Lecture_15_DulaiS09_Meiosis - Reproduction& Development...

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Unformatted text preview: Reproduction & Development Meiosis Lecture 15 – Chapter 10 Learning concepts Learning Sexual vs. Asexual reproduction 2. Homologous chromosomes 3. What are Alleles 4. Meiosis 1. Why Sex? Asexual reproduction is easier and faster Asexual Sexual reproduction can be an alternative Sexual adaption in changing environments Why Sex Fig. 10-1a, p.154 Why Sex Fig. 10-1d, p.154 Impacts, Issues: Why Sex Why Sexual reproduction Sexual has advantages when other, competing organisms change (think “evolutionary arms race”) Red Queen Red hypothesis – evolutionary treadmill Meiosis Like mitosis, meiosis is another nuclear division mechanism. It is the defining characteristic of sexual reproduction Unlike mitosis, which occurs in all living organisms (prokaryotes and eukaryotes), meiosis is only found in eukaryotes that reproduce sexually (all multicellular and some unicellular eukaryotes) The critical role of meiosis in sexual reproduction The role of meiosis is to reduce the chromosome number of sex cells by half (before they fuse) Mitosis: Diploid (2n) Meiosis: Diploid (2n) Diploid (2n) Haploid (n) This allows two gametes (haploid cells, such as the sperm and egg) to fuse (fertilization), producing a diploid cell The diploid cell inherits genetic material from both oocytes—in humans, half the DNA is maternal (mother) and half is paternal (father). Asexual Reproduction – without sex without Single parent cell produces offspring Single All offspring are genetically identical to one All another and to parent Most cells in humans divide by mitosis, Most these are somatic cells (soma=body) somatic Sexual Reproduction Sexual Involves Involves Gamete (haploid) production via meiosis from Gamete germ (diploid) cells Fertilization of two haploid gametes Fertilization Produces genetic variation among Produces offspring Concept Slide Concept Your mother’s and father’s body cells = Your each has 46 chromosomes (23 pairs) The egg she produced and the sperm he The produced had just 23 chromosomes each (one of each type) You were conceived when fertilization took You place 23 + 23 = 46 chromosomes Your body is 46 chromosomes (23 pairs) – Your just like you parents! Fig. 10-3, p.156 Human Human Karyotype Karyotype 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 XX (or XY) Fig. 10-4, p.157 Remember, there are two “copies” of each chromosome in every cell nucleus in your body But, these are nonnon identical copies (they are homologous) homologous One came from father’s sperm, the other from mother’s egg cell Fig. 10-2, p.156 Homologous Chromosomes Carry Different Alleles Alleles Cell has two of each chromosome Cell One chromosome in each pair from One mother, other from father Paternal and maternal chromosomes may Paternal carry different alleles Two homologous maternal and paternal chromosomes have a different combination of alleles. genes A Paternal Chromosome 1 Maternal Chromosome 1 bc D E f genes A Bc D e J F K GHI Paternal Chromosome 2 Maternal Chromosome 2 g hi j k The importance of sexual The reproduction is that it shuffles alleles Through sexual reproduction, offspring Through inherit new combinations of alleles, which leads to variations in traits This variation in traits is the basis for This evolutionary change Chromosome Number Chromosome Sum total of chromosomes in a cell Sum Germ cells are diploid (2n) = 46 in human Germ 46 somatic cells and germ cells somatic Gametes are haploid (n) = 23 in human gametes Gametes 23 (produced from germ cells) (produced Meiosis halves chromosome number Meiosis Meiosis: Two Divisions Meiosis: Two consecutive nuclear divisions Two Meiosis I – variation Meiosis Meiosis II – duplication/division Meiosis DNA is not duplicated between Meiosis I and DNA Meiosis II Results in four total nuclei (each is haploid) Results Meiosis I Meiosis Following duplication in interphase stage S, each homologue in the cell pairs with its partner, then the partners separate p. 158 Meiosis II Meiosis The two sister chromatids of each The duplicated chromosome are separated from each other two chromosomes (unduplicated) one chromosome (duplicated) p. 158 Meiosis I – Stages Meiosis Meiosis 1 is all about introducing variation to offspring Prophase I Metaphase I Anaphase I Telophase I Prophase I Prophase Each duplicated Each chromosome pairs with homologue Homologues swap Homologues segments in a process called crossing over crossing Each chromosome Each becomes attached to spindle Fig. 10-5, p. 158 Crossing Over Crossing • Each chromosome becomes zippered to its homologue • All four chromatids are closely aligned • Non-sister chromatids exchange segments Fig. 10.5 Effects of Crossing Over After crossing over, each chromosome contains both maternal and paternal segments. Crossing over creates new allele combinations in sex cells, and ultimately in offspring. g hi J K Note: Only 2 of the 4 chromatids are represented here GHI J K g hi j k G HI j k Metaphase I Metaphase Chromosomes are Chromosomes pushed and pulled into the middle of cell The spindle is fully The formed Fig. 10-5, p. 158 Anaphase I Anaphase Homologous Homologous chromosomes segregate The sister The chromatids remain attached Fig. 10-5, p. 158 Telophase I Telophase The chromosomes The arrive at opposite poles Usually followed by Usually cytoplasmic division Fig. 10-5, p. 158 Prophase II Prophase Microtubules attach Microtubules to the kinetochores of the duplicated chromosomes Fig. 10-5, p. 158 Metaphase II Metaphase Duplicated Duplicated chromosomes line up at the spindle equator, midway between the poles Fig. 10-5, p. 158 Anaphase II Anaphase Sister chromatids Sister separate to become independent chromosomes Fig. 10-5, p. 158 Telophase II Telophase The chromosomes The arrive at opposite ends of the cell A nuclear envelope nuclear forms around each set of chromosomes Four haploid cells Four Fig. 10-5, p. 158 Random Alignment Random During the transition between prophase 1 and metaphase 1, the initial contacts between microtubules and chromosomes are random. are Either the maternal or paternal Either member of a homologous pair can end up at either pole. end As a result, the chromosomes in a As gamete are a mix of chromosomes from the two parents. from Possible chromosome combinations As a result of random alignment in meiosis 1, the number of possible combinations of chromosomes in a gamete is: 2n (n is the haploid number of chromosomes, is or the number of chromosome pairs) or 1 2 3 combinations possible or Possible Chromosome Combinations or or Fig. 10-7, p.161 Factors Creating Chromosome (allele combination) Factors Variation among Offspring at each generation 1- Crossing over during prophase I 2- Random alignment of chromosomes at metaphase I 3- Random combination of sex cells (e.g., gametes, Random spores) at fertilization (actually cytogamy) cytogamy ...
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