Cell cycle checkpoints Chromosome and · Have the Is cell of sufficient size? centrosome duplication chromosomes been Have proper signals from environment been received completely duplicated? THEN: Decision to duplicate THEN: Decision to chromosomes and enter mitosis centrosomes Prophase Interphase Mitosis Ongoing protein synthesis and e Metaphase cell growth N Have all chromosomes Telophase arrived and aligned at and the metaphase plate? cytokinesis Anaphase THEN: Decision to initiate anaphase
Sexual reproduction: Meiosis Germ cells undergo two special nuclear divisions with only one chromosome replication These meiotic divisions reduce the number of chromosomes in the gametes Chromosome behavior in meiosis is the first evidence that genes reside on chromosomes
Sexual reproduction: Meiosis • Germ cells undergo two special nuclear divisions with only one chromosome replication. • These meiotic divisions reduce the number of chromosomes in the gametes. • Chromosome behavior in meiosis is the first evidence that genes reside on chromosomes
Drosophila melanogaster X Diploid cells X 2n=8 Haploid cells (gametes) n=4
Meiosis: An overview 2n Chromosomes duplicate Meiosis i No duplication Meiosis l No duplication n
Meiosis I a. Earty prophase I ORient chromosome During prophase I, homologous chromosomes condense The condensing chromosomes farst become visible during earty prophase I(in the leptotene substage), and the paIr s besome fully con (becoming bivalents), and recombine Each chromosome hAs twe chron. The bivalents attach to spindle fibers. During anaphase I the homologous During telophase I two daughter nuclei form m,可ane Each of the two daughter cells from meiosis I chromosome from esch bivalent nters meiosis I
Meiosis I • During prophase I, homologous chromosomes condense, pair (becoming bivalents), and recombine. • The bivalents attach to spindle fibers