Understanding Mutation and Mutagenic Agents
Concept of Mutation
Mutation refers to any modifications or changes occurring in the genetic material that can lead to new manifestations of characteristics in organisms. The individual or cell carrying the mutation is called a mutant.
Key Features of Mutations:
- They are the source of new genetic characters and increase the genetic variability of organisms.
- They are heritable, persisting in the offspring when they occur in reproductive cells (germline mutations), transmitting the positive or negative effects of the mutation to descendants. If the mutation occurs in somatic cells, it only affects the daughter cells (somatic mutations).
- They occur naturally in the genome of organisms, although only some manifest in the phenotype, while the rest remain hidden or inactive in the genetic material. They can also be induced by mutagens.
- Mutation is a random phenomenon occurring with a characteristic frequency and can be used to measure the evolutionary distance between species by comparing existing mutations or changes in certain molecules.
- According to their effects, mutations can be:
- Silent: Undetectable because they cause no changes in the characteristics.
- Neutral: When a change in an amino acid in a protein does not alter its activity because the new amino acid has similar properties.
- Harmful, lethal, or deleterious: The most common type of mutations, with negative effects causing diseases, malformations, and, in some cases, the death of the organism.
- Beneficial: Have positive effects on certain characteristics, giving an advantage to the organism.
Mutagenic Agents
Mutagenic agents are those capable of producing a mutation or change in DNA sequences, i.e., in the genes. Both techniques, social habits, and food have increased exposure to mutagens. Some notable examples include:
- Thermal fluctuations (Physical – F): Human cells, simply by being at 37°C, spontaneously lose about 5000 purine bases daily and suffer deaminations that provoke the transformation of cytosine to uracil and adenine to hypoxanthine.
- Ultraviolet radiation from the sun (F): Induces the formation of a covalent bond between two consecutive pyrimidine bases on the same strand, leading to thymine or cytosine dimers. This results in the breakage of hydrogen bonds with complementary bases, and the DNA double helix is disrupted around the dimers.
- Ionizing radiation (F): These are electromagnetic radiations with very short wavelengths and high energy, like X-rays, gamma rays, and neutron and proton flows originating from nuclear reactors. When they collide with atoms and molecules in their path, they transform them into highly reactive ions and radicals, attacking and breaking cellular molecules like DNA.
- Particle radiation (F): These particles are emitted in the decay process of radioactive isotopes, and their effects on DNA are similar to ionizing radiation, causing the rupture of DNA strands.
- Reactive metabolites (Chemical – Q): Some metabolic waste products, particularly free radicals derived from O2, are highly reactive and capable of inducing damage to DNA. Mitochondrial DNA is especially vulnerable to these attacks.
- Chemical substances (Q): A large number of substances that we consume or inhale are potential mutagens.
Types of Mutations
Gene Mutations (Molecular or Point Mutations)
These are mutations that alter nucleotide sequences, sometimes affecting only a single nucleotide. The changes involve alterations in the sequence of bases and can cause two types of changes according to the mechanism of mutation:
- By substitution or change of bases: The sequence is altered, changing the DNA code, the mRNA, and the resulting protein.
- By shifting the reading frame: When a base is inserted (an extra nucleotide) or lost, the genetic message is altered, leading to a dramatically different protein.