A mutagen is a substance or agent that induces heritable change in cells or organisms. Mutagenesis is the processes that result in genetic change. A mutation is any change in a cell or in an organism that is transmitted to subsequent generations. Mutations can occur spontaneously or be induced by chemical or physical agents. The cause of mutations is usually some form of damage to DNA or chromosomes. However, damage can occur in a segment.of DNA that is a noncoding region and thus will not result in a mutation. Mutations may or may not be harmful. They may occur in either somatic or germ cells. Mutations that occur in germ cells may be transmitted to subsequent generations.
There are two types of mutagens: Chemical mutagens and physical mutagens
(a) Chemical Mutagens Chemical mutagen are
I. Nitrosoguanidine (NTG)
- Base analogs (e.g. BrdU)
- Simple chemicals (e.g. acids)
- Alkylating agents (e.g. N-ethyl-N-nitrosourea (ENU): These agents can mutate both replicating and non-replicating DNA. In contrast, a base analog can only mutate the DNA when the analog is incorporated in replicating the DNA. Each of these classes of chemical mutagens has certain effects that then lead to transitions, transversions, or deletions.
- Methylating agents (e.g. ethane methyl sulfonate (EMS))
- Polycyclic hydrocarbons (e.g. benzpyrenes found in internal combustion engine exhaust)
- DNA intercalating agents (e.g. ethidium bromide)
- DNA crosslinker (e.g. platinum)
- Oxidative damage caused by oxygen radicals.
(b) Physical Mutagens
Physical mutagens arc non-ionizing and ionizing radiations:
I. Ultraviolet radiation (nonionizing radiation): These radiations excite electrons to a higher energy level. DNA absorbs ultraviolet light. Two nucleotide bases in DNA – cytosine and thymine-are most vulnerable to excitation that can change base-pairing properties. UV light can induce adjacent thymine bases in a DNA strand to pair with each other, as a bulky dimer.
2. Ionizing radiation: These radiations are X- rays. gamma rays etc. DNA has so-called hotspots. where mutations occur up to 100 times more than the normal mutation rate. A hotspot can be
at an unusual base, e.g.. 5-methylcytosine. Ionizing radiations attacks on these hot spot and break the DNA.
The process by which certain traits or gene are selected due to some environmental advantage is called selection. Selection is classified into natural and artificial selection.
In the context of evolution, certain traits or alleles of a species may be subject to selection. Under selection, individuals with advantageous or “adaptive” traits tend to be more successful. They contribute more offspring to the succeeding generation than others do. The offspring will inherit those traits from their parents. Therefore, selection can increase the prevalence of certain traits. When selection is intense and persistent, then adaptive traits become universal to the population or species.
Factors affecting selection
Selection depends on the conditions in which the individuals of a species find themselves. Adults, juveniles, embryos. and even eggs and sperm ‘nay undergo selection. Factors affecting selection include Iimits on resources (nourishment, habitat space. mates) and the existence of threats (predators. disease, adverse weather). Biologists called such factors as selective pressures.
Natural selection is the most familiar type of selection. In this case, nature selects the fittest organisms. There are different types of natural selection: sexual selection, ecological selection, stabilizing selection, disruptive selection and directional selection.
Conditions for selection
Selection occurs only when the individuals of a population are diverse in their characteristics. In the absence of individual variation, or when variations are selectively neutral, selection does not occur. Significance of selection
Selection does not guarantee that advantageous traits or alleles will become prevalent within a population. Through genetic drift, such traits may become less common or disappear. In the face of selection even a so-called deleterious allele may become universal to the members of a species. This is a risk primarily in the case of “weak” selection or small populations. Though deleterious alleles may sometimes become established, selection may act “negatively” as well as “positively.
I. Negative selection decreases the prevalence of traits that diminish individuals’ capacity to succeed reproductively (i.e. their fitness)
2. Positive selection increases the prevalence of adaptive traits. Patterns of selection Aspects of selection may be divided into effects on a phenotype and their causes. The effects are called patterns of selection. They do not necessarily result from particular causes (mechanisms). In fact each pattern can arise from a number of different mechanisms.
- Stabilizing selection favors individuals with intermediate characteristics.
- Disruptive Selection favors those with extreme characteristics.
- Directional Selection occurs when characteristics lie along a phenotypic spectrum and the individuals at one end are more successful.
- Balancing Selection is a pattern in which multiple characteristics may be favored.
Mechanisms of selection
Distinct from patterns of selection are mechanisms of selection; for example, disruptive selection often is the result of disassortative sexual selection. Similarly balancing selection may result from frequency-dependent selection and overdominance.