MEIOSIS Production of Gametes 1 Meiosis Facts 2

MEIOSIS Production of Gametes 1

Meiosis Facts 2

Meiosis • The form of cell division by which gametes, with half the number of chromosomes, are produced • Diploid (2 n) haploid (n) • Meiosis is sexual reproduction • Two divisions (meiosis I and meiosis II) II 3

Meiosis • Sex cells divide to produce gametes (sperm or egg) • Gametes have half the number of chromosomes • Meiosis is similar to mitosis with some chromosomal differences 4

Meiosis • Occurs only in gonads (testes or ovaries) • Male: Spermatogenesis • Female: Oogenesis 5

Spermatogenesis Human sex cell n=23 Sperm 2 N=46 n=23 4 N=92 After S phase Of cell cycle 2 N=46 n=23 Haploid (n) n=23 Meiosis II 6

Interphase I • Similar to mitosis interphase • Chromosomes replicate (S phase) • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres • Centriole pairs also replicate. 7

Interphase I • Nucleus and nucleolus visible. Chromatin Nuclear membrane Cell membrane Nucleolus 8

Meiosis I 9

Stages of Meiosis I • Cell division that reduces the chromosome number by one-half • Four phases: phases a. Prophase I b. Metaphase I c. Anaphase I d. Telophase I 10

Prophase I • Longest and most complex phase (90%) • Chromosomes condense • Synapsis occurs: homologous chromosomes come together to form a tetrad • Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids) 11

Prophase I - Synapsis Homologous chromosomes Sister chromatids Tetrad Sister chromatids 12

Homologous Chromosomes • Pair of chromosomes (maternal or paternal) paternal that are similar in shape and size • Homologous pairs (tetrads) carry genes controlling the same inherited traits • Each locus (position of a gene) is in the same position on homologues 13

Homologous Chromosomes • Humans have 23 pairs of homologous chromosomes • First 22 pairs of chromosomes called autosomes • Last pair called sex chromosomes • XX female or XY male 14

Homologous Chromosomes eye color locus hair color locus Paternal Maternal 15

Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata • Crossing over: over segments of nonsister chromatids break and reattach to the other chromatid • Chiasmata (chiasma) are the sites of crossing over 16

Crossing Over - Variation nonsister chromatids chiasmata: site of crossing over Tetrad 17 variation

Sex Chromosomes XX chromosome - female XY chromosome - male 18

Prophase I Spindle fiber Centrioles Aster fibers 19

Metaphase I • Shortest phase • Tetrads align on the metaphase plate • Independent assortment occurs • Chromosomes separate randomly to the poles of the cells 20

Metaphase I • Independent assortment causes variation in the forming cells • Orientation of homologous pair to poles is random • Formula for determining variation: Formula: 2 n Example: 2 n = 4 then n = 2 thus 22 = 4 combinations 21

Metaphase I OR Metaphase plate 22

Question: • In terms of Independent Assortment, how many different combinations of sperm could a human male produce? 23

Answer • • Formula: 2 n Human chromosomes: 2 n = 46 n = 23 223 = ~8 million combinations 24

Anaphase I • Homologous chromosomes separate and move towards the poles • Sister chromatids remain attached at their centromeres 25

Anaphase I 26

Telophase I • Each pole now has haploid set of chromosomes • 1 n = 23 (human) • Cytokinesis occurs and two haploid daughter cells are formed 27

Telophase I 28

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Meiosis II 30

Meiosis II • • Short or No interphase II DNA NOT replicated again • Remember: Meiosis II is similar to mitosis • Prophase I, Metaphase II, Anaphase II, and Telophase II 31

Prophase II • Same as prophase in mitosis • Nuclear envelope breaks down and the spindle apparatus forms 32

Metaphase II • Same as metaphase in mitosis • Chromatids lined up at equator Metaphase plate 33

Anaphase II • Same as anaphase in mitosis • Sister chromatids separate 34

Telophase II • Same as telophase in mitosis • Nuclei reform • Cytokinesis occurs • Remember: Four haploid daughter cells produced • Gametes = sperm or egg 35

Telophase II 36

Meiosis sex cell n=2 Sperm 2 N=4 n=2 4 N=8 Haploid (n) 2 N=4 n=2 Meiosis II 37

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Genetic Variation 39

Variation • Important to the survival of populations • Aids in natural selection • Strongest individuals are able to survive and reproduce 40

Question • What are three sources of genetic variation in sexual reproduction? 41

Answer: 1. Crossing Over (Prophase I) 2. Independent Assortment (Metaphase I) 3. Random Fertilization (sperm joins with egg) 42

Remember: Variation is helpful to the survival of a species! 43

Question: • A diploid cell containing 20 chromosomes (2 n = 20) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes 44

Answer: • 10 chromosomes (haploid) • 1 n = 10 45

Karyotype • A method of organizing the chromosomes of a cell in relation to number, size, and type. 46

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Fertilization • The fusion of a sperm and egg to form a zygote • A zygote is a fertilized egg n=23 egg sperm n=23 2 n=46 zygote 48

Question: • A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes 49

Answer: • Four cells with 10 chromosomes each 50

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