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An organised sequence of bases on a chromosome that "spells out" the information necessary to construct a specific protein using the genetic code. Every cell contains thousands of genes that act as blueprints to produce proteins that are essential to the cell's function.

Genes vary in length - the smallest human gene is about 250 bases long; the longest gene is over one million bases. There are approximately 25,000 genes in the human genome.

Updated: 17 September 2009