Structural Organization of Bacterial Genome
Prokaryotic genome is composed of circular DNA molecules. Single bacterial chromosome is called nucleoid (the details of nucleoid structure and function are given in corresponding part of General Microbiology section). Its size varies from 500 kbp up to 5000 kbp of DNA.
The possible number of chromosomes per growing cell is from one to four. Unlike eukaryotes, bacteria don’t possess mitotic spindle apparatus that provides the segregation of eukaryotic chromosomes to the different progeny cells.
Bacterial genome is haploid.
Many bacteria harbor additional genetic elements (plasmids and episomes) that are capable of autonomous replicating. All the bacterial genomic structures that contain genetic information sufficient for their own replication are termed replicons.
The replication of bacterial DNA begins at starting point (ori locus) and spreads in both directions from the initiation site. Chromosome duplication is stopped in the locus named as ter. The newly formed nucleoids are separated before cell division, and each offspring cell gains one DNA molecule.
There are two basic modes of replication for the nucleoid or plasmids in bacterial cells. One is determined as theta-type of replication of bacterial circular chromosome through the replicative intermediate resembling Greek letter θ (theta). This bidirectional replication results in formation of two identical circular replicons (e.g., nucleoids). Theta-type of replication is characteristic for standard bacterial division by binary fission providing the daughter microbial cells with two identical copies of genomes.
Another type of genomic replication in bacteria is known as rolling circle or sigma-type of replication (from Greek letter σ – sigma). This unidirectional DNA replication primarily results in creation of one copy of circular DNA and one copy of linear DNA molecule. Rolling circle mode of DNA replication is essential for bacterial conjugation since the linear DNA copy can move into the conjugation tube. It ensures the transfer of genetic information from donor to the recipient cell via their direct contact (e.g., by structures of type 4 secretion system).
The genes required for essential bacterial metabolism are usually found within the nucleoid, whereas plasmids predominantly harbor genes with certain specialized functions.