MEIOSIS

Meiosis Cell division making gametes containing half the number of chromosomes. Meiosis is sexual reproduction. Two divisions (meiosis I and meiosis II).

Homologous Chromosomes Each chromosomes has a twin. You inherit one chromosome from each pair from your mom and one from your dad. The two chromosomes of the pair are homologous chromosomes. Each carries the same sequence of genes. Example: #5 carries the gene for red hair.

Homologous Chromosomes Homologous pairs (tetrads) have genes for 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. 22 pairs of autosomes b. 1 pair of sex chromosomes, c. XX for girls and XY for boys.

Karyotypes-picture of homologous pairs

Meiosis All cells in the body are diploid-two sets of chromosomes. So 2N= 46. Sex cells divide to produce gametes-sperm or egg. Gametes have half the # of chromosomes or one set of chromosomes-haploid. So N=23. Occurs in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis Meiosis is similar to mitosis with some chromosomal differences.

Meiosis: Haploid and Diploid Examples Ex. Dogs:78 chromosomes Diploid #?? Haploid#(# of pairs?) Ex. Horses:64 chromosomes Ex. Tomatoes:24 chromosomes

Homologous Chromosomes eye color locus hair color Paternal Maternal

Meiosis Parent cell – chromosome pair Chromosomes copied 1st division - pairs split (meiosis one) 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes (meiosis two)

Spermatogenesis n=23 n=23 2n=46 sperm haploid (n) meiosis II human sex cell diploid (2n) n=23 meiosis I

Oogenesis n=23 n=23 2n=46 human sex cell Egg haploid (n) diploid (2n) Polar body Polar Body Egg haploid (n) meiosis II 2n=46 human sex cell diploid (2n) n=23 meiosis I

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

Interphase I Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I

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).

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

Crossing Over Crossing over (variation) may occur between nonsister chromatids. Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.

Crossing Over - variation Tetrad nonsister chromatids variation

Prophase I centrioles spindle fiber aster fibers

Metaphase I Shortest phase Tetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations

Metaphase I metaphase plate OR metaphase plate

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

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

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

Anaphase I

Telophase I Each pole now has haploid set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed.

Telophase I

Meiosis II No interphase II (or very short - no more DNA replication) Remember: Meiosis II is similar to mitosis

Prophase II same as prophase in mitosis

Metaphase II same as metaphase in mitosis metaphase plate

Anaphase II same as anaphase in mitosis sister chromatids separate

Telophase II Same as telophase in mitosis. Nuclei form. Cytokinesis occurs. Remember: four haploid daughter cells produced. gametes = sperm or egg

Telophase II

Meiosis n=2 n=2 2n=4 sperm haploid (n) meiosis II meiosis I sex cell diploid (2n)

Variation Important to population as the raw material for natural selection. Question: What are the three sexual sources of genetic variation?

Answer: Remember: variation is good! 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good!

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

Answer: 10 chromosomes (haploid)

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

Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes

Compare Mitosis and Meiosis Both have interphase to duplicate chromosomes. Mitosis starts with diploid cell, makes two identical diploid cells. Meiosis starts with diploid cells, makes four different haploid cells. Meiosis has tetrads during prophase I and crossing over can occur.