by MR. ITOPA

Created on: January 06, 2011   Last Updated: January 10, 2011

A Reduction Division

How does an organism with diploid body cells produce haploid gametes?

MEIOSIS:

A process of cellular division in which the number of chromosomes is reduced by half is responsible for these.  This change occurs through a process called meiosis, the type of nuclear division that gives rise to four reproductive cells (gametes) which with half the chromosome number of the parent. Two consecutive divisions occur; in the first phase, homologues chromosome become paired and may exchange genetic materials (crossing over) before moving away from each other into separate daughter nuclei: this is the actual reduction division because each  the two nuclei contain only half of the original chromosome. The daughter nuclei then divide by mitosis and four haploid cells are produced.

In animals, meiosis takes place in the testis/ovary; the testis produces sperm and the ovary produces egg cell or ovum.  Plants also form sex cells or gametes by meiosis.  The pollen cells of flowering plants are male sex cells while the ovary produces the egg cell. The development of sex cells is known as gamatogesis i.e. oogenesis and spermatogenesis.

During meiosis, the chromosomes of a diploid cell replicate. The appearance of the chromosomes during meiosis is similar to their appearance in mitosis. Unlike mitosis; however, meiosis includes two cell divisions that yield four haploid cells. These two divisions are distinguished by the Roman numerals I and II.

The Stages of meiosis are as follows

Meiosis involves two cell divisions that produce four haploid cells. During the first division called meiosis I, homologues chromosomes move to opposite poles, it is during this division that diploid cells separate into haploid cells.

Meiosis I is often called reduction division because the chromosome number is reduced to half. The diploid or 2n is haploid or n, the chromosome number following the first division.  The second phase or meiosis II is marked by a separation of the two chromatids.

The two stages of meiosis are referred to as meiosis I & meiosis II. Like mitosis, each stage of meiosis is subdivided further into; Prophase, Metaphase, Anaphase and Telophase. Therefore, you would have Prophase I, Metaphase I, Anaphase I and Telophase I; then Prophase II, Metaphase II, Anaphase II and Telophase II.

Interphase- Before meiosis begins, the chromosomes replicate much as they did at the start of mitosis. As a result, the nucleus now contains double

• 1
• 2
• 3
• Next Page >>