Replicon principle and genome composition
The genetic organization of prokaryotes is based on a fundamental principle: the presence of elements capable of absolutely independent replication. Any such genetic element possessing the ability for autonomous replication (self-duplication) is called 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 main and absolutely essential element.
- Plasmids — accessory genetic units.
All hereditary information of a microbe is encoded in DNA, specifically in the strict 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 a gene — a discrete segment of DNA with a unique sequence and number of nucleotides that encodes one specific protein.
Bacterial chromosome: morphology and topology
It is the bacterial chromosome that acts as the primary carrier of genetic data in the cell. Structurally, 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
The number of chromosomes is also an important feature. Usually, a bacterium makes do with a single chromosome. But nature has provided exceptions: microorganisms of the genus Brucella (brucellae) and the causative agent of cholera (Vibrio cholerae) are found to have two chromosomes at once.
Regardless of quantity and shape, the chromosome always carries a haploid set of genes. It encodes vital functions without which the microbial cell is simply unable to exist. Inside the cell, this DNA forms a special compact structure called the nucleoid.
Scale and genetic capacity
The mass and length of the bacterial chromosome are values that clearly demonstrate the evolutionary position of microbes. The molecular weight varies from $3 \times 10^8$ to $2.5 \times 10^9$ Daltons.
If the size of the genetic material is measured 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 interesting to consider these figures in a comparative characteristic:
- Viruses: possess a significantly smaller amount of information (about $10^3$ bp).
- Bacteria: occupy an intermediate position ($10^6$ bp).
- Eukaryotes: have huge genomes. For example, yeast contains $10^7$ bp, and humans have an impressive $3 \times 10^9$ bp.
Briefly: answers to topical questions
What is a nucleoid and how is its spatial compaction achieved?
The nucleoid is a compact structure formed by the bacterial chromosome within the cell, serving as the equivalent of the bacterial cell nucleus.
What functions do bacterial plasmids perform?
Bacterial plasmids encode additional properties that confer a selective advantage to the bacterium. They provide antibiotic resistance, control the ability to conjugate, are responsible for the production of bacteriocins that suppress the growth of competitor flora, and also determine the synthesis of pathogenicity factors such as enterotoxins and hemolysins.
What classes of plasmids are distinguished in microorganisms?
The following classes of plasmids are distinguished in microorganisms depending on their functions and properties: R-plasmids, which provide antibiotic resistance; F-plasmids, which control the ability to conjugate; Col-plasmids, responsible for the production of bacteriocins; and pathogenicity plasmids, which encode virulence factors. According to their transmission capacity, plasmids are divided into transmissible and non-transmissible.
How does the prokaryotic nucleoid differ from the eukaryotic nucleus?
The prokaryotic nucleoid and the eukaryotic nuclear apparatus have fundamental differences in the structure and organization of genetic material.
| Sign | Prokaryotes | Eukaryotes |
|---|---|---|
| Nuclear envelope | Absent | Present |
| Nucleolus | Absent | Present |
| Histone proteins | Absent | Present (form chromatin) |
| Chromosomes | 1 circular DNA | 2 or more linear ones |
| Localization | Free in the cytoplasm | Inside the nucleus |
The nucleoid also lacks compartmentalization, and the volume of genetic information is thousands of times smaller.
By what mechanism does bacterial chromosome replication occur?
Bacterial chromosome replication occurs via a semiconservative mechanism involving an enzymatic complex. The process is bidirectional and begins at a specific initiation site. Helicase unwinds the double helix, and topoisomerase prevents excessive supercoiling. DNA polymerase synthesizes new strands, with Okazaki fragments synthesized for the lagging strand.
VCC: Vibrio cholerae and Brucella are exceptions with TWO chromosomes (instead of the standard one).
Class and exam questions
What is a gene in microbiology?
This is a discrete segment of DNA with a unique nucleotide sequence that encodes a specific protein.
Do all bacteria have a circular DNA molecule?
No, the majority have a circular chromosome (like E. coli), but exceptions exist, such as Borrelia burgdorferi, which has a linear chromosome.
Are bacteria diploid organisms?
No, bacteria have a haploid set of genes located in the primary information carrier — the bacterial chromosome.
What else to study on this topic
- Mechanisms of autonomous plasmid replication
- The process of DNA compaction into the nucleoid
- Differences between transcription and translation in microbes
- Mutations in the haploid genome and their consequences
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