Occasionally an error occurs during the division: for example, the egg or sperm might be missing a chromosome (22 chromosomes) or have an extra one (24 chromosomes), so at conception the baby has either too few (45) or too many (47) chromosomes. A well-known example of this type of genetic disorder is Down syndrome, where a person has 47 chromosomes rather than 46.

Over 1,000 known disorders are caused by chromosome abnormalities. A chromosome disorder means there is a change in either the structure or the number of chromosomes. This can happen in three main ways: The altered chromosome is passed from the parent to the child The abnormality happens when either the sperm or egg (germ cells) is created Soon after conception.

Around 6,000 known genetic disorders are caused by inheriting an altered gene.

the DNA in a cell is really just a pattern made up of four different parts called nucleotides.

Imagine a set of blocks that has only four shapes, or an alphabet that has only four letters. DNA is a long string of these blocks or letters. Each nucleotide consists of a sugar (deoxyribose) bound on one side to a phosphate group and bound on the other side to a nitrogenous base.

Watson and Crick discovered that DNA had two sides, or strands, and that these strands were twisted together like a twisted ladder -- the double helix.

A subunit of the DNA polymerase proofreads the new DNA

Before a cell can reproduce, it must first replicate, or make a copy of, its DNA.

DNA replication occurs in the cytoplasm of prokaryotes and in the nucleus of eukaryotes. Regardless of where DNA replication occurs, the basic process is the same.