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* * * * * * * * * * 9.1 What is Meiosis and why it is important to evolution and adaptation? Meiosis (sexual) serves 2 major functions Production of gametes/spores by reducing from 2n to n the chromosome number of the species Produce genetic variation by shuffling chromosomes in the cell to produce genetically different combinations Eukaryote’s chromosomes come often in pairs of homologous chromosomes (2n) Also called homologues Have the same size, shape, and construction (location of centromere) Contain the same genes for the same traits The offspring receives one member of each homologous pair from each parent Homologous pairs may contain different versions of the same gene Alleles – alternate forms of a gene Both males and females have 23 pairs of chromosomes 23 pairs or 46 total chromosomes = diploid (2n) Haploid number (n) in gametes – 23 total chromosomes 22 pairs of autosomes 1 pair of sex chromosomes XX female or XY male Figure 9.1 Homologous chromosomes homogenous autosome pair The 46 chromosomes of a male sister chromatids centromere Sex chromosomes are different lengths in males. Human life cycle Life cycle – in sexually reproducing organisms refers to all the reproductive events that occur from one generation to the next Involves both mitosis and meiosis Mitosis involved in continued growth of a child and repair of tissues throughout life Somatic (body) cells are diploid Meiosis reduces the chromosome number from diploid to haploid Gametes (egg and sperm) have only 1 member of each homologous pair Spermatogenesis produces sperm in the testes Oogenesis produces eggs in the ovaries Egg and sperm join to form diploid zygote Figure 9.2 Life cycle of humans FERTILIZATION Mitosis Mitosis 2n 2n MEIOSIS sperm n egg n 2n=46 diploid(2n) haploid (n) n=23 zygote2n 2n 9.2 The Phases of Meiosis Meiosis involves two divisions: meiosis I and meiosis II Each division is broken down into four phases: Prophase (I and II) Metaphase (I and II) Anaphase (I and II) Telophase (I and II) Results in 4 daughter cells 2 divisions Meiosis I Homologous pairs line up during synapsis resulting in tetrad Homologous chromosomes of each pair then separate Meiosis II No duplication of chromosomes Chromosomes are dyads – composed of 2 sister chromatids Sister chromatids are separated 2 daughter nuclei separate Figure 9.3 Overview of meiosis Chromosome duplication Meiosis II n=2 n=2 n=2 n=2 Meiosis II dyad Meiosis I Chromosome duplication 2n=4 sister chromatids centromere tetrad prophase I Figure 9.6 Meiosis I Meiosis I: Homologous chromosomes separate nuclear envelope fragment Prophase I Metaphase I Anaphase I Telophase I sister chromatids Prophase I Tetrads form, and crossing-over occurs as chromosomes condense; the nuclear envelope fragments. Metaphase I Tetrads align at the spindle equator. Either homologue can face either pole. Anaphase I Homologues separate, and dyads move to poles. Telophase I Daughter nuclei are haploid ,having received one duplicated chromosome from each homologous pair. n=2 centromere 2n=4 crossing-over spindle forming chromosome attached to a spindle fiber crossing over chromosomes still duplicated cleavage furrow tetrad Figure 9.4 Prophase I spindle poles tetrad During prophase I, replicated homologous chromosomes pair up and form a tetrad, a process called synapsis Each tetrad consists of 2 duplicated chromosomes (dyads), with each chromosome containing 2 sister chromatids, for a total of 4 chromatids Dyad Two sister chromatids Figure 9.5 Crossing-over chromosomes in four different gametes chromatids after exchange crossing-over between nonsister chromatids synapsis nonsister chromatids Crossing-over When a tetrad forms during synapsis, chromatids from homologous chromosomes (nonsister chromatids) may exchange genetic material Increases variability of the gametes and, therefore, the offspring The importance of meiosis Chromosome number stays constant in each new generation Generates diversity Crossing-over All possible combinations of haploid number of chromosomes Fertilization produces new combinations (2)n (2)3 chromosomally different zygotes without crossing-over (2)n=(2)3=8 Figure 9.7 Meiosis II Prophase II Chromosomes condense, and the nuclear envelope fragments. Metaphase II Tetrads align at the spindle equator. Anaphase II Sister chromatids separate ,becoming daughter chromosomes that move to the poles. n=2 Telophase II haploid daughter cells forming sister chromatids separate Telophase II Four haploid daughter cells are genetically different from each other and from the parent cell. Prophase II Metaphase II Anaphase II Meiosis II: Sister chromatids separate 9.3 Meiosis Compared with Mitosis Meiosis requires two nuclear divisions, but mitosis requires only one nuclear division Meiosis produces four daughter nuclei, and there are four daughter cells following cytokinesis; mitosis followed by cytokinesis results in two daughter cells Following meiosis, the four daughter cells are haploid and have half the chromosome number as the parent cell; following mitosis, the daughter cells have the same chromosome number as the parent cell Following meiosis, the daughter cells are genetically dissimilar to each other and to the parent cell; following mitosis, the daughter cells are genetically identical to each other and to the parent cell Figure 9.8 Meiosis compared with mitosis Meiosis Mitosis Daughter chromosomes have separated. Haploid daughter nuclei are not genetically identical to parent cell. Diploid daughter nuclei are genetically identical to parent cell. Daughter cells have formed. Daughter cells have formed. Homologues separate. Daughter chromosomes separate. Tetrads are at spindle equator. Dyads are at spindle equator. Synapsis and crossing-over occur. Meiosis occurs only at certain times of the life cycle of sexually reproducing organisms and only in specialized tissues Mitosis is much more common Occurs in all tissues during embryonic growth Also occurs during growth and repair 92 92 46 Chromosomes Chromatids Chromosomes Chromatids Number of Chromosomes and Number of Chromatids after DNA replication (duplication) 9.4 Abnormal Chromosome Inheritance Nondisjunction Meiosis I when both members of a pair go into the same daughter cell Meiosis II when sister chromatids fail to separate Trisomy – 3 copies of a chromosome Monosomy – single copy of a chromosome Figure 9.9a Nondisjunction during meiosis Meiosis II Meiosis I pair of homologous chromosomes non disjunction Gamete will have one less chromosome. Gamete will have one extra chromosome. Non disjunction during meiosis I (2n-1) (2n+1) Meiosis II Meiosis I normal meiosis I non disjunction pair of homologous chromosomes Gamete will have either one less or one extra chromosome. normal meiosis II Gamete will have normal number of chromosomes. Non disjunction during meiosis II Figure 9.10 Down syndrome Bella, 3 yo Photo: 2012 Santorum, 53 yo Garver, 52 yo Autosomal trisomy 18 (Edwards Syndrome Abnormal sex chromosome number Too few or too many X or Y chromosomes Newborns with abnormal sex chromosome numbers are more likely to survive than those with abnormal autosome numbers Extra X chromosomes become Barr bodies – inactivated Y determines maleness SRY (sex-determining region Y) gene on Y chromosome Turner syndrome (45, XO) Absence of second sex chromosome Female Klinefelter syndrome (47, XXY) Extra X inactivated as Barr body Male Figure 9.11 Abnormal sex chromosome number no facial hair some breast development very long arms less-developed testes very long legs a. A female with Turner (XO) syndrome b. A male with Klinefelter (XXY) syndrome webbed neck less-developed breasts less-developed ovaries * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * When a tetrad forms during synapsis, chromatids from homologous chromosomes (nonsister chromatids) may exchange genetic material Increases variability of the gametes and, therefore, the offspring The importance of meiosis Chromosome number stays constant in each new generation Generates diversity Crossing-over All possible combinations of haploid number of chromosomes Fertilization produces new combinations (2)n (