ABSTRACT
Concept of the gene By the 1960s, the gene was clearly defined as the region of DNA that gives rise to a single polypeptide (or to a single RNA for genes whose final product is RNA not protein, e.g. ribosomal RNA genes). The existence of operons in prokaryotes (Section G3) did not challenge this concept since, although several clustered genes produced a single polycistronic mRNA, one could still identify single DNA regions as genes based on the distinct polypeptides they encoded. The concept even accommodated the discovery that many protein-coding genes in eukaryotes comprise coding regions (exons) separated by noncoding sequences (introns; see Section G5) since, again, only one polypeptide was encoded by this region of DNA. More recently, however, other mechanisms have come to light in eukaryotic cells that can lead to a variety of polypeptides being produced from a single DNA sequence; for example, alternative RNA splicing, alternative polyadenylation sites and RNA editing (Section G7). Nevertheless, in each of these cases, the protein products are closely related by sequence and all are derived from the same single region of DNA. Thus the original definition perhaps needs amending to indicate that a proteincoding gene is a region of DNA that encodes a single polypeptide or a set of closely related polypeptides, but otherwise the definition is intact. The alternative scenario, to regard a single DNA sequence that gives rise to, say, 10 closely related polypeptides by posttranscriptional processing as representing 10 genes, would not be reasonable.
