Sunday, December 5, 2010

cell Division


Cell Division
Most cells divide at some time during their life cycle, and some divide dozens of times before they die. Organisms rely on cell division for reproduction, growth, and repair and replacement of damaged or worn out cells. Three types of cell division occur: binary fission, mitosis, and meiosis. Binary fission, the method used by prokaryotes, produces two identical cells from one cell. The more complex process of mitosis, which also produces two genetically identical cells from a single cell, is used by many unicellular eukaryotic organisms for reproduction. Multicellular organisms use mitosis for growth, cell repair, and cell replacement. In the human body, for example, an estimated 25 million mitotic cell divisions occur every second in order to replace cells that have completed their normal life cycles. Cells of the liver, intestine, and skin may be replaced every few days. Recent research indicates that even brain cells, once thought to be incapable of mitosis, undergo cell division in the part of the brain associated with memory.

It is process in which a cell’s nucleus replicates and divides in preparation for division of the cell. Mitosis results in two cells that are genetically identical, a necessary condition for the normal functioning of virtually all cells. Mitosis is vital for growth; for repair and replacement of damaged or worn out cells; and for asexual reproduction, or reproduction without eggs and sperm.
Mitosis occurs in five steps: prophase, prometaphase, metaphase, anaphase, and telophase .
 In prophase the replicated, linked DNA strands slowly wrap around proteins that in turn coil and condense into two short, thick, rodlike structures called chromatids, attached by the centromere. Two structures called centrioles, both located on one side of the nucleus, separate and move toward opposite poles of the cell. As the centrioles move apart, they begin to radiate thin, hollow, proteins called microtubules. The microtubules arrange themselves in the shape of a football, or spindle, that spans the cell, with the widest part at the center of the cell and the narrower ends at opposite poles.
Prometaphase is marked by the disintegration of the nuclear membrane. As the spindle forms, the nuclear membrane breaks down into tiny sacs or vesicles that are dispersed in the cytoplasm. The spindle fibers attach to the chromatids near the centromeres, and tug and push the chromatids so that they line up in the equatorial plane of the cell halfway between the poles. Like two individuals standing back to back at the equator, one chromatid faces one pole of the cell, and its linked partner faces the opposite pole.
In metaphase, exactly half of the chromatids face one pole, and the other half face the other pole. This equilibrium position is called the metaphase plate.

Anaphase begins when the centromeres split, separating the identical chromatids into single chromosomes, which then move along the spindle fibers to opposite poles of the cell. At the end of anaphase, two identical groups of single chromosomes congregate at opposite poles of the cell.

In telophase, the final stage of mitosis, a new nuclear membrane forms around each new group of chromosomes. The spindle fibers break down and the newly formed chromosomes begin to unwind. If viewed under a light microscope, the chromosomes appear to fade away. They exist, however, in the form of chromatin, the extended, thin strands of DNA too fine to be seen except with electron microscopes. Mitosis accomplishes replication and division of the nucleus.

The final phase of the cell cycle is known as cytokinesis. The timing of cytokinesis varies depending on the cell type. It can begin in anaphase and finish in telophase; or it can follow telophase. In cytokinesis, the cell’s cytoplasm separates in half, with each half containing one nucleus. Animals and plants accomplish cytokinesis in slightly different ways. In animals, the cell membrane pinches in, creating a cleavage furrow, until the mother cell is pinched in half. In plants, cellulose and other materials that make up the cell wall are transported to the midline of the cell and a new cell wall is constructed. The process of DNA replication, the precise alignment of the chromosomes in mitosis, and the successful separation of identical chromatids in anaphase results in two new cells that are genetically identical. The new cells enter interphase, and the cell cycle begins again.

Meiosis, process of cell division in which the cell’s genetic information, contained in chromosomes, is mixed and divided into sex cells with half the normal number of chromosomes. The sex cells can later combine to form offspring with the full number of chromosomes. The random sorting of chromosomes during meiosis assures that each new sex cell, and therefore each new offspring, has a unique genetic inheritance.
Meiosis differs from normal cell division, or mitosis, in that it involves two consecutive cell divisions instead of one and the genetic material contained in chromosomes is not copied during the second meiotic division. Whereas mitosis produces identical daughter cells, meiosis randomly mixes the chromosomes, resulting in unique combinations of chromosomes in each daughter cell.
Prior to meiosis, the corn cell undergoes Interphase, in which it synthesizes materials needed for cell growth and prepares for cell division. During this stage the cell’s genetic information, in the form of deoxyribonucleic acid (DNA), is replicated. Each of the two consecutive cell divisions consists of four stages: prophase, metaphase, anaphase, and telophase.
Prophase I each long DNA strand wraps around proteins that in turn coil and condense to form a chromosome. Since the DNA was copied during interphase, each chromosome condenses to form two identical chromatids, joined at a centromere. A corn cell has 20 chromosomes at this stage, each with two identical chromatids, making a total of 40 chromatids.
Chromosomes exist in pairs; one is inherited from the mother (maternal) and one from the father (paternal). When the chromosomes duplicate, two maternal and two paternal chromatids are produced. These two pairs of chromatids gather together in groups of four called tetrads. Each corn cell contains 10 tetrads. While grouped together in tetrads, sections of the chromatids from the maternal pair may randomly exchange, or cross over, with sections of the paternal chromatid pair. Called genetic recombination, this process is the first of two ways that meiosis mixes genetic information during sexual reproduction.
Also in Prophase I, two structures called centrioles, both located on one side of the nucleus, separate and move toward opposite sides of the cell. As the centrioles move apart, they radiate thin hollow structures called spindle fibers. The membrane around the nucleus of the cell breaks down, marking the beginning of the next stage.
During metaphase I, the spindle fibers attach to the chromatids near the centrioles. The spindle fibers move the tetrads so that they line up in a plane halfway between two centrioles.
Anaphase I begins when the spindle fibers pull the tetrads apart, pulling the maternal and paternal chromosomes toward opposite sides of the cell. The first meiotic division concludes with Telophase I, when the two new groups of chromosomes reach opposite sides of the cell. A nuclear membrane may form around the two new groups of chromosomes and a division of cell cytoplasm forms two new daughter cells.
Each daughter corn cell receives 10 chromosomes made up of a random mixture of maternal and paternal chromosomes. This second mixing of genetic information is called independent assortment. Genetic recombination and independent assortment make it possible for parents to have many offspring who are all different from each other.
In the second meiotic division the cell moves directly into prophase II, skipping the interphase replication of DNA. Each corn cell begins the second division with 10 chromosomes. Once again the centrioles radiate spindle fibers as they move to opposite sides of the cell. During metaphase II, the chromosomes line up along the plane in the center of the cell, and in anaphase II the pairs of chromatids are pulled apart, each moving toward opposite ends of the cell.
Telophase II completes meiosis. The spindle fibers disappear and a new nuclear membrane forms around each new group of chromosomes to form four haploid cells. The original diploid corn cell with 20 chromosomes has undergone meiosis to form four haploid daughter cells, each containing 10 chromatids. It is now possible for two haploid sex cells to join during fertilization to form one egg cell with the normal diploid number of chromatids. After fusion and DNA replication, two haploid corn cells will yield one diploid egg cell with 10 pairs of chromosomes.
In humans meiosis occurs only in the reproductive organs, the testes in males and the ovaries in females. In males, each of the meiotic divisions result in four equally sized haploid cells that mature into functional sperm cells. In females, the meiotic divisions are uneven, resulting in three tiny cells called polar bodies and one large egg that can be fertilized.

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