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cell division GAMETES (SEX CELLS) HALF CHROMOSOMES  The form of cell division by which GAMETES (SEX CELLS) 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

GONADS (testes or ovaries).  Occurs only in GONADS (testes or ovaries).  Male: SPERMATOGENESIS – creates 4 sperm  Female: OOGENESIS – creates 1 egg or ova and 3 polar bodies that die 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.  G1 phase- Cell growth  S phase- Chromosomes are replicated chromosometwo identical SISTER CHROMATIDS CENTROMERES  Each duplicated chromosome consists of two identical SISTER CHROMATIDS attached at their CENTROMERES.  G2 phase- Cell prepares for cell division. 6

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

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 are called autosomes sex chrom osomes b.Last pair are sex chrom osomes 8

PaternalMaternal eye color locus eye color locus hair color locus hair color locus 9

XX chromosome - female XY chromosome - male 10

 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 11 Prophase I

 Chromosomes  Chromosomes condense.  SynapsisHomologous chromosomes tetrad  Synapsis occurs - Homologous chromosomes come together to form a tetrad.  Tetrad two chromosomes four chromatids  Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids). 12

Homologous chromosomes sister chromatids Tetrad 13

 Crossing over chromatids  Crossing over may occur between non-sister chromatids only during Prophase I  Crossing over chromatids chromatid  Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. Genetic Recombination or gene exchange.  Causes Genetic Recombination or gene exchange.  Creates genetic variation. 14

nonsister chromatids site of crossing over variation Tetrad 15

Homologs separate 16 Meiosis I

Nucleolus & nuclear membrane Nucleolus & nuclear membrane disappear Spindle fibers Spindle fibers forms coilSynapsis Chromosomes coil & Synapsis (pairing) occurs Tetrads Crossing over Tetrads form & Crossing over Occurs centrioles spindle fiber aster fibers 17 TETRAD

Tetrads align on the equator. Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION 18 Homologs line up at equator or metaphase plate

Homologous chromosomes Homologous chromosomes separate and move towards the poles. Sister chromatids centromeres Sister chromatids remain attached at their centromeres. 19 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. 20

21 cytokinesis

Sister Chromatids Separate 22 Meiosis II

 No Interphase or very short  No DNA Replication  Remember: Meiosis II is similar to mitosis but 2 cells go through the process now. 23

 Same as Prophase in mitosis  Nuclear membrane & nucleolus disappear  Chromosomes condense  Spindle fibers forms 24

 Same as Metaphase in mitosis Chromosomes (not homologs) line up at equator 25

Anaphasemitosis  Same as Anaphase in mitosis  SISTER CHROMATIDS separate and move toward ends of each cell. 26

 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) 27 1n Sperm cell fertilizes 1n egg to form 2n zygote

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