What are chromosomes?

Chromosomes are small structures inside cells which package long strings of DNA (and genes). Chromosomes can only be seen under a microscope when a cell is dividing.

In normal human cells there are 46 chromosomes. There is one pair of sex chromosomes (two X chromosomes for females and one X and one Y chromosome for males) and 22 pairs of autosomes (non-sex chromosomes) numbered 1 through 22.

When a cell divides the DNA is copied and 1 copy of each chromosome is distributed into each new cell. The new cells are an identical copy of the previous cell. This biological process is called mitotis. When producing sperm and eggs, the cells divide so that there are only 23 chromosomes in each new cell - one of each pair (a process called meiosis).

At fertilization, the chromosomes from the sperm and egg come together to give a new full complement of 46 chromosomes in the embryo. Usually the DNA is copied exactly and the chromosomes are distributed perfectly, but sometimes errors are made by the cell, see Genetic mistakes.