Cell Division Health Article

Definition

The process by which a cell distributes its genetic material (DNA) and cytoplasm to daughter cells.

Description

In higher organisms including humans, there are two types of cell division, mitosis and meiosis. Strictly speaking, mitosis and meiosis refer to division of the DNA and associated materials in the nucleus of the cell. In mitosis, the cells produced by division have exactly the same genetic information as the original cell, while in meiosis the cells produced by division have only half the genetic information as the original cell. Both processes are accompanied by cytokinesis, the division of the cytoplasm.

Mitosis

Mitosis produces two daughter cells, each of which has the same genetic information as the parent cell. The entire process includes a series of precise steps to insure that the genetic material is accurately duplicated and distributed. The life of a cell is generally made up of two parts, interphase and mitosis. During interphase, DNA synthesis occurs. Since the process uses the original DNA as a template, the copy is exact (or nearly exact if mutations occur). After a pause, the cell then enters mitosis. Although the lifespan of a cell varies in length depending on the cell type, mitosis itself usually takes about one to two hours and involves four stages: prophase, metaphase, anaphase, and telophase.

PROPHASE. During prophase, the chromosomes, which contain the DNA, condense in length and become visible under a microscope. Humans have 23 chromosome pairs, for a total of 46 chromosomes. Since DNA duplication has already occurred, each of the 46 chromosomes at this stage is present in two copies referred to as sister chromatids. The two sister chromatids of a pair are attached to each other at a point called the centromere. As the chromosomes condense, the membrane surrounding the nucleus disappears, and fibers appear, which come together to form a spindle within the cell. The spindle has two opposite poles and a mid-section, the equatorial plate.

METAPHASE. At the beginning of metaphase, the chromosomes line up individually on the equatorial plate. Fibers emanating from the poles of the spindle attach to the centromeres of the sister chromatids. One member of each pair of sister chromatids is attached to a spindle fiber that radiates from one pole, and the other is attached to a fiber that radiates from the opposite pole.

ANAPHASE. After all chromosomes (92 sister chromatids in 46 pairs) have aligned on the equatorial plane of the spindle, the centromere of each chromosome splits, and the fibers begin to contract. One sister chromatid of each pair is pulled to one pole of the spindle and the other is pulled to the opposite pole.

TELOPHASE. Separate membranes form around the chromosome sets at each pole to form two nuclei. The chromosomes elongate and the spindle disappears. Cytokinesis then occurs, resulting in two daughter cells each with 46 chromosomes and roughly half the cytoplasm of the parent cell.

Function and role in human health

Mitosis is the process by which a single human zygote (fertilized egg cell) becomes a complex organism consisting of over 100 trillion cells. During the lifetime of an individual, mitosis continues. In some tissues such as epithelium (skin, mucous membranes), mitosis actively occurs to replace cells and repair damage. Other cell types such as nerve cells do not readily undergo mitosis after a certain point in development. Thus the capacity for mitosis is programmed into each cell type and is cell- specific. In addition, there are many molecules within the body that can influence cell division. Scientists are just beginning to learn about some of these and their possible roles in human health. For example, cancer occurs when the normal pattern of cell division within a tissue or organ is disrupted, and the cells begin to repeatedly undergo mitosis. Changes within the cell as well as external influences can play a part in disrupting the normal control of mitosis.