iGen3 05-03

What is a "Gene" ?

    A "gene" occurs over a particular physical region (locus) of a double-stranded dsDNA molecule. It includes an RNA-coding region, where transcription occurs from the DNA template strand so as to synthesize an RNA transcript in the 5'3' direction, running between the initiation and termination sites.

    In scientific papers and databases, a gene sequence is typically shown as that of the single-stranded sense (non-template, partner) strand, in the 5'3' direction, between the first base (+1) corresponding to the beginning of the start codon and the last base corresponding to the end of the stop codon. Control regions for transcription (promoters and terminators) occur immediately outside the coding sequence. By convention, the gene is written "left to right" in the 5'3' direction of the sense strand: the region to the 'left' of the gene is called 'upstream' and the region to the 'right' is 'downstream'.

    Note again that "left / right' and "top / bottom" are arbitrary, whereas 5'3' is a physical property of the molecule. The coding region of eukaryotic genes typically consists of a series of expressed exons with intervening introns. Other promoters and control elements may occur well outside the region shown here, both upstream and downstream.

Figure © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2012 by Steven M. Carr