GENERAL MICROBIOLOGY - T.P. Pirog - 2004

19. BACTERIAL GENETICS: STABILITY, VARIATION, AND HEREDITY OF TRAITS

19.1. PROTEIN SYNTHESIS AND THE GENETIC CODE

19.1.1. Definition of the Gene

It is well known that Proteins are the foundation of life. Studies, initially on insects and later on microorganisms, demonstrated that the manifestation of specific traits depends on enzyme activity. In microorganisms, Enzymes could be linked to specific traits subject to precise biochemical identification. Consequently, as early as the 1930s, the "one Gene — one protein (enzyme)" rule was formulated. This rule implies that a single gene contains the information required to synthesize a particular enzyme. However, when it became clear that genes are composed of DNA, a new definition emerged: a gene is a linear segment of DNA that encodes a single protein (enzyme). It was subsequently discovered that many Proteins are built from multiple polypeptide chains (subunits). This necessitated another modification to the definition of a gene: a gene is a linear segment of DNA that encodes a single polypeptide chain of a protein. Nevertheless, about 20–25 years ago, it became evident that this definition

also needed revision. It was established that the coding sequences of Many eukaryotic genes contain insertions that bear no relation to the protein in question. In other words, a gene consists of both coding and non-coding regions. The coding regions of a gene are called exons, and the non-coding insertions are called introns. The size of introns can range from a few to several thousand nucleotide residues. This led to a revised Definition of the gene: a gene is the set of DNA segments that encode a single polypeptide chain of a protein. Yet, even this is insufficient, as there are genes for transfer and Ribosomal RNAs that do not encode proteins. Accordingly, the following definition was proposed: a gene is a nucleotide sequence that performs a specific function. Today, it is generally accepted that every structural gene encodes a specific polypeptide chain.