Section 11-4: Meiosis

Meiosis – cell division that halves the number of chromosomes in new cells, making haploid cells

*Recall, haploid cells in human are egg/sperm cells, called gametes, and contain 23 chromosomes.

Stages of MeiosisI. Interphase

A. cells go through G1, S, and G2 phases;

B. DNA is copied and cells grow larger.

II. Meiosis IA. Prophase I

1. DNA is coiled into chromosomes & nucleus breaks down

2. Centrioles move to ends of cell; spindle fibers appear

3. Chromosomes pair up (in homologous pairs) and twist together.– Portions of a chromatid can break off and

reattach to the identical chromatid on its homologous chromosome = crossing over

Prophase I and Crossing Over

B. Metaphase I1. Homologous chromosomes line up

randomly in the center of the cell2. Spindle fibers attach to the centromeres

C. Anaphase I1. Spindle fibers pull homologous

chromosomes apart.2. The direction the chromosomes are pulled

is random = independent assortment

D. Telophase I1. Chromosomes move to opposite ends of

the cell2. Cytokinesis occurs

**Meiosis I results in 2 new haploid cells

III. Meiosis II• Begins immediately after Meiosis I.• Both cells from Meiosis I go through

Meiosis II.• DNA is not copied this time!!!!!!!

A. Prophase II – spindle fibers reform, centrioles move to opposite ends of cell

B. Metaphase II – single chromosomes line up in the middle of the cell

C. Anaphase II – spindle fibers attach at centromeres and pull chromatids apart

D. Telophase II – chromatids move to opposite ends, and nucleus forms around them and cytokinesis occurs

**Meiosis II results in 4 new Haploid cells.

Stages of Meiosis II

IV. Meiosis Forms GametesA. The 4 haploid cells produced in Meiosis

are gametes (sex cells = egg/sperm)B. Spermatogenesis – production of sperm

cellsC. Oogenesis – production of mature egg

cells or ova (singular = ovum)

V. Genetic Variation in MeiosisA. Crossing Over – switching of DNA

between chromatids of homologous pairs; occurs during prophase I

B. Independent Assortment – each homologous pair is separated independent of how the other pairs are separated.

C. Random Fertilization – of the 4 gametes produced in meiosis, any one can fertilize (join) with another gamete