apLectureNotes13 - CHAPTER 13 MEIOSIS AND SEXUAL LIFE...

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Unformatted text preview: CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES _____________——————-——-———- OUTLINE I. An Introduction to Heredity A. Offspring acquire genes from parents by inheriting chromosomes B. Like begets like, more or less: a comparison of asexual versus sexual reproduction II. The Role of Meiosis in Sexual Life Cycles A. Fertilization and meiosis alternate in sexual life cycles B. Meiosis reduces chromosome number from diploid to haploid: a closer look Ill. Origins of Genetic Variation A. Sexual life cycles produce genetic variation among offspring B. Evolutionary adaptation depends on a population’s genetic variation OBJECTIVES After reading this chapter and attending lecture, the student should be able to: 1. Explain why organisms only reproduce their own kind, and why offspring more closely resemble their parents than unrelated individuals of the same species. 2. Explain what makes heredity possible. 3. Distinguish between asexual and sexual reproduction. 4. Diagram the human life cycle and indicate where in the human body that mitosis and meiosis occur; which cells are the result of meiosis and mitosis; and which cells are haploid. 5. Distinguish among the life cycle patterns of animals, fungi, and plants. 6. List the phases of meiosis I and meiosis II and describe the events characteristic of each phase. 7 7. Recognize the phases of meiosis from diagrams or micrographs. 9. Describe the process of synapsis during prophase I, and explain how genetic recombination occurs. 10. Describe key differences between mitosis and meiosis; explain how the end result of meiosis differs from that of mitosis. ll. Explain how independent assortment, crossing over, and random fertilization contribute to genetic variation in sexually reproducing organisms. 12. Explain why inheritable variation was crucial to Darwin's theory of evolution. 13. List the sources of genetic variation. 168 Unit III Genetics KEY TERMS heredity karyotype zygote meiosis II variation homologous diploid cells synapsis genetics chromosomes meiosis tetrad gene sex chromosomes alternation of chiasmata asexual reproduction autosome generations chiasma clone gamete sporophyte crossing over sexual reproduction haploid cell spores life cycle fertilization gametophyte somatic cell syngamy meiosis I LECTURE NOTES Reproduction is an emergent property associated with life. The fact that organisms reproduce their own kind is a consequence of heredity. Heredity = Continuity of biological traits from one generation to the next - Results from the transmission of hereditary units, or genes, from parents to offspring. - Because they share similar genes, offspring more closely resemble their parents or close relatives than unrelated individuals of the same species. Variation = Inherited differences among individuals of the same species 0 Though offspring resemble their parents and siblings, they also diverge somewhat as a consequence of inherited differences among them. - The development of genetics in this century has increased our understanding about the mechanisms of variation and heredity. Genetics = The scientific study of heredity and hereditary variation. Beginning students often compartmentalize their knowledge, which makes it difficult to transfer and apply information learned in one context to a new situation. Be forewarned that unless you point it out, some students will never make the connection that meiosis, sexual reproduction, and heredity are all aspects of the same process. I. An Introduction to Heredity A. Offspring acquire genes from parents by inheriting chromosomes DNA = Type of nucleic acid that is a polymer of four different kinds of nucleotides. Genes = Units of hereditary information that are made of DNA and are located on chromosomes. ~ Have specific sequences of nucleotides, the monomers of DNA - Most genes program cells to synthesize specific proteins; the action of these proteins produce an organism's inherited traits. Inheritance is possible because: - DNA is precisely replicated producing copies of genes that can be passed along from parents to offspring. - Sperm and ova carrying each parent's genes are combined in the nucleus of the fertilized egg. The actual transmission of genes from parents to offspring depends on the behavior of chromosomes. Chapter 13 Meiosis and Sexual Life Cycles 169 Chromosomes = Organizational unit of heredity material in the nucleus of eukaryotic organisms ' Consist of a single long DNA molecule (double helix) that is highly folded and coiled along with proteins - Contain genetic information arranged in a linear sequence - Contain hundreds or thousands of genes, each of which is a specific region of the DNA molecule, or locus Locus = Specific location on a chromosome that contains a gene - Each species has a characteristic number of chromosomes; humans have 46 (except for their reproductive cells). B. Like begets like, more