Meiosis Meiosis Two rounds of cell division Meiosis

Meiosis

Meiosis • Two rounds of cell division (Meiosis I and Meiosis II) • Results in 4 haploid cells • We need this to create gametes (sex cells) – Occurs in the testes in males (spermatogenesis) – Occurs in the ovaries in females (oogenesis)

Meiosis in Humans • Start with 46 double-stranded chromosomes (2 n) • After 1 division– 23 double or duplicate stranded chromosomes (n) • After 2 nd division— 23 single stranded chromosomes (n)

Why do we need Meiosis? • Fundamental basis of sexual reproduction • Two haploid (n) gametes are brought together through fertilization to form a diploid (2 n) zygote

Key Terms • Homologous chromosomes – Two separate chromosomes that have the same genes – One from your mother and one from your father • Sister chromatids – Each half of one chromosome • Homologous chromosomes are divided in meiosis I (reduction division) and sister chromatids will be divided in meiosis II.

Meiosis I: Reduction Division Spindle fibers Nucleus Early Prophase I (Chromosome number doubled) Nuclear envelope Late Prophase I Metaphase I Anaphase I Telophase I 8

Prophase I

Crossing Over

Metaphase I • Homologous chromosomes line up along the middle of the cell. • Each side has chromosomes from both parents.

Anaphase I • Homologous chromosomes separate and move to opposite ends of the cell. • Sister chromatids remain attached at their centromeres

Telophase I • Two nuclei are formed • Spindle disappears • Cytokinesis

Meiosis II • During Meiosis I, we divided homologous chromosomes (diploid haploid) • Meiosis I leads immediately into Meiosis II – There is no interphase, cell does not grow or duplicate DNA • In Meiosis II, we will divide sister chromatids. – Produces gametes with one copy of each chromosome and gene

Spermatogenesis • Occurs in the testes • Two divisions produce 4 spermatids, which mature into sperm • Men produce about 250, 000 sperm per day

Oogenesis • Occurs in the ovaries • Two divisions produce 3 polar bodies that die and 1 egg • Polar bodies die because of unequal division of cytoplasm • Immature egg called oocyte • Starting at puberty, one oocyte matures into an ovum (egg) every 28 days