Meiosis: Defining Attributes n Homologous chromosomes pair in Prophase I n Recombination occurs in Prophase I n Homologs separate (“segregate”) in Anaphase I Prophase I Metaphase I Anaphase I

Meiotic chromosome dynamics Prophase Metaphase IAnaphase I Metaphase IIAnaphase II

Meiosis I Segregates Homologous Chromosomes Reductional Division 2N to N Meiosis II Segregates Chromatids Equational Division 1C Meiosis II Segregates Chromatids Equational Division 1C Meiosis I and II: Reductional and equational divisions

Mitosis (and Meiosis II): Sister centromere cohesion provides force to develop tension and align chromosomes at metaphase

Meiosis I: Homologs pair From Griffiths et al., From Griffiths et al., Klug & Cummings, 6th Ed., 2000

Meiosis I: Homologs recombine Hartl and Jones, 5th Ed., 2001

Meiosis I: Non-sister centromeres not associated; chiasmata and sister-chromatid cohesion provide force to develop tension and align homologous pairs of chromosomes at metaphase

Meiosis I: Homologs separate Hartl and Jones, 5th Ed., 2001

Random Metaphase I alignment leads to independent assortment Hartl and Jones, 5th Ed., 2001

Meiosis II: Equational separation of chromatids Hartl and Jones, 5th Ed., 2001

Meiotic error: Nondisjunction Griffiths et al., 1999

Human Down syndrome: Trisomy 21 n Nondisjunction of Chr 21 during meiosis leads to a gamete with an extra chromsome Klug and Cummings, 6th Ed., 2000