or less: a comparison of asexual versus sexual reproduction Asexual Reproduction Sexual Reproduction Single individual is the sole parent. Two parents give rise to offspring. Single parent passes on all its genes to Each parent passes on half its genes, its offspring. to its offspring. Offspring are genetically identical to Offspring have a unique combination the parent. of genes inherited from both parents. Results in a clone, or genetically Results in greater genetic variation; identical individual. Rarely, genetic offspring vary genetically from their differences occur as a result of siblings and parents (see Campbell, mutation, a change in DNA (see Figure 13.2). Campbell, Figure 13.1). What generates this genetic variation during sexual reproduction? The answer lies in the process of meiosis. III. The Role of Meiosis in Sexual Life Cycles A. Fertilization and meiosis alternate in sexual life cycles 1. The human life cycle Follows the same basic pattern found in all sexually reproducing organisms; meiosis and fertilization result in alternation between the haploid and diploid condition (see Campbell, Figure 13.3). Life cycle = Sequence of stages in an organism's reproductive history, from conception to production of its own offspring Somatic cell = Any cell other than a sperm or egg cell 0 Human somatic cells contain 46 chromosomes distinguishable by differences in size, position of the centromere, and staining or banding pattern. - Using these criteria, chromosomes from a photomicrograph can be matched into homologous pairs and arranged in a standard sequence to produce a karyotype. Karyotype = A display or photomicrograph of an individual's somatic—cell metaphase chromosomes that are arranged in a standard sequence. (See Campbell, Methods Box: Preparation of a Karyotype) - Human karyotypes are often made with lymphocytes. 0 Can be used to screen for chromosomal abnormalities Homologous chromosomes (homologues) = A pair of chromosomes that have the same size, centromere position, and staining pattern. 0 With one exception, homologues carry the same genetic loci. 170 Unit Ill Genetics Homologous autosomes carry the same genetic loci; however, human sex chromosomes carry different loci even though they pair during prophase of meiosis I. Autosome = A chromosome that is not a sex chromosome. Sex chromosome = Dissimilar chromosomes that determine an individual's sex Females have a homologous pair of X chromosomes. Males have one X and one Y chromosome. Thus, humans have 22 pairs of autosomes and one pair of sex chromosomes. Chromosomal pairs in the human karyotype are a result of our sexual origins. One homologue is inherited from each parent. Thus, the 46 somatic cell chromosomes are actually two sets of 23 chromosomes; one a maternal set and the other a paternal set. Somatic cells in humans and most other animals are diploid. Diploid = Condition in which cells contain two sets of chromosomes; abbreviated as 2n Haploid = Condition in which cells contain one set of chromosomes; it is the chromosome number of gametes and is abbreviated as n Gamete = A haploid reproductive cell Sperm cells and ova are gametes, and they differ from somatic cells in their chromosome number. Gametes only have one set of chromosomes. Human gametes contain a single set of 22 autosomes and one sex chromosome (either an X or a Y). Thus, the haploid number of humans is 23. The diploid number is restored when two haploid gametes unite in the process of fertilization. Sexual intercourse allows a haploid sperm cell from the father to reach and fuse with an ovum from the mother. Fertilization = The union of two gametes to form a zygote Zygote = A diploid cell that results from the union of two haploid gametes Contains the maternal and parental haploid sets of chromosomes from the gametes and is diploid (2n) As humans develop from a zygote to sexually mature adults, the zygote's genetic information is passed with precision to all somatic cells by mitosis. Gametes are the only cells in the body that are not produced by mitosis. Gametes are produced in the ovaries or testes by the process of meiosis. Meiosis is a special type of cell division that produces haploid cells and compensates for the doubling of chromosome number that occurs at fertilization. Meiosis in humans produces sperm cells and ova which contain 23 chromosomes. When fertilization occurs, the diploid condition (2n=46) is restored in the zygote. 