Transcript Meiosis
Meiosis Advanced Animal Science Meiosis Type of cell division One germ cell makes 4 gametes with ½ the # of chrom. Occurs only in germ cells in gonads ○ testes /ovaries Occurs in flowers ○ ovary and anther Sperm surrounding an egg This shows how only one single sperm gets to penetrate the egg, releasing its nucleus of 23 chromosomes to merge with the nucleus of the egg and its 23 chromosomes. Homologous Chromosomes Pair of chrom. similar in shape , size, and types of genes. Each locus (location of the gene) in same position on chrom. Humans have 23 pairs of homologues Housefly – 6 prs Chicken – 39 prs Apple – 17 prs Dog – 39 prs Cat – 19 prs This is a karyotype (an image of an organism’s chromosomes) This is a karyotype of a normal human male Homologous Chromosomes eye color locus eye color locus hair color locus hair color locus Paternal (from Dad) Maternal (from Mom) Prophase I Longest and most complex phase (90%). Chromosomes condense. Synapsis - a process: when hom. chrom. come together, pair up, form a tetrad. Prophase I - Synapsis Nonsister chromatids sister chromatids Tetrad sister chromatids Prophase I - Crossing Over Crossing over may occur in the tetrad: between nonsister chromatids, ends break and reattach Crossing Over - Provides Variation nonsister chromatids Chiasma: site of crossing over Tetrad variation Metaphase I Shortest phase; paired homologues align. INDEPENDENT ASSORTMENT occurs pairs of homologues line up independently of other pairs’ orientation toward the poles -- random. Adds variation. Anaphase I Homologous chromosomes separate towards the poles (Tetrads separate) Sister chromatids remain attached Telophase I Each pole now has haploid set of chromosomes (however – still doubled). Cytokinesis occurs: two haploid daughter cells formed. Meiosis II No interphase II ( no more DNA replication) Remember: Meiosis II is similar to mitosis Prophase II / Metaphase II Anaphase II Telophase II Same as telophase in mitosis. Nuclei form. Cytokinesis occurs (2nd time). Four haploid daughter cells produced (chromosomes now back to single condition). gametes ~ sperm or egg; ovule or pollen grain Gamete Formation in Animals Diff. bet. male and female gametes. Male: spermatogenesis all 4 develop into sperm cells. Female: oogenesis cytokinesis in meiosis is uneven. most of cytoplasm goes into 1 of the 4 meiotic products (forms large egg cell) 3 other cells are small “polar bodies”, break down (extra chrom. lost). Spermatogenesis n=23 human germ cell in testes sperm n=23 Still doubled chromosomes n=23 2n = 46 haploid (n) n=23 n=23 diploid (2n) Still doubled chromosomes n=23 meiosis I meiosis II Oogenesis human germ cell in ovary 23 n=23 23 Still doubled 2n = 46 23 n=23 Still doubled diploid (2n) Ovum n=23 meiosis I meiosis II Polar Bodies Mitosis vs Meiosis Mitosis Body (somatic cells) 2 daughter cells made (identical) Each w/ same # & kind of chrom. as parent cell 1 division process 1 cytokinesis No synapsis or crossing over Are diploid (2n) Meiosis Germ cells of gonads 4 gamete cells made (all different) Each w/ ½ chrom. # as parent cell 2 divisions 2 cytokineses events Synapsis & crossing over occurs in Prophase 1 Are haploid (n) Chromosome numbers: However many “types” of chromosomes an organism has, that number is the “n” number of chromosomes it has. Body cells (somatic cells) therefore have a “2n” # (diploid) Gametes have “n” # (haploid) For humans, 2n = 46 n = 23 Nondisjunction When the tetrad (in Anaphase I) or the sister chromatids (in Anaphase II) do not separate, creating an abnormal # of chrom. to occur in the gametes. Lethal most of the time Karyotype… male Karyotype… female