MEIOSIS ReductionDivision Genetic Recombination 1 Mitosis replicates an

MEIOSIS Reduction-Division Genetic Recombination 1

Mitosis replicates an EXACT COPY of a cell with a complete set of 46 chromosomes!!! (23 pairs) Mitosis starts with ONE parents cell and divides to create 2 IDENTICAL daughter cells One DIPLOID cell creates 2 DIPLOID cells 2

In Meiosis, we will be taking ONE parent cell and creating 4 daughter cells, each with different DNA! This is SEXUAL REPRODUCTION One DIPLOID cell creates 4 HAPLOID cells 3

Diploid cell- Cell that has 46 chromosomes Haploid cell – Cell that has 23 chromosomes Diploid – 2 n Haploid- n 4

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 5

MEIOSIS Sex cells divide to produce GAMETES (sperm or egg) Gametes have HALF the # of chromosomes. Occurs only in GONADS (testes or ovaries). Male: SPERMATOGENESIS -sperm Female: OOGENESIS - egg or ova 6

SPERMATOGENESIS human sex cell n=23 sperm n=23 2 n=46 diploid (2 n) n=23 haploid (n) n=23 Meiosis II 7

OOGENESIS 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 8

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. 9

INTERPHASE I Nucleus and nucleolus visible. chromatin Nucleus cell membrane nucleolus 10

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 Prophase I d. Telophase I 11

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 non-sister chromatids). 12

“ 1 set” of chromosomes (2 X’s) Chromosomes that carry genes to code for a specific trait 2 chromosomes carry same gene to code for trait (hair color, eye color…. etc. . )? They are homologous chromosomes! 13

NON-SISTER CHROMATIDSHOMOLOGS • Homologs contain DNA that codes for the same genes , but different versions of those genes • Genes occur at the same LOCATION on the chromosome 14

PROPHASE I Homologous chromosomes sister chromatids Tetrad sister chromatids 15

HOMOLOGOUS CHROMOSOMES Pair of chromosomes 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. Humans have 23 pairs of homologous chromosomes: a. First 22 pairs of autosomes b. Last pair of sex chromosomes 16

HOMOLOGOUS CHROMOSOMES eye color locus hair color locus Paternal Maternal 17

CROSSING OVER Crossing over may occur between non -sister chromatids at sites called chiasmata Crossing over: over segments of nonsister chromatids break and reattach to the other chromatid Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over. ) over Causes Genetic Recombination 18

Crossing over is why all egg and sperm have slightly different genetic code!! This is why you DO NOT look identical to your older/younger brother or sister! 19

GENETIC RECOMBINATION nonsister chromatids chiasmata: site of crossing over Tetrad variation 20

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SEX CHROMOSOMES XX chromosome - female XY chromosome - male 22

MEIOSIS I Meiosis I Homologs separate 23

PROPHASE I • Nucleus & Nucleolus disappear • Spindle forms • Chromosomes coil & Synapsis (pairing) occurs • Tetrads form & Crossing over Occurs spindle fiber centrioles aster fibers TETRAD 24

METAPHASE I Shortest phase Homologou chromosomes align on the equator. 25

METAPHASE I OR Homologous chromosomes line up at equator 26

Formula: 2 n Example: 2 n = 4 then 1 n = 2 thus 22 = 4 combinations 27

ANAPHASE I Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres 28

ANAPHASE I Homologs separate 29

TELOPHASE I Each pole now has haploid (1 n) set of chromosomes Cytokinesis occurs and two haploid daughter cells are formed. 30

TELOPHASE I cytokinesis 31

MEIOSIS II Meiosis II Sister Chromatids Separate 32

MEIOSIS II No Interphase II or very short No DNA Replication Remember: Meiosis II is SAM AS mitosis 33

PROPHASE II Same as Prophase in mitosis Nucleus & nucleolus disappear Chromosomes condense Spindle forms 34

METAPHASE II Same as Metaphase in mitosis Chromosomes (not homologs) line up at equator 35

ANAPHASE II Same as Anaphase in mitosis SISTER CHROMATIDS separate 36

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 37

TELOPHASE II 38

VARIATION 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 39

QUESTION: What are the 2 sources of genetic recombination or variation? 40

ANSWER: 1. 2. CROSSING OVER (prophase I) RANDOM FERTILIZATION 41

QUESTION: A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes 42

ANSWER: 10 chromosomes (haploid or 1 n) 43

KARYOTYPE An organized picture of the chromosomes of a human arranged in pairs by size from largest to smallest. Pairs 1 -22 called AUTOSOMES Last pair are SEX CHROMOSOMES Male - XY 44

KARYOTYPE Female - XX 45

KARYOTYPE Down Syndrome – Trisomy 21 Female - XX 46

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 47

QUESTION: A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes 48

ANSWER: 10 chromosomes 49