Illustration of the placement of genes in a chromosome.
A gene can be defined as a region of DNA that controls a hereditary
characteristic. It usually corresponds to a sequence used in the production
of a specific protein or RNA.
A gene carries biological information in a form that must be copied and
transmitted from each cell to all its progeny. This includes the entire
functional unit: coding DNA sequences, non-coding regulatory DNA sequences,
Genes can be as short as 1000 base pairs or as long as several hundred
thousand base pairs. It can even be carried by more than one chromosome.
The estimate for the number of genes in humans has decreased as our knowledge has increased. As of 2001, humans are thought to have between 30,000 and 40,000 genes.