The DNA contained in the cell's nucleus forms a thin threadlike substance called cromatin, by combining with proteins called histone. Because of their thinness, they cannnot be seen under the microscope.

However when in cell division, cromatin condenses and becomes a thicker strand, enabling vision through microscopes. This string-like structure is called chromosome. The name comes from its characteristic of being easily stained by dyes for observation (chromo means color).

The number of chromosomes observed in cell division differs depending on the kind of organism, but in humans, there are 23 sets of chromosomes. Organisms like humans, where chromosomes are counted in pairs are called diploids.

Humans have 23 pairs of the same chromosomes, each pair with the genes for the same characteristics (alleles) lined up in the same order. These two corresponding chromosomes stick to one another when in meiosis (pairing).

There are two kinds of chromosomes: 22 autosomes and a sex chromosome. Autosomes are made upof 2 sets of chromosomes- one from the father, and one from the mother.

There are two kinds of sex chromosomes: an X and a Y chromosome, which determine the sex of the organism. Normally, males have an X and Y pair and females a pair of X chromosomes. Y chromosomes are much smaller than X chromosomes, and their shapes differs as well. However, since the two makes up a pair, it can be said that they are homologous chromosomes.