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Why the Mutation’s Location Matters
For point mutations, it is very important where the mutation happens in the DNA. A mutation can occur inside a gene or between genes. Mutations that happen outside genes often have little or no effect, because large parts of the genome do not directly code for proteins and contain repeated or regulatory sequences (Alberts et al., 2015).
When a mutation occurs inside a protein-coding gene, its effect depends on whether it changes the protein. If the DNA change does not alter the amino acid sequence of the protein, it is called a synonymous mutation. This is possible because the genetic code is redundant, meaning that different DNA triplets (codons) can code for the same amino acid (Griffiths et al., 2019).
For example, the codons TCA and TCG both code for the amino acid serine. If a mutation changes the last letter from A to G, the protein remains exactly the same. As a result, the mutation usually has no effect on how the protein works.
Many synonymous mutations occur in the third position of a codon, also known as the wobble position. Changes in this position often do not affect the protein structure or function (Strachan & Read, 2018).
There are also neutral mutations, where one amino acid is replaced by another with similar chemical properties, such as charge or size. Because the new amino acid behaves in a similar way, the protein can often still function normally. These small changes usually do not harm the organism and help explain why genetic variation can exist without causing disease (National Human Genome Research Institute, 2023).