternal or paternal homologs end up together in a gamete. Thus far in this chapter. we have focused on mitotic and mei- otic chromosomes. emphasizing their behavior during cell di- vision and gamete formation. You might wonder why chromosomes are invisible during interphase but visible during the various stages of mitosis and meiosis. Studies using elec— tron microscopy cieariy show why chromosomes are visible only during division stages. How Do WE Know? How do we know that mitotic chromosomes are derived from interphase chromatin? Mitosis and Meiosis 34 Chapter 2 (d) (C) FIGURE 2—13 Comparison of (a) the chromatin fibers characteristic of the interphase nucleus with (b) and (c) metaphase chromosomes that are derived from chromatin during mitosis. Part (d) diagrams the mitotic chromosome and its various components, showing how chromatin is condensed into it. Parts (3) and (c) are transmisséon electron micrographs, while part (b) is a scanning electron micrograph. The Synaptonemal Complex The electron microscope has also been used to visualize an- other ultrastructural component of the chromosome found only in cells undergoing meiosis. This structure, first intro- duced during our earlier discussion of the first meiotic prophase stage. is found between synapst homologs and is called the synaptonemal complex.‘:" in 1956, Montrose Moses observed this complex in spermatocytes of crayfish, and Don Faweett saw it in pigeon and human spermatocytes. Because there was not yet any satisfactory explanation of the mechanism of synapsis or of crossing over and chiasmata formation. many researchers became interested in this structure. With few ex- ceptions, the ensuing studies revealed the synaptonemal com- plex to be present in most plant and animal cells Visualized during meiosis. As you can see in the electron micrograph in Figure 2—] die), the synaptonernal complex is a tripartite structure. The central element is usually less dense and thinner (100—150 A) than the two identical outer elements [500 A). The outer structures, called lateral elements. are intimately assoeiated with the *An alternative spelling of this term is synaptinemal complex. synapsed homologs on either side. Selective staining has re- vealed that these lateral elements consist priinaril y of DNA and protein. suggesting that chromatin is an essential part of them. Some DNA fibrils traverse these lateral elements. making con- nections with the central element, which is composed primer ily of protein. Figure 2—l4tb) provides a diagrammatic interpretation of the electron micrograph consistent with the foregoing description. The formation of the synaptonemal complex begins prior to the pachytene stage. As early as leptonema of the first meiotic prophase. lateral elements are seen in assodation with sister chromatids. Homologs have yet to associate with one another and are randomly dispersed in the nucleus. As we saw earlier, by the next stage, zygonema. homologous chromosomes begin to align with one another in what is called rough pairing. but they remain distinctly apart by some 300 nm. Then, during pachynema, the intimate asso- ciation between homologs1 characteristic of synapsis, oc- curs as Formation of the complex is completed. In some diploid organisms, this occurs in a zipperlike fashion. be- ginning at the ends ofthe chromosomes. which may be at- tached to the nuclear envelope. The synaptonemal complex is the vehicle for the pairing of homologs and their subsequent segregation during meiosis. where no synaptonemal complexes are formed. Thus, it is possible that the function of this structure may go beyond its involvement in the formation of bivalents. in certain instances where no synaptonemal complexes are formed during meiosis, synapsis is not complete and crossing over is reduced or eliminated. For ex- ample, in male Drosopliit'a meianugaster; where synaptone- mal complexes are not usually seen, meiotic crossing over rarely. if ever, occurs. This observation suggests that the synaptonemal complex may be important in order for chiasntata to form and crossing over to occur. The study of sip]. a mutation in the yeast S'acchru‘omyces cere- visiae. has provided further in- sights into chromosome pairing. Cells bearing this mutation can undergo the initial alignment stage [rough pairing) and full— length central and lateral element formation, but fail to achieve the intimate pairing charac- teristic of synapsis. It has been suggested that the gene prod- uct of the zip] locus is a protein component of the central element of the synaptonemal complex. since it is absent in Chapter Summary 35 Lateral , elements / Synaptonemal complex Central element Chromatin fiber FIGURE 2—14 (a) Electron micrograph of a portion of a synaptonemal complex found between synapsed bivalents of Neotiella