Replicon Principle and Genome Composition
The genetic organization of prokaryotes is based on a fundamental principle: the presence of elements that can duplicate completely independently. Any such genetic element capable of autonomous replication (self-duplication) is termed a replicon.
The genome of a microbial cell is a collection of these replicons. The genetic apparatus of a microorganism includes two fundamentally different types of elements:
- Bacterial chromosome — the primary and essential element.
- Plasmids — additional genetic units.
All hereditary information of a microbe is encoded in DNA, specifically in the precise sequence of its nucleotides. This sequence serves as a direct instruction determining the order of amino acids during protein synthesis. The unit of information is the gene — a discrete region of DNA with a unique sequence and number of nucleotides that encodes one specific protein.
Bacterial Chromosome: Morphology and Topology
The bacterial chromosome serves as the primary carrier of genetic data in the cell. Molecularly, a bacterial chromosome consists of a single double-stranded DNA molecule.
In terms of spatial form (topology), the molecule is most often closed. A classic example of a microorganism with a circular chromosome is Escherichia coli. However, exceptions exist: for example, the bacterium Borrelia burgdorferi possesses a linear chromosome.
The number of chromosomes is also an important characteristic. Typically, a bacterium has a single chromosome. But nature provides exceptions: microorganisms of the genus Brucella and the cholera pathogen (Vibrio cholerae) each possess two chromosomes.
Regardless of number and shape, the chromosome always carries a haploid set of genes. It encodes vital functions without which the microbial cell cannot survive. Inside the cell, this DNA forms a specialized compact structure called the nucleoid.
Scale and Genetic Capacity
The mass and length of the bacterial chromosome clearly illustrate the evolutionary position of microbes. Molecular weight varies from $3 \times 10^8$ to $2.5 \times 10^9$ Daltons.
When measuring the size of genetic material in base pairs (bp), the average indicator is about $3.2 \times 10^6$ bp. For comparison, in a typical representative — E. coli — the chromosome contains $5 \times 10^6$ base pairs.
It is informative to examine these figures in a comparative context:
- Viruses: Possess a significantly smaller volume of information (about $10^3$ bp).
- Bacteria: Occupy an intermediate position ($10^6$ bp).
- Eukaryotes: Have massive genomes. For example, yeast contain $10^7$ bp, and humans harbor an impressive $3 \times 10^9$ bp.