Understanding Genetic Mutations: Types and Causes
Mutations
Mutation refers to the sudden changes that appear in individuals of a plant species. Derived from an alteration that occurs in the sequence of nitrogenous bases of DNA, a sequence change in the amino acid is the corresponding protein. The protein encoded by that DNA can change its biological function or act inappropriately. We are now fully aware of the role that mutations play in numerous biological processes, from species evolution to the development of cancer.
Mutations can be classified according to several criteria:
- Affected cells:
- Somatic cells (which are not transmitted to offspring)
- Germ cells (passed to offspring)
- Causes:
- Spontaneous (natural causes)
- Induced (introduced by mutagens)
- Effects:
- Neutral
- Beneficial
- Harmful
- Lethal (death occurring in at least 90% of individuals)
- Sublethal (death in less than 10% of the affected individuals)
- Pathological (produces disease)
- Gene expression:
- Dominant (compared to the normal non-mutated allele)
- Recessive (normal allele compared to non-mutated)
- Genetic alteration:
- Gene-induced (affecting the nucleotide sequence of a gene)
- Chromosomal (alters the structure of chromosomes)
- Genomic (changes the number of chromosomes)
Gene Mutations
Mutations in the strict sense consist of chemical changes in DNA, so they cannot be observed under a microscope. Gene mutations occur mostly for two reasons: uncorrected errors produced during DNA replication, and the action of certain physical or chemical agents, such as radiation or certain substances from outside the cell or from the cell’s own metabolism, that alter DNA. There are several mechanisms to repair DNA alterations caused by mutations, for example, enzymes responsible for correcting errors that occur in the DNA replicative process, either during the incorporation of nucleotides or after completion of the synthesis of a new strand.
- Mutations by substitution of one base for a different one:
- Transitions: when a purine base is replaced by another purine base, or a pyrimidine base is substituted by another pyrimidine base.
- Transversions: change of a purine base by a pyrimidine base or vice versa.
- Mutations by loss and insertion of bases: These are more serious than the previous ones. From the point of deletion or addition, all base triplets will be changed and, therefore, the encoded message will be completely different.
- Mutations by changing the location of some DNA segments (transpositions): The displacement of nucleotide sequences of the chain causes the appearance of new triplets, thus modifying the genetic message.
Chromosomal Mutations
Chromosomal mutation affects the structure of chromosomes; it is possible to detect it under the microscope. There are changes in the number of genes or in the linear arrangement of chromosomes.
- Altered by the existence of an incorrect number of genes: Occurs by a failure in meiotic pairing that can produce a wrong crossover, leaving a fragment with an extra chromosome and the other with a deficit. After fertilization, several anomalies originate from gametes, including:
- Deficiencies and deletions: Loss of a fragment of chromosome genes.
- Overlaps: A segment of a chromosome is repeated, so there is an excess of the corresponding genes.
- Alterations in the order of genes: Causing abnormal gametes that give rise to offspring with a deficit or excess of genes. There are two types:
- Inversions: The arrangement of the genes of a chromosome segment is reversed.
- Translocations: A segment of a chromosome changes position, moving to another part of the chromosome, to its counterpart, or to any other.
Genome Mutations
Genome mutations, or numerical alterations, consist of a change in the number of chromosomes of a species, either higher or lower, and always produce severe alterations. There are two types of numerical mutations: euploidies and aneuploidies.
- Euploidies: Change in the number of chromosomal sets. A chromosome set is the combination of one chromosome of each type.
- Monoploidies: There is only one complete chromosome set, i.e., n chromosomes.
- Polyploidy: The anomaly is the existence of more than two chromosome sets (3n, 4n, 6n). These changes cause an increase in cell size and increased body size; it often happens in plants.
- Aneuploidies:
- Nullisomies: (2n – 2)
- Monosomies: (2n – 1) Lack of a specific chromosome pair. Lethal effects.
- Trisomy: (2n + 1) Morphological changes.
- Tetrasomy: (2n + 2)
Aneuploidies are produced by the fusion of a normal gamete (with n chromosomes) with one that has (n – 1), (n + 1), or (n + 2) chromosomes.
Physical Mutagenic Agents
Physical mutagenic agents are radiation, which are divided into ionizing and non-ionizing.
- Ionizing radiation: They have a very short wavelength and are very energetic. Among these are radiation and X-rays, and their effects are:
- Physiological
- Cytogenetic (in the structure of chromosomes)
- Genetic (cause new ionizations and the emergence of highly reactive free radicals, chemical changes in DNA resulting in gene mutations)
- Non-ionizing radiation: These are ultraviolet (UV) radiation.