Meiosis Cellular Reproduction Part II Ploidy • Ploidy (N) describes the number of copies of chromosomes a cell contains. Haploid (1N), Diploid (2N), Triploid (3N), Tetraploid (4N). • Humans are diploid organisms, each cell has 2 sets of chromosomes. • In order to conserve the diploid requirement for humans, sex cells must be haploid (1N). That way when two sex cells join they create a diploid organism. • Meiosis is the process of creating the haploid sex cells called gametes. • If meiosis didn’t occur, when our sex cell combine they would create a tetraploid zygote, which would be incompatible for human life and not survive. Human Spermatogenesis Humans Oogenesis Meiosis Meiosis I • • • • • • Prophase I Metaphase I Anaphase I Telophase I Interkinesis Results in one diploid primary oocytes and a polar body in females. • Results in two diploid primary spermatocytes in males. Crossing Over: exchange of genetic material between homologous chromosomes. Occurs during Prophase I. Responsible for genetic diversity. Meiosis II • • • • • Prophase II Metaphase II Anaphase II Telophase II Cytokinesis • Results in 4 haploid secondary spermatocyes in males. • Results in 1 haploid secondary oocyte and 3 polar bodies in females. • Occurs in anaphase II, when the sister chromatids are not separated correctly. This results in one n+1 gamete, one n-1 gamete, and two normal haploid gametes. • If the n+1 gamete gets fertilized it will result in trisomy, If the n-1 gamete gets fertilized it will result in monosomy. Non-disjunction Chromosomal Diseases • Monosomy – Most monosomy zygotes are incompatible with life and spontaneously abort. – Turner’s Syndrome: XO http://ghr.nlm.nih.gov/condition/turner-syndrome • Trisomy’s: – 21: Down’s syndrome http://ghr.nlm.nih.gov/condition/down-syndrome – XXY: Kleinfelter’s syndrome http://ghr.nlm.nih.gov/condition/klinefelter-syndrome – XYY syndrome http://ghr.nlm.nih.gov/condition/47xyysyndrome Overview • Meiosis occurs in sex cells to create gametes. • It is two events of cell division with only one even of DNA synthesis which results in haploid gametes. • In human men it results in 4 sperm. Men constantly produce sperm through meiosis once they reach puberty. • In human females it results in 1 oocyte and 3 polar bodies. Females create all the oocytes they will ever have in while they are a fetus. The oocytes are rested in the primary oocyte phase until puberty, then each ovulation cycle they mature 1 oocyte to the secondary oocyte stage. • The division events have the same stages as mitosis. (prophase I, metaphase I, anaphase II, telophase I, interkinesis, prophase II, metaphase II, anaphase II, telophase II, cytokinesis) • The gametes are all unique. • Nondisjunction is an error in separating chromosomes (anaphase), it can happen during meiosis II, and results in monosomy or trisomy diseases.