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

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.