Problem: Our cells have 46 chromosomes—so why don’t fertilized eggs have 92 chromosomes?

Answer: Meiosis

Meiosis The form of cell division by which gametes, with half the number of chromosomes, are produced. Diploid (2n)  haploid (n) Meiosis is sexual reproduction. Two divisions (meiosis I and meiosis II).

Fertilization n=23 egg 2n=46 zygote The fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote

Meiosis Sex cells divide to produce gametes (sperm or egg). Gametes have half the # of chromosomes. Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis Meiosis is similar to mitosis with some chromosomal differences.

Spermatogenesis n=23 n=23 2n=46 sperm haploid (n) meiosis II human sex cell diploid (2n) n=23 meiosis I

Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes

Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

Crossing Over Crossing over (variation) may occur between nonsister chromatids at the chiasmata. Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. Chiasmata (chiasma) are the sites of crossing over.

Crossing Over - variation Tetrad nonsister chromatids chiasmata: site of crossing over variation

Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

Metaphase I metaphase plate OR metaphase plate

Anaphase I

Telophase I

Meiosis II No interphase II (or very short - no more DNA replication) Remember: Meiosis II is similar to mitosis

Prophase II same as prophase in mitosis

Metaphase II same as metaphase in mitosis metaphase plate

Anaphase II same as anaphase in mitosis sister chromatids separate

Telophase II