Causes and Mechanisms of Occurrence
Mutations occur when alterations in the primary structure of DNA go unnoticed and are uncorrected by cellular repair systems. Key causes include errors made by DNA polymerases during replication, failures in DNA repair systems, and exposure to environmental stressors (such as radiation) and internal chemical mutagens.
Chemical mutagens act through various mechanisms:
- Alkylating agents. They attach an alkyl group to the N7 position of the guanine purine ring. This alters its ionization properties: during subsequent DNA replication, thymine is incorporated opposite this modified guanine (instead of the normal cytosine). Consequently, in the next cell generation, the G—C base pair is permanently replaced by an A—T pair.
- Intercalating agents. These are substances that physically insert (intercalate) between the nitrogenous bases of the DNA double helix, disrupting its geometry and preventing normal replication.
Classification of Point Mutations (Substitutions)
The most studied and widespread type of damage involves point mutations. The most frequent variant is the substitution of a single nucleotide for another. Depending on how this change affects the encoded protein, point mutations are divided into three types.
| Type of Substitution | Change in mRNA | Effect on Protein Structure |
|---|---|---|
| Silent | Codon meaning is preserved (synonymous substitution) | None. The protein remains normal due to the degeneracy of the genetic code. |
| Missense | Change in codon meaning | One amino acid is replaced by another, which may impair protein function. |
| Nonsense | Formation of a premature stop codon | Synthesis is terminated, producing a functionally inactive, truncated molecule. |
Reading Frame Shift
In addition to substitutions, point defects include insertions and deletions. The scale of their consequences depends on the number of nucleotides affected:
- Without a reading frame shift. This occurs when the number of added or lost nucleotides is a multiple of three (e.g., a whole triplet). The meaning of the remaining codons is unchanged. The protein will simply be lengthened or shortened by the corresponding number of amino acids while preserving its core structure.
- With a reading frame shift (frameshift mutation). This occurs when the number of deleted or inserted nucleotides is not a multiple of three. This is a catastrophic change: all downstream genetic information is misread by the ribosome. As a result, a peptide with a random amino acid sequence is synthesized. Often, such a shift leads to the accidental formation of a stop codon shortly after the mutation site, preventing the formation of a full-length protein.
Impact on the Organism and Evolution
Mutations can affect not only coding regions but also regulatory elements. For example, defects in promoter CpG islands lead to decreased or complete absence of gene expression, even if the coding sequence remains intact.
Clinical consequences heavily depend on the cell type:
- Mutations in germ cells are heritable and cause hereditary diseases in offspring by altering protein structure and function.
- Mutations in somatic cells are not inherited, but they cause functional disorders (metabolic, morphological) in the affected organism itself.
At the same time, genetic variability is the driving force of evolution (phylogenesis). Gene duplication, independent mutations in duplicated copies (divergence), and chromosomal recombination during meiosis (crossing over) lead to the emergence of new traits and related protein families.