rutr'lans. (b) Schematic interpretation of the components making up the synaptonemal complex. The lateral elements, central element. and chromatin fiber are labeled. D. van WettsteinlAnnual Reviews, lncJWith permission, from "Annual Review of Genetics, " Volume 6. ©1972 by Annual Reviews, inc. W.AnnualReviews.org. Photo from D. Van Wertstei'n. mutant cells. This observation further suggests thal a com- plete and intact synaptonemal complex is essential during the transition from the initial rough alignment stage to the intimate pairing characteristic of synapsis. CHAPTER SUMMARY 1. The structure of cells is elaborate and complex. Many compo- are pulled apart and directed toward opposite poles. Telophase 2. nents of cells are involved directly or indirectly with genetic processes. In diploid organisms, chromosomes exist in homologous pairs. Each pair shares the same size, centromere placement, and gene Sites. One member of each pair is derived from the maternal parent and one is derived from the paternal parent. Mitosis and meiosis are mechanisms by which cells distribute genetic information contained in their chromosomes to progeny cells in a precise. orderly fashion. Mitosis. or nuclear division, is part of the cell cycle and is the basis of cellular reproduction. Daughter cells are produced that are genetically identical to their progenitor cell. Mitosis may be subdivided into discrete stages: prophase. prometaphase. metaphase. anaphase, and telophase. Condensa- tion of chromatin into chromosome structures occurs during prophase. During prometaphase, chromosomes appear as dou— ble structures. each composed of a pair of sister chromatids. In metaphase. chromosomes line up on the equatorial plane of the cell. During anaphase, sister chromatids 01" each chromosome completes daughter cell formation and is characterized by cy- tokinesis. the division of the cytoplasm. Meiosis, the underlying basis of sexual reproduction. results in the conversion 01" a diploid cell to a haploid gamete or spore. As a result at chromosome duplication and two subsequent divi- sions. each haploid cell receives one member of each homologous pair of chromosomes. A major dil'l’erence exists between meiosis in males and females. Spermatogenesis partitions cytoplasmic volume equally and pro duces four haploid sperm cells. OogenEsis, on the other hand, ac- cumulates the cytoplasm in one egg cell and reduces the other haploid sets of genetic material to polar bodies. The extra cyto- plasm contributes to zygote development following fertilization. Meiosis results in extensive genetic variation by virtue of the ex- change during crossing over between maternal and paternal chro— matids and their random segregation into gametes. In addition. meiosis plays an important role in the life cycles of fungi and plants, serving as the bridge between alternating generations. Mitotic chromosomes are produced as a result of the coiling and condensation of chromatin fibers characteristic of interphuse. Mitosis and Meiosis 36 Chapter 2 With this initiai appearance of "Insights and Solutions,” it is ap- propriate to describe its value to you as a student. This section pre- cedes the “Problems and Discussion Quest-fans" in each chapter and provides sample pmblems and solutions that demonstrate ap- proaches usefiti in genetic anaiysis. The insights you gain by work- ing through this section will help you arrive or correct solutions to ensuing problems in each chapter: 1. In an organism with diploid number of 6, how many individual chromosomal structures will align on the metaphase plate during (a) mi- tosis, (b) meiosis I, and (c) meiosis [1? Describe each configuration. Solution: (3) In mitosis, Where homologous chromosomes do not synapse, there will be 6 double structures. each consisting of a pair of sister chromatids. The number of structures is equiva- lent to the diploid number. (b) In meiosis I, the homologs have synapsed. reducing the number of structures to 3. Each is called a tetrad and consists of two pairs of sister chromatids. (cl In meio» sis II, the same number of structures exist (3). but in this case they are called dyads. Each dyad is a pair of sister chromatids. When crossing over has occurred, each ehromatid may contain parts of one of its uonsister chromatids, obtained during exchange in prophase I. 2. Consider two pairs of chromosomes, one larger and metacen- trio and the other smaller and metacentn’c. Draw all possible align- ment configurations that can occur during metaphase of meiosis 1. Solution: As shown in the diagram below, four configurations are possible when it = 2. Case lV 3. For the genes and chromosomes in the previous problem, as- sume one gene is present on both of the larger chromosomes with two alleles. A and a. as shown. Also assume a second