A gene is a basic unit of heredity in biology, as well as a sequence of nucleotides in DNA that encodes the synthesis of a gene product, either RNA or protein.
During gene expression, DNA is transcribed into RNA first. Genes in humans range in size from a few hundred DNA bases to over 2 million bases.
According to the HGP (Human Genome Project), which attempted to discover the sequence of the human genome and identify the genes it contains, humans have between 20,000 and 25,000 genes.
The dystrophin gene is the biggest known gene in humans.