Often as a research project moves forward, a new pathway or gene will be tied into the developing story. Today it’s straightforward to look up almost any gene and find a wealth of sequencing information, including patient data if the gene is implicated in disease. Frequently there will be mutant sequences identified in the gene of interest that may or may not be relevant to other projects. When multiple mutations such as point mutations are known, they must all be evaluated to determine which ones should be prioritized for further study.
Point mutations alter a single nucleotide in the genetic code. The effects of such a change can range from completely disrupting the gene’s function to causing essentially no functional change at all. Most genes encode a protein sequence and the function of the gene is to produce the correct amount of properly-made protein. Point mutations can affect the amount of protein that is made or the ability of that protein to function properly. Thus the different types of point mutations are referred to by their effects on protein function and expression level.
At one end of the spectrum of possible effects are silent and conservative mutations. These cause no change in the function of the protein encoded by the gene, or a change that’s so minor it’s hard to measure. Silent mutations are possible because of the way genetic code is translated into protein sequence. The genetic code has redundancy built in, so sometimes changes to genes don’t result in changes to the final protein. Conservative mutations occur when the protein sequence is changed but the new amino acid is similar to the original one.
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Non-conservative mutations can be more severe because they cause a greater change to the protein sequence or expression level. An example would be changing an acidic amino acid into a basic one. However, this kind of mutation can also be mild if the protein sequence is changed in a domain that’s not crucial. Evaluating this kind of mutation requires extensive knowledge about the protein and which parts of the sequence and structure are vital.
Then there are the point mutations with the greatest effect which alter the code that marks the start or stop of the gene’s coding sequence. Mutations of this type may occur in the existing “start” or “stop” codons of the gene’s sequence or they can happen somewhere else and create a new “start” or “stop” codon in the wrong place. A final type of point mutation affects a non-coding region of a gene. These can affect the gene in unpredictable ways, for example if the mutation is in the promoter region that controls the gene’s activity. Such a mutation could alter the level of protein that is made from the gene but not change the protein sequence. This varied list of different effects from point mutations highlights why it’s so important to evaluate them carefully, especially if considering them for use in experiments.
The different effects from point mutations listed above are only an introduction to the complex ways that a single nucleotide change can alter a gene’s function. There are even more layers of complexity when looking at the effects of a mutation in the context of an animal or person because a gene may have different functions in different tissues or at different stages of life. However, a basic understanding of the different types of point mutation can provide guidance when looking at a list of different mutations and deciding which ones are most likely to be interesting for your specific research project.
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