GENETICS - Textbook - A.V. Sivolob - 2008

CHAPTER 5. Genetics of Bacteria, Viruses, and Unicellular Eukaryotes

ORGANIZATION OF THE PROKARYOTIC GENETIC APPARATUS

Typically, the bacterial genome is represented by a single circular DNA molecule (the bacterial chromosome). This DNA interacts with Proteins to form a nucleoid, which occupies a specific region within the bacterial Cell. The total length of the chromosome in Escherichia coli, for instance, is 4.6 million Base Pairs, with about 90% corresponding to coding sequences (see Table 1.1). The average size of an E. coli Gene is 950 base pairs (prokaryotic genes lack introns), and the average intergenic distance is 118 base pairs. However, intergenic regions are quite unevenly distributed in length, varying from 0 to 1,730 base pairs.

The bacterial genome also contains mobile elements belonging to the Class of DNA Transposons, which are entirely analogous to transposons in Eukaryotic Genomes (see Chapter 6). Bacterial transposons are divided into two types that differ somewhat in sequence Structure and length: insertion sequences (IS elements), ranging from 700 to 1,400 base pairs in size, and larger canonical transposons (Tn elements). Transposons of both types frequently carry transposase genes. Transposase catalyzes the Chemical Reactions that drive the transposition of the element to another site within The Genome (see Chapter 6). In addition to transposase genes, Tn transposons may harbor other genes as well.

Replication of the bacterial chromosome initiates at the origin region (typically a single origin, see Chapter 1) and proceeds bilaterally via two replication forks (Fig. 5.1). The origin is believed to interact with The Cell membrane during this process; new Cell wall material is synthesized between the newly synthesized and old origins during replication, and upon completion of replication, Cell Division occurs, distributing a complete set of parental genes to each daughter cell.

Fig. 5.1. Synchronization of replication and cell division in a bacterial cell

A characteristic feature of Bacteria is the presence of small autonomous genetic elements known as Plasmids, which exist within the cell alongside the main bacterial chromosome. A plasmid is a circular DNA molecule (typically around 3 kb in size) that carries several genes and replicates independently of the bacterial chromosome, possessing its own origin of replication. Plasmids are widely used as convenient tools in molecular biology research (see Chapter 9).