Gene

A gene is the unit of heredity in living organisms. Genes are encoded in an organism's genome, composed of DNA or RNA, and direct the physical growth and behavior of the organism. Most genes encode proteins, which are biological macromolecules comprising linear chains of amino acids that affect most of the chemical reactions carried out by the cell. Some genes do not encode proteins, but produce non-coding RNA molecules that play key roles in protein biosynthesis and gene regulation. Molecules that result from gene expression, whether RNA or protein, are collectively known as gene products.
Most genes have non-coding regions that do not code for the gene products, but often regulate gene expression. A critical non-coding region is the promoter, a short DNA sequence that is required for initiation of gene expression. The genes of eukaryotic organisms often contain non-coding regions called introns which are removed from the messenger RNA in a process known as splicing. The regions that actually encode the gene product, which can be much smaller than the introns, are known as exons.