Cell Division (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 No. of chromosomes. Occurs only in germ cells Meiosis is similar to mitosis with some chromosomal differences.
Spermatogenesis human sex cell n=23 n=23 n=23 sperm 2n=46 diploid (2n) n=23 n=23 haploid (n) n=23 meiosis I 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.
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%). It has 5 sub stages: Leptotene,Zygotene,Pachytene,Diplotene& diakinesis Synapsis occurs: homologous chromosomes come together to form a tetrad. Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).
Zygotene - 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
Pachytene - 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 Tetrad chiasmata: site of crossing over variation
Prophase I spindle fiber centrioles aster fibers
Shortest phase Metaphase I Tetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2 n Example: 2n = 4 then n = 2 thus 2 2 = 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
Significant results of meiosis: 1. Haploid cells are produced because two rounds of division follow one round of chromosome replication. 2. Alignment of paternally and maternally inherited chromosomes is random in metaphase I, resulting in random combinations of chromosomes in each gamete. Number of possible chromosome arrangements = 2 n-1. 3. Crossing-over between maternal and paternal chromatids during meiosis I provides still more variation. Moreover, the crossing-over sites vary from one meiosis to another.
Meiosis n=2 sex cell n=2 n=2 sperm 2n=4 diploid (2n) n=2 n=2 haploid (n) n=2 meiosis I meiosis II