Cell Division - Meiosis

Meiosis Process of cell division that reduces the number of chromosomes in new cells to half the number in the original cell. The haploid cells produced are called gametes.

Meiosis Cells preparing to divide by meiosis undergo the G1, S and G2 phases of interphase.

Meiosis Cells will then begin meiosis with a duplicate set of chromosomes. Meiosis is divided into two stages called Meiosis I and Meiosis II.

Meiosis I Prophase I DNA coils tightly into chromosomes, spindles appear, and the nuclear membrane disassembles.

Prophase I Each chromosome lines up next to its homologue. This is called synapsis. Each pair of homologous chromosomes is called a tetrad.

Prophase I During this stage, crossing-over occurs. This is when genetic material breaks off and attaches to the adjacent chromatids. The result is genetic recombination.

Metaphase I Tetrads line up randomly along the midline of the dividing cell. Spindles attach to the centromeres of the homologous chromosomes.

Anaphase I Each homologous chromosome moves to an opposite pole. The chromosomes are separated randomly. This is called independent assortment.

Telophase I Chromosomes reach the opposite ends of the cell and cytokinesis begins

Meiosis II Meiosis II is not started by the copying of DNA. There may be a short resting stage but usually there is no true interphase.

Prophase II Spindle fibers form and begin to move the chromosomes toward the midline of the cell

Metaphase II Chromosomes move to the midline of the cell.

Anaphase II The chromatids separate and move toward opposite poles of the cell.

Telophase II Cytokinesis occurs and produces 4 cells A nuclear membrane forms around the chromosomes in each of the 4 new cells.

Formation of Gametes In males, meiosis occurs in the testes. This is called spermatogenesis. Four sperm cells result from meiosis.

Formation of Gametes In females, meiosis occurs in the ovaries. This is called Oogenesis.

Formation of Gametes During cytokinesis for females, the cytoplasm is unequally divided. This results in only 1 egg cell being formed and 3 polar bodies.

Asexual Reproduction Offspring from only one parent Offspring are genetically identical Occurs from binary fission or mitosis

Sexual Reproduction Offspring from two parents Offspring are genetically different Meiosis