Reduction-Division Genetic Recombination 1

GAMETES, HALF CHROMOSOMES, (  Creation of GAMETES, with HALF the number of CHROMOSOMES, (HAPLOID)  MeiosisSEXUAL reproduction.  Meiosis is SEXUAL reproduction.  TWO divisionsMEIOSIS I MEIOSIS II  TWO divisions (MEIOSIS I and MEIOSIS II). 2

mitosis  Similar to mitosis interphase.  CHROMOSOMES (DNA) S phase  CHROMOSOMES (DNA) replicate in the S phase 3

 Cell division chromosome number one- 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 4 Prophase I

Chromosomes Chromosomes condense. Nuclear membrane Nuclear membrane disappears Spindles Spindles forms  Synapsis Homologous chromosomes tetrad  Synapsis occurs - Homologous chromosomes come together to form a tetrad. Crossing over Crossing over Occurs 5

Homologous chromosomes sister chromatids Tetrad 6

 Crossing over chromatids chiasmata  Crossing over may occur between non-sister chromatids at sites called chiasmata.  Crossing over chromatids chromatid  Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.  Chiasmata (chiasma) exchange genes crossing over  Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.) Genetic Recombination  Causes Genetic Recombination 7

nonsister chromatids chiasmata: site of crossing over variation Tetrad 8

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 Tetrads align on the equator.  Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION 10

Homologs line up at equator or metaphase plate OR 11

 In terms of Independent Assortment -how many different combinations of sperm could a human male produce? 12

 Formula: 2 n  Human chromosomes:2n = 46  n = 23  2 23 = ~8 million combinations 13

 Homologous chromosomes  Homologous chromosomes separate and move towards the poles.  Sister chromatids centromeres  Sister chromatids remain attached at their centromeres. 14

 Nuclear membrane  Nuclear membrane reforms around each group of chromosomes 15

 Cytokinesis  Cytokinesis occurs and two haploid daughter cells are formed. 16

17 cytokinesis

 Interphase II or very short  No DNA Replication  Remember: Meiosis II is similar to mitosis 18

 Same as Prophase in mitosis  Nucleus & nucleolus disappear  Chromosomes condense  Spindle forms 19

 Same as Metaphase in mitosis Chromosomes line up at equator 20

Anaphasemitosis  Same as Anaphase in mitosis  SISTER CHROMATIDS separate 21

 Same as Telophase in mitosis.  Nuclear membrane reforms, spindle disappears 22

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 The cells split  FOUR HAPLOID DAUGHTER cells are produced.  Called GAMETES (eggs and sperm) 24

2n=46 human sex cell diploid (2n) n=46 Meiosis I n=23 sperm haploid (n) Meiosis II 25

2n=46 human sex cell diploid (2n) n=23 Meiosis I n=23 egg Haploid (1n) Meiosis II 26 Polar Bodies (die)

 Also known as GENETIC RECOMBINATION  Meiosis allows for much variation due to:  1. Independent assortment  2. Crossing over  3. Random fertilization 27

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? 28

 10 chromosomes (haploid or 1n) 29

 Aorganized picture arranged in pairs by size  An organized picture of the chromosomes of a human arranged in pairs by size from largest to smallest.  P1-22 AUTOSOMES  Pairs 1-22 called AUTOSOMES  LSEX CHROMOSOMES  Last pair are SEX CHROMOSOMES 30 Male - XY

31 Female - XX

32 Female - XX Down Syndrome – Trisomy 21

spermegg zygote  The fusion of a sperm and egg to form a zygote. FERTILIZED EGG  A zygote is a FERTILIZED EGG  64 trillion combinations for the zygote n=23 egg sperm n=23 2n=46 zygote 33