CELL DIVISION

The scientists have determined that the new cells necessary for growth and tissue repair are produced by cell division. A parent cell divides to form two daughter cells, each of which has the same amount and type of DNA as the parent cell. Because DNA determines the structure and function of cells, the daughter cells have the same structure and perform the same functions as the parent cell.

Cell division involves two major events: the division of the nucleus to form two new nuclei, and the division of the cytoplasm to form two new cells, each of which contains one of the newly formed nuclei. The division of the nucleus occurs by mitosis, and division of the cytoplasm is called cytokinesis.

Physiologists have estimated that mitosis is the division of the nucleus into two nuclei, each of which has the same amount and type of DNA (deoxyribonucleic acid) as the original nucleus. The DNA, which was dispersed as chromatin in interphase, condenses in mitosis to form chromosomes. In each of the human somatic cells, which include all cells except the sex cells, there are 46 chromosomes, which are referred to as a diploid number of chromosomes. Sex cells have half the number of chromosomes as somatic cells. The 46 chromosomes in somatic cells are organized into 23 pairs of chromosomes. Twenty-two of these pairs are called autosomes. Each member of an autosomal pair of chromosomes looks structurally alike, and together they are called a homologous pair of chromosomes. One member of each autosomal pair is derived from the person's father, and the other is derived from the mother. The remaining pair of chromosomes comprises the sex chromosomes. In females sex chromosomes look alike, and each is called an X chromosome. In males the sex chromosomes do not alike, one is an X chromosome, and other is smaller and is called a Y chromosome.

Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase. Although each stage represents major events, mitosis is a continuous process, and there are no jumps from one stage to another.

Cytokinesis begins in anaphase, continues through telophase, and ends in the following interphase. The first sign of cytokinesis is the formation of a cleavage furrow, or puckering of the cell membrane, which forms midway between the centrioles. A contractile ring composed primarily of actin filaments pulls the plasma membrane inward, dividing the cell into two halves. Cytokinesis is complete when the two halves separate to form two new cells.

Diagram showing the changes which occur in the centrosomes and nucleus of a cell in the process of mitotic division. I to III, prophase; IV, metaphase; V and VI, anaphase; VII and VIII, telophase.