All organisms have a cell or cells. Each cell contains a nucleus, and in the nucleus are strands of DNA organized into structures called chromosomes.

A cell is like a tiny box that constructs all organisms, and is the minimum unit of life. In a single cell organism, a cell has the same meaning as an individual. In a multicellular organism, multiple cells gather up to create the structures of an organism.

　

There are two kinds of cells: a somatic cell and a reproductive (sex) cell

In most of the nucleus of human cells, there are two sets of identical chromosomes. This is called a diploid cell. However, there is one type of cell that has only one set of chromosome. They are called reproductive (sex) cells, and exists only in the sperms and the eggs. All the other cells in which the chromosomes exist in pairs are called human body cells, and most cells are these.

Since reproductive cells have only one set of choromosome, whereas human body cells have two, cells are obliged to reduce to half when becoming a sex chromosome. This process is called meiosis.