Why Are Chromosomes Important for Cell Division?

Chromosomes

• Chromosomes are tightly packed structures that hold a cell's DNA wrapped around a protein. They reside in the nucleus of a cell. All cells, except the egg and sperm, have 46 chromosomes. Sex cells have 23 chromosomes, so when they combine they end up with the usual 46.

  Chromosomes have a constriction point, known as the centromere, and two arms. The short arm is known as the "p arm" and the long arm is called the "q arm." Chromosomes become even more tightly packed during cell division, making them visible under a microscope.

Cell Division

• There are two types of cell division: mitosis and meiosis. Mitosis is the most common type of cell division as it creates new body cells, whereas cells undergo meiosis only to make new egg and sperm cells.

  A cell completely replicates itself during mitosis and then divides into two daughter cells. Each daughter cell is identical to the parent cell and contains the same number of chromosomes. During meiosis, four daughter cells are created, each with half the number of chromosomes as the parent cell.

DNA Replication in Mitosis

• DNA replication is an essential part of both mitosis and meiosis. It ensures that each daughter cell has the correct number of chromosomes. To replicate the DNA in mitosis, each chromosome replicates so that the new chromosome is attached to the original one at the centromere. The two chromosomes are called sister chromatids. They divide in two before the cell divides, and each daughter cell gets one chromosome from each of the sister chromatids.

DNA Replication in Meiosis

• Meiosis is more complicated that mitosis and requires two cell divisions. In the first stage, the chromosomes replicate as in mitosis. However, then the chromatid arms of sister chromatids may overlap with other sister chromatids and cause crossovers --- swapping of DNA between chromatids, so that the each chromatid is no longer identical to its sister. The cell then divides twice, so that the sister chromatids separate and the daughter cells have 23 chromosomes each.