Transcript Meiosis II
MEIOSIS Meiosis • The form of cell division by which gametes, with half the number of chromosomes, are produced. • Diploid (2n) haploid (n) • Meiosis is sexual reproduction. • Two divisions (meiosis I and meiosis II). 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 Female: oogenesis • Meiosis is similar to mitosis with some chromosomal differences. Spermatogenesis n=23 human sex cell 2n=46 2n=46 sperm n=23 4n=92 haploid (n) n=23 2n=46 diploid (2n) Replication meiosis I n=23 meiosis II 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. 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. 92 to 46 • 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 sister chromatids Homologous Chromosomes • Pair of chromosomes (maternal and 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. • Humans have 23 pairs of homologous chromosomes. a. 22 pairs of autosomes b. 01 pair of sex chromosomes Homologous Chromosomes eye color locus eye color locus hair color locus hair color locus Paternal Maternal Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata. • Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. • Chiasmata (chiasma) are the sites of crossing over. Crossing Over - variation nonsister chromatids chiasmata: site of crossing over Tetrad variation Sex Chromosomes XX chromosome - female XY chromosome - male Prophase I spindle fiber aster fibers centrioles 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 OR metaphase plate metaphase plate Question: • In terms of Independent Assortment how many different combinations of sperm could a human male produce? Answer • 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 diploid set of chromosomes. 92 to 46 • Cytokinesis occurs and two diploid 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 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. 46 to 23 gametes = sperm or egg Telophase II Meiosis n=23 human sex cell 2n=46 2n=46 sperm n=23 4n=92 haploid (n) n=23 2n=46 diploid (2n) Replication meiosis I n=23 meiosis II Variation • Important to population as the raw material for natural selection. • Question: What are the three sexual sources of genetic variation? Answer: 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) Karyotype • A method of organizing the chromosomes of a cell in relation to number, size, and type. Fertilization • 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 Question: • A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: • 10 chromosomes