Meiosis 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 cellsGametes = sperm or egg

Animation Link

• Mitosis vs. Meiosis