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 - sperm  Female: OOGENESIS - egg or ova 3

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

2n=46 human sex cell diploid (2n) n=23 Meiosis I n=23 egg Haploid (1n) Meiosis II 5 Polar Bodies (die)

mitosis  Similar to mitosis interphase.  CHROMOSOMES (DNA) S phase  CHROMOSOMES (DNA) replicate in the S phase chromosometwo identical SISTER CHROMATIDS CENTROMERES  Each duplicated chromosome consist of two identical SISTER CHROMATIDS attached at their CENTROMERES.  CENTRIOLE  CENTRIOLE pairs also replicate. 6

visible  Nucleus and nucleolus visible. Nucleus nucleolus cell membrane chromatin 7

Homologs separate 8 Meiosis I

 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 9 Prophase I

 Longest and most complex phase (90%).  Chromatin  Chromatin condenses.  SynapsisHomologous chromosomes tetrad  Synapsis occurs - Homologous chromosomes come together to form a tetrad.  Tetradtwo chromosomes four chromatids  Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids). 10

11 Homologs contain DNA that codes for the same genes, but different versions of those genes Genes occur at the same loci

Homologous chromosomes sister chromatids Tetrad 12

chromosomesmaternal paternalsimilar in shape and size.  Pair of chromosomes (maternal and paternal) that are similar in shape and size. (tetrads) carry GENES controlling the SAME inherited traits  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.  Humans23 pairs homologous chromosomes:  Humans have 23 pairs of homologous chromosomes: 22 pairs autosomes a.First 22 pairs of autosomes sex chrom osomes b.Last pair of sex chrom osomes 13 LOCI

PaternalMaternal eye color locus eye color locus hair color locus hair color locus 14

 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 15

nonsister chromatids chiasmata: site of crossing over variation Tetrad 16

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XX chromosome - female XY chromosome - male 18

 Shortest phase  Tetrads align on the equator.  Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION 19

Homologs line up at equator or metaphase plate OR 20

 Formula: 2 n  Example:2n = 4 then 1n = 2 then 1n = 2 thus2 2 = 4 combinations thus2 2 = 4 combinations 21

 In terms of Independent Assortment -how many different combinations of sperm could a human male produce? 22

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

 Homologous chromosomes  Homologous chromosomes separate and move towards the poles.  Sister chromatids centromeres  Sister chromatids remain attached at their centromeres. 24

25 Homologs separate Homologs separate

haploid chromosomes  Each pole now has haploid (1n) set of chromosomes.  Cytokinesis  Cytokinesis occurs and two haploid daughter cells are formed. 26

27 cytokinesis

Sister Chromatids Separate 28 Meiosis II

 No Interphase II may have interkinesis (very brief or very long)  No DNA Replication 29

 Same as Prophase in mitosis  Nucleus & nucleolus disappear  Chromatin condense  Spindle forms 30

 Same as Metaphase in mitosis Chromosomes (not homologs) line up at equator 31

Anaphasemitosis  Same as Anaphase in mitosis  SISTER CHROMATIDS separate after centromere divides 32

 Same as Telophase in mitosis.  Nuclei and Nucleoli reform, spindle disappears  CYTOKINESIS occurs.  Remember: FOUR HAPLOID DAUGHTER cells are produced.  Called GAMETES (eggs and sperm) 33 1n Sperm cell fertilizes 1n egg to form 2n zygote

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 Also known as GENETIC RECOMBINATION  Important to population as the raw material for NATURAL SELECTION.  All organisms are NOT alike  Strongest “ most fit” survive to reproduce & pass on traits 36

 What are the 3 sources of genetic recombination or variation? 37

1. CROSSING OVER (prophase I) 2. INDEPENDENT ASSORTMENT (metaphase I) 3. RANDOM FERTILIZATION 38

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? 39

 10 chromosomes (haploid or 1n) 40

 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 41 Male - XY

42 Female - XX

43 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 n=23 egg sperm n=23 2n=46 zygote 44

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