2. The variety of sexual life cycles Altemation of meiosis and fertilization is common to all sexually reproducing organisms; however, the timing of these two events in the life cycle varies among species. There are three basic patterns of sexual life cycles (see Campbell, Figure 13.4): a. Animal In animals, including humans, Chapter 13 Meiosis and Sexual Life Cycles 171 gametes are the only haploid cells. - Meiosis occurs during gamete production. The resulting gametes undergo no further cell division before fertilization. MEIOSIS FERTILIZATION ° Fertilization produces a diploid zygote that divides b} . mitosis to produce a diploid Zygotc multicellular animal. Multicellular MITOSIS ‘ organism b. Fungi and some protists In many fungi and some protists, the only diploid stage is the zygote. - Meiosis occurs immediately after the zygote forms. - Resulting haploid cells divide by mitosis to produce a haploid multicellular organism. - Gametes are produced by mitosis from the already haploid organism. c. Plants and some algae Plants and some species of algae alternate between multicellular haploid and diploid generations. - This type of life cycle is called an alternation of generations. 0 The multicellular diploid stage is called a sporophyte, or spore—producing plant. Meiosis in this stage produces haploid cells called spores. ° Haploid spores divide Mumoenular mitotically to generate a Sporophyte multicellular haploid stage ' called a gametophyte, or gamete-producing plant. MITOSIS ............................................................................................. - Haploid gametophytes produce gametes by mitosis. - Fertilization produces a diploid zygote which develops into the next sporophyte generation. B. Meiosis reduces chromosome number from diploid to haploid: a closer look Meiosis and sexual reproduction significantly contribute to genetic variation among offspring. Meiosis includes steps that closely resemble corresponding steps in mitosis (see Campbell, Figure 13.5). 0 Like mitosis, meiosis is preceded by replication of the chromosomes. - Meiosis differs from mitosis in that this single replication is followed by two consecutive cell divisions: meiosis I and meiosis II. 172 Unit 111 Genetics ' These cell divisions producefour daughter cells instead of two as in mitosis. - The resulting daughter cells have half the number of chromosomes as the original cell; whereas, daughter cells of mitosis have the same number of chromosomes as the parent cell. 0 Campbell, Figure 13.6 shows mitosis and meiosis in animals. The stages of meiotic cell division: Interphase I. Interphase I precedes meiosis. 0 Chromosomes replicate as in mitosis. - Each duplicated chromosome consists of two identical sister chromatids attached at their centromeres. - Centriole pairs in animal cells also replicate into two pairs. Meiosis I. This cell division segregates the two chromosomes of each homologous pair and 5 Nuclear Chromatin reduces the chromosome number by one-half. It envelope includes the following four phases: . Prophase I. This is a longer and more " _ " = complex process than prophase of mitosis. Tetrad (Pa‘red homOIOgUCS with two chromatids each) Chromosomes condense. Synapsis occurs. During this process, homologous chromosomes come together as pairs. Chromosomes condense further until they are distinct structures that can be seen with a microscope. Since each chromosome has two . . . Sister Nonsister chromatids, each homologous palr 1n chromatids chromatids Synapsis appears as a complex of .- ............................................................................ :. four chromatids or a tetrad. In each tetrad, sister chromatids of the same chromosome are attached at their centromeres. Nonsister chromatids are linked by X-shaped chiasmata, sites where homologous strand exchange or crossing-over occurs. Chromosomes thicken further and detach from the nuclear envelope. As prophase I continues, the cell prepares for nuclear division. Centriole pairs move apart and spindle microtubules form between them. Nuclear envelope and nucleoli disperse. Chromosomes begin moving to the metaphase plate, midway between the two poles of the spindle apparatus. Prophase I typically occupies more than 90% of the time required for meiosis. Chapter 13 Meiosis and Sexual Life Cycles 173 Metaphase I. Tetrads are aligned on the metaphase plate. Each synaptic pair is aligned so that centromeres of homologues point toward opposite poles. Each homologue is thus attached to kinetochore microtubules emerging from the pole it faces, so that the two homologues are destined to separate in anaphase and move toward opposite poles. Anaphase I. Homologues separate and are moved toward the poles by the spindle apparatus. Sister chromatids remain attached at their centromeres and move as a unit toward the same pole, while the homologue moves toward the opposite pole. This differs from mitosis during which chromosomes line up individually on the metaphase plate (rather than in pairs) and sister chromatids are moved apart toward opposite poles of the cell. T elophase I and Cytokinesis. The spindle apparatus continues to separate homologous chromosome pairs until the chromosomes reach the poles. Each pole now has a haploid set of chromosomes that are each still composed of two sister chromatids attached at the centromere. Usually, cytokinesis occurs simultaneously with telophase I, forming two haploid daughter cells. Cleavage furrows form in animal cells, and cell plates form in plant cells. In some species, nuclear membranes and nucleoli reappear, and the cell enters a