gene with two alleles (B, b} is present on the smaller chromosomes. Calculate the probability of generating each gene combination (A8, A11. oB‘ ab) lollowing meiosis I. Solution: Case I AB and ab Case ll Ab and a8 Case [11 dB and A!) Case [V ab and AB Total: AB = 2 (p = U4) Ab = 2 (p = 1/4) (:8 = 2 [p = U4) ctb=2 (p=1/¢l) 4. How many different chromosome configurations can occur fol— lowing meiosis i it' three different pairs of chromosomes are present (I: = 3')? Solution: If n = 3. then eight diiferent coni‘tgurations would be possible. The formula 2". where n equals the hapioid number. will allow you to calculate the number of potential alignment patterns. As we will see in the next chapter, these patterns are produced as a result of the Mendelian postulate called segregation, and they serve as the physical basis of the Mendelian postulate of independent (tssormtcnr. 5. Describe the composition of a meiotic tetrad as it exists during prophase 1. assuming no crossover event has occurred. Problems and Discussion Questions 37 What impact would a single crossover event have on this structure"? Solution: Such a tenad contains four chromaticis. existing as two pairs. Members of each pair are replicas of one another and are called sister chromatids. They are held together by a common centromere. Members of one pair are maternally derived, whereas members of the other an: paternally derived. Matemal and paternal members are called nonsistcr cbt'ornatids. A single crossover event has the effect of ex- changing a portion of a maternal and a paternal chromaticl. leading to a chiasma. where the two involved cltrontatids overlap physically in the tetrad. The process of exchange is referred to as crossing over. PROBLEMS AND DISCUSSION QUESTIONS 1. Explain the role the following cellular components play in the storage, expression. or transmission of genetic inl’ortnar tion: (a) chromatin. (b) nucleolus, (c) ribosome. (d) mito- chondrion. (e) centriolc. (i) centromere. 2. Discuss the concepts of homologous chromosomes. diploidy. and haploidy. What characteristics are shared between two chromosomes considered to be homologous? 3. If two chromosomes of a species are the same length and have similar centromere placements yet are not homologous. what is different about them 1’ 4. Describe the events that characterize each stage of mitosis. if an organism has a diploid number 0116, how many chromatids are visible at the end of mitotic prophase? How many chromo- somes are moving to each pole during anaphase of mitosis? Ln 6. Describe how chromosomes are named on the basis of theircen- tromerc placement. 7. Contrast telophase in plant and animal mitosis. 8. Describe the phases of the cell cycle and the events that charac- terize each phase. 9. An organism has a diploid number of 16 in a primary ooeyte. (a) How It]. 11. 12. many tetrads are present in the first meiotic pt'ophase? (b) How in any dyads are present in the second meiotic prophase? (c) How many monads migrate to each pole during the second meiotic artaphasc‘.’ Contrast the end results of meiosis with those of mitosis. Define and discuss these terms: (a) synapsis, (b) bivalents, (c) chi- asnutta. (d) crossing over, (e‘) chromomeres, (l) sister chromatids. (g) tetrads, (h) dyads, (i) monads. Contrast the genetic content and the on' gin of sister versus nonsis- tcr chromatids during their earliest appearance in prophetse l of 38 Chapter 2 Mitosis and Meiosis meiosis. How might the genetic content of these change by the time tetrads have aligned at the equatorial plate during metaphase l‘? 13. Given the end results of the two types of division. why is it nec- essary for homologs to pair during meiosis and not desirable for them to pair during mitosis? l4. Examine Figure 2—1 l. which shows oogenesis in animal cells. Will the genotype of the second polar body (derived from meior sis II) always be identical to that of the ootid'i Why or why not? 1:). Contrast spennatogenesis and oogerresis. What is the significance of the formation of polar bodies“? 16. Explain why meiosis leads to significant genetic variation while mitosis does not. 17. A diploid cell contains three pairs of homologous chromosomes designated C1 and (‘2. M1 and M2. and Si and SB. No crossing over occurs. Viihat possible combinations of chromosomes will be present in (a) daughter cells following mitosis'? (b) the first meiotic metaphase'? (cl haploid cells following both divisions of meiosis? 