Reduction-Division Genetic Recombination 1

cell division GAMETES, HALF CHROMOSOMES,  The form of cell division by which GAMETES, with HALF the number of CHROMOSOMES, are produced.  DIPLOID (2n)  HAPLOID (n)  MeiosisSEXUAL reproduction.  Meiosis is SEXUAL reproduction.  TWO divisionsMEIOSIS I MEIOSIS II  TWO divisions (MEIOSIS I and MEIOSIS II). 2

GAMETES(sperm or egg)  Sex cells divide to produce GAMETES (sperm or egg). HALF  Gametes have HALF the # of chromosomes. GONADS (testes or ovaries).  Occurs only in GONADS (testes or ovaries).  Male: SPERMATOGENESIS  Female: OOGENESIS 3

2n=46 human sex cell diploid (2n) n=23 Meiosis I n=23 sperm haploid (n) Meiosis II 4

mitosis  Similar to mitosis interphase.  CHROMOSOMES S phase.  CHROMOSOMES replicate in the S phase. chromosome two identical sister CHROMATIDS CENTROMERES  Each duplicated chromosome consist of two identical sister CHROMATIDS attached at their CENTROMERES.  CENTRIOLE  CENTRIOLE pairs also replicate. 5

 Nucleusnucleolus  Nucleus and nucleolus visible. nuclear membrane nucleolus cell membrane chromatin 6

 Cell division chromosomeone-half.  Cell division that reduces the chromosome number by one-half.  four phases  four phases: a.prophase I b.metaphase I c.anaphase I d.telophase I 7

 Longest and most complex phase (90%).  Chromosomes  Chromosomes condense.  Synapsis homologous chromosomes tetrad  Synapsis occurs: homologous chromosomes come together to form a tetrad.  Tetradchromosomes chromatids  Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids). 8

Homologous chromosomes sister chromatids Tetrad 9

chromosomesmaternalpaternal  Pair of chromosomes (maternal and paternal) that are similar in shape and size. (tetrads)  Homologous pairs (tetrads) carry genes controlling the same inherited traits. locus(position of a gene)  Each locus (position of a gene) is in the same position on homologues. homologous chromosomes.  Humans have 23 pairs of homologous chromosomes. autosomes a.22 pairs of autosomes sex chromosomes b.01 pair of sex chromosomes 10

PaternalMaternal eye color locus eye color locus hair color locus hair color locus 11

 Crossing over (variation) chromatidschiasmata  Crossing over (variation) may occur between nonsister chromatids at the chiasmata.  Crossing over chromatids chromatid  Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.  Chiasmata (chiasma) crossing over  Chiasmata (chiasma) are the sites of crossing over. 12

nonsister chromatids chiasmata: site of crossing over variation Tetrad 13

XX chromosome - femaleXY chromosome - male 14

centrioles spindle fiber aster fibers 15

 Shortest phase  Tetradsmetaphase plate  Tetrads align on the metaphase plate.  INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2 n Example:2n = 4 then n = 2 thus 2 2 = 4 combinations thus 2 2 = 4 combinations 16

metaphase plate OR metaphase plate 17

 In terms of Independent Assortment -how many different combinations of sperm could a human male produce? 18

 Formula: 2 n  Human chromosomes:2n = 46 n = 23 n = 23  2 23 = ~8 million combinations 19

 Homologous chromosomes  Homologous chromosomes separate and move towards the poles.  Sister chromatids centromeres  Sister chromatids remain attached at their centromeres. 20

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haploid chromosomes  Each pole now has haploid set of chromosomes.  Cytokinesis  Cytokinesis occurs and two haploid daughter cells are formed. 22

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 No interphase II DNA replication (or very short - no more DNA replication)  Remember: Meiosis II mitosis  Remember: Meiosis II is similar to mitosis 24

prophasemitosis  same as prophase in mitosis 25

metaphasemitosis  same as metaphase in mitosis metaphase plate 26

anaphasemitosis  same as anaphase in mitosis  sister chromatids separate 27

telophasemitosis  Same as telophase in mitosis.  Nuclei form.  Cytokinesis  Cytokinesis occurs.  Remember:four haploid daughter cells produced. gametes = sperm or egg 28

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2n=4 sex cell diploid (2n) n=2 meiosis I n=2 sperm haploid (n) meiosis II 30

 Important to population as the raw material for natural selection.  Question: What are the three sexual sources of genetic variation? 31

1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good! 32

20 chromosomes(diploid) chromosomes  A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? 33

 10 chromosomes (haploid) 34

 A method of organizing the chromosomes of a cell in relation to number, size, and type. 35

spermeggzygote  The fusion of a sperm and egg to form a zygote.  A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote 36

40 chromatids chromosomes  A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? 37

 10 chromosomes 38