Transcript Meiosis II
Meiosis A double cell division to produce sex cells (sperm and egg) Meiosis • The form of cell division by which gametes, with half the number of chromosomes, are produced. • diploid (2n) haploid (1n) • Two divisions (meiosis I and meiosis II) Meiosis • Sex cells divide to produce gametes (sperm or egg) • Gametes have half the number of chromosomes • Occurs in the gonads (testes or ovaries) – Male: spermatogenesis – Female: oogenesis • Meiosis is similar to mitosis with some chromosomal differences Interphase I • Similar to mitosis interphase • Chromosome replicated (S phase) • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres • Centriole pairs also replicate Prophase I • Chromosomes condense • Synapis occurs: homologous chromosomes come together to form a tetrad • Tetrad is four chromatids (sister and nonsister chromatids) Homologous chromosomes • Pair of chromosomes (material and paternal) that are similar in shape and size. • Homologous pairs (tetrads) carry genes controlling the same inherited traits. • Each locus (position of the gene) is in the same position on the homologous chromosomes. • Humans have 23 pairs of homologous chromosomes – 22 pairs of autosomes – 1 pair of sex chromosomes Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chismata. • Crossing over: segments of nonsister chromatids break and reattach to the other chromatids • Chismata (chiasma) are the sites of crossing over. Metaphase I • Tetrads align on the metaphase plate (middle) Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromere Telophase I • Each pole now has sister chromatids that will be separated Meiosis II • No interphase II – Very short: no need for DNA replication • Remember: Meiosis II is similar to mitosis Telophase II • • • • Same as telophase in mitosis Nuclei form Cytokinesis will occur at the end The final product should be four haploid daughter cells Gametes = sperm or egg Animation Link • Mitosis vs. Meiosis