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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.
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
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/47xyy-syndrome
• Monosomy– Most monosomy zygotes are incompatible with life and
spontaneously abort.– Turner’s Syndrome: XO
http://ghr.nlm.nih.gov/condition/turner-syndrome
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.