A Reduction Division
How does an organism with diploid body cells produce haploid gametes?
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 the normal number of chromosomes. The chromosomes at this time are still in an uncoiled form, much like coiled wire.
In prophase I of meiosis, the NDA coiled tidily, the chromosomes condense into short thick rods that can be seen through a light microscope. Each chromosome is now double consisting of two chromatids. Homologues pairs of chromosomes come together on the spindle fiber; this paring of chromosomes is called synapsis. In each synapsing group, there are homologous four chromatids - two chomatids composing each of the homologous chromosomes. Each group of four chromatids is called a tetrad.
The tetrads move along spindle fibers until they reach the equator of the cell.
The homologous chromosomes are pulled apart so that the pairs of chromatids from each tetrad move toward opposite poles of the cell. The pattern of chromatid distribution is random. What is important, however, is that homologous chromosomes segregate and move in opposite directions.
The cell divides into two smaller cells. Each new cell contains one homologous chromosome from each original pair. The new cells are not identical since homologous chromosomes do not contain identical information.
The chromatids do not replicate at this point. They uncoil and become invisible. Soon the two newly created cells enter into the second stage of meiosis. The first cell division of meiosis is different from mitosis because the number of chromosomes is reduced by one half. Thus meiosis is sometimes referred to as a reduction division. The phases in the second meiotic division are similar to the phases of mitosis.
The chromatid pair condense and become visible
Each chromatid pair moves along a spindle fiber until it reaches the equator
Once the chromosomes reach their destinations, a nuclear envelope forms around each set of chromosomes.
The chromatids of each pair separate and move toward opposite poles of the cell. Each individual chromatid is now called a chromosome. At this point, each of the two cells divides to form two smaller cells. As a result of meiosis, one diploid cell undergoes two divisions to form four haploid cells. Thus each cell has only half as many chromosomes as the original diploid nucleus; more importantly, each new cell contains a new combination of genetic materials. The new combination makes offspring different from their parents. This is why siblings may resemble each other, but are not identical (unless they are identical twin).
Significance of meiosis
- In meiosis, there is crossing over which is the exchange of genetic material leading to variation.
- The halfing of chromosomes ensures that the chromosome number is maintained.