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.
Non-disjunction 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.
Chromosomal Diseases Trisomy’s: – 21: Down’s syndrome – XXY: Kleinfelter’s syndrome – XYY syndrome syndromehttp://ghr.nlm.nih.gov/condition/47xyy- syndrome Monosomy – Most monosomy zygotes are incompatible with life and spontaneously abort. – Turner’s Syndrome: XO
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.