MEIOSIS ReductionDivision Genetic Recombination 1 MEIOSIS The form

MEIOSIS Reduction-Division Genetic Recombination 1

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 2

MEIOSIS Sex cells divide to produce GAMETES (sperm or egg) Gametes have HALF the # of chromosomes. Occurs only in testes or ovaries. 3

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

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

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

INTERPHASE I Nucleus and nucleolus visible. chromatin Nucleus cell membrane nucleolus 7

MEIOSIS I (FOUR PHASES) Cell division that reduces the chromosome number by onehalf. Four phases: phases a. Prophase I b. Metaphase I c. Anaphase I d. Telophase I Prophase I 8

PROPHASE I Longest and most complex phase (90%). Chromosomes condense. Homologous chromosomes come together to form a tetrad Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids). 9

NON-SISTER CHROMATIDSHOMOLOGS • Homologs contain DNA that codes for the same genes , but different versions of those genes • Genes occur at the same loci 10

PROPHASE I Homologous chromosomes sister chromatids Tetrad sister chromatids 11

HOMOLOGOUS CHROMOSOMES Pair of chromosomes (maternal and 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 LOCI the same position on homologues. Humans have 23 pairs of homologous chromosomes: a. First 22 pairs of autosomes b. Last pair of sex chromosomes 12

HOMOLOGOUS CHROMOSOMES eye color locus hair color locus Paternal Maternal 13

SEX CHROMOSOMES XX chromosome - female XY chromosome - male 14

MEIOSIS I Meiosis I Homologs separate 15

PROPHASE I • Nucleus & Nucleolus disappear • Spindle forms • Chromosomes coil & Synapsis (pairing) occurs • Tetrads form spindle fiber centrioles aster fibers TETRAD 16

METAPHASE I Shortest phase Tetrads align on the equator. 17

METAPHASE I OR Homologs line up at equator or metaphase plate 18

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

ANAPHASE I Homologs separate 20

TELOPHASE I Each pole now has haploid (1 n) set of chromosomes Cytokinesis occurs and two haploid daughter cells are formed. 21

TELOPHASE I cytokinesis 22

MEIOSIS II Meiosis II Sister Chromatids Separate 23

MEIOSIS II No Interphase II or very short No DNA Replication Remember: Meiosis II is similar to mitosis 24

PROPHASE II Same as Prophase in mitosis Nucleus & nucleolus disappear Chromosomes condense Spindle forms 25

METAPHASE II Same as Metaphase in mitosis Chromosomes (not homologs) line up at equator 26

ANAPHASE II Same as Anaphase in mitosis SISTER CHROMATIDS separate 27

TELOPHASE II 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) 1 n Sperm cell fertilizes 1 n egg to form 2 n zygote 28

TELOPHASE II 29

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QUESTION: A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes 31

ANSWER: 10 chromosomes (haploid or 1 n) 32

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 33

QUESTION: A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes 34

ANSWER: 10 chromosomes 35

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