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Transcript
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). 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. • 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 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 nonsister chromatids chiasmata: site of crossing over Tetrad variation 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 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 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 sex cell n=2 sperm n=2 n=2 2n=4 haploid (n) n=2 diploid (2n) n=2 n=2 meiosis I meiosis II Meiosis 1 First division of meiosis • Prophase 1: Each chromosome dupicates and remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage. • Metaphase 1: Homologous chromosomes align at the equatorial plate. • Anaphase 1: Homologous pairs separate with sister chromatids remaining together. • Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair. Meiosis II Second division of meiosis: Gamete formation • Prophase 2: DNA does not replicate. • Metaphase 2: Chromosomes align at the equatorial plate. • Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. • Telophase 2: Cell division is complete. Four haploid daughter cells are obtained. Meiosis KM 27 Meiosis KM 28