MEIOSIS

Meiosis It is a two-stage cell division in which the chromosome number of the parental cell is reduced by half. It is the process by which gametes (or sex cells) are formed.

Chromosome Number The number of chromosomes in a gamete is called the haploid chromosome number or (n). The number of chromosomes in all other cells is twice the haploid number and is called the diploid number or (2n). In humans, the haploid chromosome number is 23 and the diploid chromosome number is 46.

Genetic Information Offspring carry genetic information from each of the parents. Each of the 23 chromosomes you receive from your father is matched by 23 chromosomes from your mother. These paired chromosomes are called homologous chromosomes as they are similar in shape, size and gene arrangement.

Homologous Chromosomes Homologous chromosomes interact during meiosis. This interaction determines the characteristics you receive. During fertilization a haploid (n=23) sperm cell unites with a haploid (n=23) egg cell to produce a diploid (2n=46) zygote.

Stages of Meiosis Meiosis involves 2 nuclear divisions that produce 4 haploid cells. Meiosis I involves the diploid chromosome number being reduced to the haploid chromosome number. Meiosis II is marked by a separation of the 2 chromatids.

Prophase I The chromosomes condense. The chromosomes come together in homologous pairs (referred to as synapsis). Each pair is composed of a pair of sister chromatids. As chromosomes synapse, chromatids undergo crossing over which permits the exchange of genetic material between homologous pairs. Chromosomes attach to the spindle

Metaphase I Chromosomes line up at the equatorial plate.

Anaphase I Each chromosome separates from its homologue They move to opposite poles of the cell.

Telophase I The nucleus completes its division. The chromosomes are still composed of sister chromatids. The cytoplasm divides after telophase

Prophase II The centrioles in the 2 new cells move to opposite poles and new spindle fibres form. The chromosomes become attached to the spindle.

Metaphase II Chromosomes line up at the equatorial plate.

Anaphase II Sister chromatids of each chromosome separate and move to opposite poles

Telophase II The cytoplasm separates, leaving 4 haploid daughter cells. The chromosome number has been reduced by half (46→23). These cells may become gametes.

Animation http://www.cellsalive.com/meiosis.htm http://highered.mcgraw- hill.com/sites/0072495855/student_view0/chapte r28/animation__how_meiosis_works.html