The Importance of Chromosomes

In eukaryotic cells, the genetic information that is passed on from one generation of cells to the next is carried by chromosomes. Chromosomes are made up of DNA – which carries the cell’s coded genetic information. The cells of every organism have a specific number of chromosomes. The cells of fruit flies, for example, have 8 chromosomes; human cells have 46 chromosomes; and carrot cells have 18 chromosomes.

Chromosomes are not visible in most cells except during cell division. This is because the DNA molecules that make up the chromosomes are spread throughout the nucleus. At the beginning of cell division, however; the chromosomes condense into compact, visible structures that can be seen through a light microscope.

Well before cell division, each chromosome is replicated, or copied. Because of this, each chromosome consists of two identicalchromatids, as shown in the diagram below. When a cell divides, the chromatids separate from each other. One chromatid goes to each of the two new cells.