Biochemistry and Molecular Biology - Belyasova N.A. 2002

Molecular Foundations and Mechanisms of Heredity
Maintenance of Genome Stability and Variability
Mutational Process

If the DNA damage described in Section 2.2 is not repaired before Replication begins, it will most likely be fixed as Mutations during METABOLISM/36.html">DNA replication. Generally speaking, mutations can be defined as abrupt, heritable Changes in the genetic material of organisms. A distinction is made between spontaneous mutations (occurring without apparent external intervention), which typically have a frequency of 10^-6 to 10^-8 per Cell, and induced mutations (arising in response to a mutagenic factor). The frequency of the latter depends on the Organism's characteristics, the type and dose of the mutagen, as well as Treatment conditions, and can reach 10^-3 per cell. Mutations are considered the primary source of genetic Variability in organisms and, alongside recombination processes, serve as a powerful driving force of evolution.

To systematically describe the full diversity of mutational changes, several Classification principles are applied:

1) by The Nature of genome alteration, distinguishing between genomic mutations (changes in chromosome number), chromosomal rearrangements (structural alterations in Chromosomes leading to changes in the number and order of genes within them), and Gene Mutations (changes in The nucleotide sequence within a single gene, i.e., mutations in the strict sense);

2) by their expression in the heterozygous state, mutations are divided into dominant (expressed in heterozygotes) and recessive (expressed only in homozygotes);

3) by the direction of action, mutations are subdivided into forward (altering the original or so-called wild type) and reverse (reversions), which represent a reversion to the wild type;

4) by intracellular localization, distinguishing between nuclear mutations, which occur in the chromosomes of eukaryotes and the nucleoids of prokaryotes, and cytoplasmic mutations, affecting extranuclear heredity;

5) by phenotypic manifestation, distinguishing lethal mutations (leading to death), morphological mutations (alterations in Morphology), biochemical mutations (metabolic changes), behavioral mutations (physiological shifts), cryptic mutations (having no visible phenotypic effect), resistance or sensitivity to damaging agents, and others.

In turn, gene mutations (which are the most common) are subdivided into deletions (loss of one or more NUCLEOTIDES), transitions (substitution of Purines for purines or Pyrimidines for pyrimidines), transversions (substitution of purines for pyrimidines and vice versa), duplications (doubling of a group of nucleotides), and amplifications (multiplication of nucleotide groups). The latter two types are more characteristic of chromosomal rearrangements, in which case we refer to the doubling or multiplication of individual genes or even groups of genes. Chromosomal mutations are also characterized by Transpositions (insertion of chromosomal segments into new locations), translocations (exchange of segments between chromosomes), and inversions (alterations in the order of genes along a chromosome).

It should be noted that mutations can lead to both the loss of function and the acquisition of new traits, especially when Gene Fusion occurs and genes come under the control of regulatory elements foreign to them.