Meiosis: Defining Attributes Homologous chromosomes pair in

Prophase I Recombination occurs in Prophase I Homologs separate (“segregate”) in

Anaphase I

Prophase I Metaphase I Anaphase I

Meiotic chromosome dynamics

Prophase Metaphase I Anaphase I

Metaphase II Anaphase II

Meiosis ISegregates Homologous Chromosomes

Reductional Division2N 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

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

DNA Synthesis & HomologousChromosome Pairing

Equational Division

ABCABCabcabcReductional DivisionABCABc abCabcABC ABc abC abc

abcABC

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

Nondisjunction of Chr 21 during meiosis leads to a gamete with an extra chromsome

Klug and Cummings, 6th Ed., 2000