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