Biology
KEY CONCEPTS:
- Mutations May Cause Loss-of-function Or Gain-of-function
KEY TERMS:A null mutation completely eliminates the function of a gene. Leaky mutations leave some residual function, for instance when the mutant protein is partially active (in the case of a missense mutation), or when read-through produces a small...
- Many Hotspots Result From Modified Bases
KEY TERMS:Modified bases are all those except the usual four from which DNA (T, C, A, G) or RNA (U, C, A, G) are synthesized; they result from postsynthetic changes in the nucleic acid. A mismatch describes a site in DNA where the pair of bases does...
- The Effects Of Mutations Can Be Reversed
KEY TERMS:Revertants are derived by reversion of a mutant cell or organism to the wild-type phenotype. Forward mutations inactivate a wild-type gene. A back mutation reverses the effect of a mutation that had inactivated a gene; thus it restores wild...
- Comparison Between Spontaneous And Induced Mutations
Spontaneous and Induced mutations: Major differences Spontaneous mutations Induced mutations 1. It is caused by natural agents, so it is also known as natural mutations or background mutations. It is caused...
- Progeria
Gene Regulation and Mutation: Vocabulary:-gene regulation: ability of an organism to control which genes are transcribed in response to the environment -operon: section of DNA thatcontains the genes for the proteins needed for a specificmetabolic pathway-mutation:...
Biology
Mutations change the sequence of DNA
KEY TERMS:
- Spontaneous mutations occur in the absence of any added reagent to increase the mutation rate, as the result of errors in replication (or other events involved in the reproduction of DNA) or by environmental damage.
- The background level of mutation describes the rate at which sequence changes accumulate in the genome of an organism. It reflects the balance between the occurrence of spontaneous mutations and their removal by repair systems, and is characteristic for any species.
- Mutagens increase the rate of mutation by inducing changes in DNA sequence, directly or indirectly.
- Induced mutations result from the action of a mutagen. The mutagen may act directly on the bases in DNA or it may act indirectly to trigger a pathway that leads to a change in DNA sequence.
- All mutations consist of changes in the sequence of DNA.
- Mutations may occur spontaneously or may be induced by mutagens.
Mutations provide decisive evidence that DNA is the genetic material. When a change in the sequence of DNA causes an alteration in the sequence of a protein, we may conclude that the DNA codes for that protein. Furthermore, a change in the phenotype of the organism may allow us to identify the function of the protein. The existence of many mutations in a gene may allow many variant forms of a protein to be compared, and a detailed analysis can be used to identify regions of the protein responsible for individual enzymatic or other functions.
All organisms suffer a certain number of mutations as the result of normal cellular operations or random interactions with the environment. These are called spontaneous mutations; the rate at which they occur is characteristic for any particular organism and is sometimes called the background level. Mutations are rare events, and of course those that damage a gene are selected against during evolution. It is therefore difficult to obtain large numbers of spontaneous mutants to study from natural populations.
The occurrence of mutations can be increased by treatment with certain compounds. These are called mutagens, and the changes they cause are referred to as induced mutations. Most mutagens act directly by virtue of an ability either to modify a particular base of DNA or to become incorporated into the nucleic acid. The effectiveness of a mutagen is judged by how much it increases the rate of mutation above background. By using mutagens, it becomes possible to induce many changes in any gene (for review see Drake and Balz, 1976).
Spontaneous mutations that inactivate gene function occur in bacteriophages and bacteria at a relatively constant rate of 3-4 × 10?3 per genome per generation (Drake, 1991). Given the large variation in genome sizes between bacteriophages and bacteria, this corresponds to wide differences in the mutation rate per base pair. This suggests that the overall rate of mutation has been subject to selective forces that have balanced the deleterious effects of most mutations against the advantageous effects of some mutations. This conclusion is strengthened by the observation that an archaeal microbe that lives under harsh conditions of high temperature and acidity (which are expected to damage DNA) does not show an elevated mutation rate, but in fact has an overall mutation rate just below the average range (Grogan, Carver, and Drake, 2001).
Figure 1.19 shows that in bacteria, the mutation rate corresponds to ~10?6 events per locus per generation or to an average rate of change per base pair of 10?9-10?10 per generation. The rate at individual base pairs varies very widely, over a 10,000 fold range. We have no accurate measurement of the rate of mutation in eukaryotes, although usually it is thought to be somewhat similar to that of bacteria on a per-locus per-generation basis (Drake et al., 1998).
- Mutations May Cause Loss-of-function Or Gain-of-function
KEY TERMS:A null mutation completely eliminates the function of a gene. Leaky mutations leave some residual function, for instance when the mutant protein is partially active (in the case of a missense mutation), or when read-through produces a small...
- Many Hotspots Result From Modified Bases
KEY TERMS:Modified bases are all those except the usual four from which DNA (T, C, A, G) or RNA (U, C, A, G) are synthesized; they result from postsynthetic changes in the nucleic acid. A mismatch describes a site in DNA where the pair of bases does...
- The Effects Of Mutations Can Be Reversed
KEY TERMS:Revertants are derived by reversion of a mutant cell or organism to the wild-type phenotype. Forward mutations inactivate a wild-type gene. A back mutation reverses the effect of a mutation that had inactivated a gene; thus it restores wild...
- Comparison Between Spontaneous And Induced Mutations
Spontaneous and Induced mutations: Major differences Spontaneous mutations Induced mutations 1. It is caused by natural agents, so it is also known as natural mutations or background mutations. It is caused...
- Progeria
Gene Regulation and Mutation: Vocabulary:-gene regulation: ability of an organism to control which genes are transcribed in response to the environment -operon: section of DNA thatcontains the genes for the proteins needed for a specificmetabolic pathway-mutation:...