18. Considering die preceding problem. predict the number ot‘difi‘er— ent haploid cells that will occur it a fourth chromosome pair (WI and W2) is considered in addition to the C. M. and S chromosomes. Extra-Spicy Problems As part oft/re “Problems and Discussion Questions " section in each chapter; we slrrilipresenr a number of "Extra-Spicy " genetics prob- lems. We here chosen to set there upon in order to identify prob iemr‘ that are particularly ci'rtriicnging. You may be asked to examine and assess octrrai data. to design genetics experiments. or to engage in cooperative learning. Like. genetic varieties ofpeppers. some of these experiences rrrejrrst spicy and some are very hot. Hopefully. nir‘ oftirem wiii leave an riflertoste rim! is pleasing to those who indulge tirerrrseit‘es. For Questions 3:;- [ii at the right. consider a dipioid cell that contains three pairs of chromosomes designated AA. BB. and CC. Each pair contains a maternal and a paternal member reg. A’" and A” ]. Using these designations. demonstrate your understanding of mitosis and meiosis by drawing chromatid combinations as requested. Be sure to indicate when chromatids are paircd as a result ot‘ replication audior synopsis. You may wish to use a large piece of brown manila wrapping paper or a cut-up paper grocery bag and work with another student as you deal with these problems. Such cooperative learning may be a useful approach as you solve problems throughout the text. SELECTED READINGS Alberts. 13.. et al. 2002. :lr'oiecnitrr biology ofn're ceii, 4th ed. New York: Garland. Brachet. 1.. and Mirsky. A.E. 19m. The cell: Meiosis onr] mitosis. Vol. 3. Orlando. FL: Academic Press. DuPraw_ EJ. 1970. DNA and chromosomes. New York: Holt. Rinehat‘t & Winston. Glover. D.M., Gonzalez. C.. and Rafi. J.W. 1993. The centrosome. Sci. Am. (lune) 268:62e68. Golomb. H.M.. and Bahr. G.F. 1971. Scanning electron microscopic observations of surface structures of isolated human chromosomes. Science 17121024—26. 19. During oogetresis in an animal species with a haploid number ot‘6. one dyad undergoes nondisjunctioo during meiosis Ll. Following the second meiotic division. the involved dyad ends up intact in the ovum. How litany chromosomes are present in (a) the mature ovum and (b) the second polar body? (c) Following fertilization by it nor- mal sperm. what chromosome condition is created? 20. What is the probability that. in an organism with a haploid number of 10. a Sperm will be l'ormed that contains all 10 chromosomes whose centromcrcs were derived from maternal homologs'i 21. During the first meiotic pmphase. (a) when does crossing over occur“? ([3) when does synapsis occur“? to) during which stage are the chro- mosomes least condensed? (d) when are chiasrnata first visible? 22. Describe the role of meiosis in the life cycle ofa vascular plant. 23. Contrast the chromatin fiber with the mitotic chromosome. How are the two structures related“? 24. Describe the “folded iiber" model of the mitotic chromosome. 25. You are given a nietaphase chromosome preparation (a slide] from an unknown organism that contains 12 chromosomes. Two are clearly smaller than the rest, appearing identical in length and centromere placement. Describe all that you can about these chromosomes. Zn. In mitosis, what chromatid combinationis) will be present dur- ing metaphase‘? What combination(s) will be present at each pole at the completion of anaphase’? 27. During meiosis I, assuming no crossing over. what chromatid combination(s) will be present at the completion of prophase'? Draw all possible alignments of chromatids as migration begins during early anaphase. 28. Are there any possible combinations present during prophase of meiosis ll other than those that you drew in Problem 27"? If so. draw them. If not. then proceed to Problem 29. 2‘}. Draw all possible combinations of chromatids during the early phases of anaphase in meiosis ll. 3t]. Assume that during meiosis l none of the C chromosomes disjoin at metaphase. but they separate into dyads (instead of monads‘i dur- ing meiosis I]. How would this change the alignments that you con- structed during the anaphasc Stages in meiosis l and Il’? Draw them. 31. Assume that each gamete resulting from Problem 30 participated in fertilization with a normal haploid gamete. What combinations will result? What percentage of zygotes will be diploid. containing one paternal and one matemal member of each chromosome pair? Hartwell. L.H.. and Karstan. MB. 1994. Cell cycle control and can cer. Science 266:182le28. Hartwell. L.l-{.. and Weinert. TA. 1989. Checkpoint controls that en- sure the order of cell cycle events. Science 246:629-34. Malia. D. 196], How cells divide. Sci. Am. (Jan) 205:101—20. 7. 197’4. The cell cycle. Sci. Am. iJan.) 235:54—64. McIntosh. J.R._. and McDonald. K.L. 1989. The mitotic spindle. Sci. Am. ('Oct.) 2612-48—56. W’estergaard, M.. and von Wettstein. D. [972. The synaptinemal com— plex. Annu. Rev. Genet. 6:71—110. ...
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Chapter 2 - 0 Genetic continuity between cells and between...

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