A very important feature of cell division is the correct distribution of genetic material among daughter cells. To aid in accomplishing this distribution, the genetic material is organized into structures called chromosomes. In prokaryotes, the chromosome is simply a molecule of double stranded DNA (in bacteria, the single chromosome is a circular piece of DNA). Eukaryotic chromosomes are much more complex. Each chromosome consists of a single linear molecule of DNA complexed with specific proteins, forming a substance called chromatin. Chromosomes exist as dispersed chromatin (and are therefore not detectable by light microscopy) during times when the cell is not actively dividing. During cell division, however, the chromatin undergoes organization and packing, becoming highly condensed, and the chromosomes become visible under a microscope (as observed at right).

Because the genetic material must be duplicated for distribution to daughter cells, during cell division each chromosome has been replicated and consists of two sibling chromatids joined by a centromere. The centromere is the position at which spindle fibers attach to the chromosome during cell division. The centromere is found at a characteristic position on each chromosome, but this position can vary between chromosomes from the center to the end of the chromosome. Because they consist of linear DNA, Eukaryotic chromosomes also have structures called telomeres. Telomeres are repetitive DNA structures at the end of the chromosomes. Prokaryotic chromosomes, because they are circular, do not have telomeres.

Each eukaryotic species has a characteristic number of chromosomes in every cell. For example, typical human cells contain 46 chromosomes. Closer examination reveals that there are actually 23 pairs of chromosomes; therefore, there are 23 different chromosomes in human cells, each of which is present in two copies in most cells. The number of different chromosomes characteristic of a species (such as 23 in humans) is called the haploid (often abbreviated 'n') number of chromosomes. The total number of chromosomes in most cells (46 in humans) is the diploid number (abbreviated '2n'). As implied above, most cells of an organism are diploid; however, the gametes, or reproductive cells, are haploid.