Genetic Polarity in Bacteria

Abstract

The major genetic apparatus of the enteric bacteria, Escherichia coli and Salmonella typhimurium, appears at first glance to be poor material for creating phenomena which would reflect polarity. Genetically it consists of a single linkage group or chromosome forming a circle (Jacob and Wollman, 1958; Taylor and Thoman, 1964; Sanderson and Demerec, 1965). Physically it consists of DNA, a Watson-Crick (Watson and Crick, 1953) double helix (see Cavalieri and Rosenberg, 1963; Thomas, 1963; Baldwin, 1964) formed into a circle with no free ends (Cairns, 1963). The two strands of the double helix have opposite chemical polarities, and the resulting symmetry eliminates an important possible structural basis for polarity. Most polarity phenomena are those which result from the sequential nature of the functioning of the genetic material. Functioning can be considered at two levels: the sequential replication of the chromosome and the expression (transcription and translation) of the genes. This discussion will be limited to the latter. Observed polarity effects upon gene function can generally be ascribed to the fact that the genetic information consists of a linear sequence of nucleotides and that the mechanisms which transcribe the information into messenger RNA and translate the messenger RNA into the final chain of amino acids do so in a sequential manner. As a consequence, there is a beginning and an endpoint to the linear sequence of purine and pyrimidine bases which make up a gene. This polarity is reinforced at the level of translation since the messenger RNA, being a single strand, possesses a chemical polarity.