period of interkinesis before meiosis Ii. In other species, the daughter cells immediately prepare for meiosis II. Regardless of whether a cell enters interkinesis, no DNA replication occurs before meiosis II. Metaphase plate 174 Unit 111 Genetics Meiosis II. This second meiotic division separates sister chromatids of each chromosome. Prophase 11 ° If the cell entered interkinesis, the nuclear envelope and nucleoli disperse. - Spindle apparatus forms and chromosomes move toward the metaphase 11 plate. Metaphase II 0 Chromosomes align singly on the metaphase plate. - Kinetochores of sister chromatids point toward opposite poles. Anaphase II - Centromeres of sister chromatids separate. :5 : i —, I , 0 5” ‘Z- 0 ii . . . . x , \_ 1/ \ - Slster chromatids of each pair (now \ INK: //n\ . . . . I \ 2 1nd1v1dual chromosomes) move ‘ ./ / toward opposite poles of the cell. T elophase II and Cytokinesis - Nuclei form at opposite poles of the cell. 0 Cytokinesis occurs producing four haploid daughter cells. Haploid daughter cells Chapter 13 Meiosis and Sexual Life Cycles 175 1. Mitosis and meiosis compared Spending class time on a comparison of mitosis and meiosis is really worth the effort. It not only brings closure to the topic, but also provides an opportunity to check for understanding. One check is to ask students to identify unlabeled diagrams of various stages in mitosis and meiosis. The ability to distinguish metaphase of mitosis from metaphase of meiosis I, is particularly diagnostic of student understanding. If you are fortunate enough to have video capability in your classroom, you can show moving sequences of mitosis and meiosis. The fact that these are dynamic processes involving chromosomal movement is not a trivial point, but is often lost in the course of a lecture where the only visuals are drawings or micrographs. Though the processes of mitosis and meiosis are similar in some ways, there are some key differences (see Campbell, Figure 13.7): - Meiosis is a reduction division. Cells produced by mitosis have the same number of chromosomes as the original cell, whereas cells produced by meiosis have half the number of chromosomes as the parent cell. ° Meiosis creates genetic variation. Mitosis produces two daughter cells genetically identical to the parent cell and to each other. Meiosis produces four daughter cells genetically different from the parent cell and from each other. 0 Meiosis is two successive nuclear divisions. Mitosis, on the other hand, is characterized by just one nuclear division. ; Comparison of Meiosis I and Mitosis _________________________———————————— Meiosis I Mitosis Prophase Synopsis occurs to form tetrads. Neither synapsis nor crossing Chiasmata appear as evidence over occurs. that crossing over has occurred. Metaphase Homologous pairs (tetrads) align Individual chromosomes align on on the metaphase plate. the metaphase plate. Anaphase Meiosis I separates pairs of Mitosis separates sister chromosomes. Centromeres do chromatids of individual not divide and sister chromatids chromosomes. stay together. Sister chromatids of each chromosome move to the same pole of the cell; only the homologues separate. g Centromeres divide and sister chromatids move to opposite poles of the cell. Meiosis II is virtually identical in mechanism to mitosis, separating sister chromatids. Origins of Genetic Variation A. Sexual life cycles produce genetic variation among offspring Meiosis and fertilization are the primary sources of genetic variation in sexually reproducing organisms. Sexual reproduction contributes to genetic variation by: - Independent assortment - Crossing over during prophase I of meiosis 176 Unit III Genetics - Random fusion of gametes during fertilization 1. Independent assortment of chromosomes At metaphase I, each homologous pair of chromosomes aligns on the metaphase plate. Each pair consists of one maternal and one paternal chromosome. The orientation of the homologous pair to the poles is random, so there is a 50-50 chance that a particular daughter cell produced by meiosis I will receive the maternal chromosome of a homologous pair, and a 50-50 chance that it will receive the paternal chromosome. Each homologous pair of chromosomes orients independently of the other pairs at metaphase I; thus, the first meiotic division results in independent assortment of maternal and paternal chromosomes (see Campbell, Figure 13.8) A gamete produced by meiosis contains just one of all the possible combinations of maternal and paternal chromosomes. Independent assortment = The random distribution of maternal and paternal homologues to the gametes. (In a more specific sense, assortment refers to the random distribution of genes located on different chromosomes.) Since each homologous pair assorts independently from all the others, the process produces 2” possible combinations of maternal and paternal chromosomes in gametes, where n is the haploid number. In humans, the possible combinati...
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