Three Factors Affecting Mutation Rate
The mutation rate is the frequency with which a gene changes from the wild type to a mutant. It is commonly expressed as the number of mutations per biological unit, which may mean per cell division, per gamete, or per round of replication.
The mutation frequency, however, is defined as the incidence of a specific type of mutation within a group of individual organisms, which is expressed as a percentage of the total population being studied.
The mutation rate can be affected by three factors in particular.
Factor 1: Frequency of Primary Changes in DNA
The mutation rate depends on the frequency of primary changes in DNA. These primary changes may arise from spontaneous molecular changes in the DNA, or be induced by chemical or physical agents in the environment.
Factor 2: Probability of Repair
The second influential factor is the probability that, when a change in DNA takes place, it will be repaired. Most cells posses a number of mechanisms to repair changes in DNA, so most alterations are repaired before they are replicated.
If these repair systems are effective, mutation rates will be low. If they are faulty, mutation rates will be increased. There are even mutation that increase the overall mutation rate for other genes. Such mutations usually occur in genes that encode components of the repair mechanisms or repair enzymes.
Factor 3: Probability of Recognition
The third factor is one that influences the ability to calculate mutation rates. It is the probability that a mutation is recognized and recorded. When DNA is sequenced, all mutations are potentially detectable. In practice, however, sequencing is still quite expensive, so most mutations are detected by their phenotypic effects. Some mutations may appear more likely to take place simply because they are easier to detect.
Even though mutation rates vary among organisms and even among genes, there are some general conclusions to be drawn as you can see:
- Spontaneous mutation rates are low for all organisms studied (1 to 100 in 10 billion cells for bacteria and viruses, and 1 to 10 per million gametes for eukaryotes).
- Within each major class of organisms, mutation rates vary considerably. This perhaps a consequence of biological differences, differences in repair mechanisms or different exposure to mutagens.
- Within a single species, mutation rates can vary per gene. Some regions of DNA seem to be more susceptible to mutations than others. The reason for this is still not entirely understood.