Genetic Alterations: A Comprehensive Look at DNA Changes
Gene Mutations: Types and Impacts
Gene mutations alter the nucleotide sequence of a single gene.
Base Substitutions
Base substitutions account for approximately 20% of all mutations. They involve the replacement of one DNA base with another, affecting only a single nucleotide.
Due to the degenerate nature of the genetic code, a mutated triplet may still encode the same amino acid; this is known as a silent mutation. If the new triplet codes for a different amino acid, it can have serious consequences, especially if it affects the active site of a protein. Otherwise, it may go unnoticed. A base substitution can also lead to a termination triplet, resulting in a shorter, often non-functional, protein.
These mutations primarily occur due to uncorrected errors during DNA replication or exposure to physicochemical agents.
There are two main types of base substitutions:
- Transitions: Substitution of one purine base (Adenine or Guanine) by another purine, or a pyrimidine base (Cytosine or Thymine) by another pyrimidine.
- Transversions: Substitution of a purine by a pyrimidine, or vice versa.
Frameshift Mutations: Insertions and Deletions
Frameshift mutations involve the addition (insertion) or loss (deletion) of one or more nucleotides in the DNA molecule. These changes alter the reading frame of the gene.
From the point of mutation onwards, all subsequent triplets are altered, leading to the production of completely different and often non-functional proteins when the gene is translated.
Genomic Mutations: Chromosome Number Variations
Genomic mutations are variations in the normal number of chromosomes within a species. They are typically produced by the abnormal separation of homologous chromosomes or sister chromatids during meiosis.
Euploidy: Changes in Chromosome Sets
Euploidy refers to a change in the number of complete sets of chromosomes (n).
Monoploidy
Monoploidy describes organisms with a single set of chromosomes (n).
Polyploidy
Polyploidy occurs when an organism has more than two complete sets of chromosomes (e.g., 3n, 4n, 6n).
Polyploid organisms often have larger cells, which can lead to an increased body size. This characteristic is frequently exploited in crop plants, such as bananas (3n), potatoes (4n), and wheat (6n). However, polyploidy is generally impractical or lethal in animals.
Aneuploidy: Abnormal Chromosome Numbers
Aneuploidy is the presence of an abnormal number of chromosomes, either more or fewer than the normal diploid count.
Down Syndrome (Trisomy 21)
A classic example of aneuploidy is Down syndrome, which is caused by trisomy 21 (the presence of an extra copy of chromosome 21).
Chromosomal Mutations: Structural Changes
Chromosomal mutations cause changes in the structure of chromosomes. These alterations can affect either the order of genes or their number.
Changes in Gene Number (Duplications & Deletions)
These mutations are often produced by a failure of crossing-over during meiosis, resulting in one chromosome fragment with an extra set of genes (duplication) and another with a deficit (deletions). Gametes originating from such abnormalities can lead to genetic disorders.
Alterations in Gene Order (Inversions & Translocations)
Alterations in the order of genes, such as inversions or translocations, typically do not cause immediate alterations in the individual carrying them. However, they may produce gametes with a deficit or excess of genes, which can lead to issues in offspring.
Mutagenic Agents: Causes of Genetic Change
Physical Mutagenic Agents
- Ionizing Radiation: This includes very short-wavelength electromagnetic radiation such as X-rays, Gamma rays, and particulate radiation like alpha particles. Ionizing radiation can cause significant chromosomal mutations (e.g., deletions and translocations) and directly modify DNA bases.
- Non-ionizing Radiation: This refers to longer-wavelength electromagnetic radiation, primarily Ultraviolet (UV) light. UV radiation causes the formation of covalent bonds between adjacent pyrimidine bases (e.g., thymine dimers), leading to DNA damage.
Chemical Mutagenic Agents
Various chemical compounds can act as mutagens, including: Polycyclic hydrocarbons, aromatic amines, alkylating agents, certain dyes, and pesticides.
The effects of chemical mutagens tend to be slower and can accumulate over time.
Biological Mutagenic Agents
Some biological agents are known to increase the frequency of mutations:
- Certain Viruses: Viruses such as retroviruses, adenoviruses, and human hepatitis B virus can integrate their genetic material into the host genome, potentially causing mutations.
- Transposons: Also known as “jumping genes,” these are mobile segments of DNA that can change their position within the genome, leading to insertions, deletions, or inversions.
It is believed that some viruses can incorporate transposons and facilitate their insertion into other cells, further contributing to genetic variability.