Phenotypic Manifestations of Mutations
Mutational changes in the bacterial genome affect external properties and vitality. The main phenotypic consequences include:
- Changes in cell shape and structure (morphology).
- Auxotrophy — the emergence of a requirement for additional growth factors (e.g., vitamins or amino acids) that the cell previously synthesized adequately.
- Development of resistance to antibacterial drugs.
- Altered response to temperature conditions.
- Attenuation — weakening of the pathogenic properties (virulence) of the bacterium.
Classification of Mutations
Depending on the scale of DNA damage, mutations are divided into two main groups:
- Point mutations — affect only a single nucleotide pair.
- Large-scale mutations (aberrations) — cover significant regions of the chromosome. These include:
- Deletion (loss of a nucleotide fragment);
- Duplication (doubling of a region);
- Translocation (movement of a chromosomal fragment to another location);
- Inversion (rotation of a DNA segment by 180 degrees).
According to origin, mutations can be spontaneous (occurring naturally due to replication errors or the activity of mobile elements) and induced (caused by mutagens).
Types of Mutagens and Mechanisms of Action
Mutagenic factors are divided into three groups: physical, chemical, and biological.
Physical mutagens (UV rays, gamma rays). Ultraviolet light affects pyrimidine bases, leading to the formation of thymine dimers — covalent cross-links between adjacent thymine residues. This disrupts chain structure and blocks normal replication.
Chemical mutagens act via various pathways:
- Base analogs (5-bromouracil) incorporate into DNA, alter structure, and provoke transition — the replacement of a purine with a purine or a pyrimidine with a pyrimidine.
- Deaminating agents (nitrous acid) remove amino groups from bases. For example, adenine is converted to hypoxanthine, which pairs with cytosine, also leading to a transition.
- Intercalating agents (acridine dyes) wedge between bases, stretching the chain. During replication, this causes a frameshift, making the correct reading of all downstream genetic information impossible.
Biological mutagens are represented by mobile genetic elements, such as transposons.
Forward Mutations and Reversions
A mutation that causes a bacterium to lose a specific function is called a forward mutation. However, bacteria can regain lost traits — a process known as reversion.
Reversions are of two types:
- True (exact) reversion — the original genotype is fully restored, along with the phenotype.
- Suppressor mutation — the primary mutation remains in the genotype, but the phenotype is restored due to a secondary compensatory mutation (which may occur in the same gene, in other genes, or affect tRNA).