Synonymous vs. Nonsynonymous Mutations

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A DNA molecule with a mutation. Getty/Marciej Frolow

Deoxyribonucleic acid (DNA) is the carrier of all the genetic information in a living thing. DNA is like a blueprint for what genes an individual has and the characteristics the individual shows (the genotype and phenotype, respectively). The processes by which DNA is translated using Ribonucleic acid (RNA) into a protein is called transcription and translation. In a nutshell, DNA's message is copied by messenger RNA during transcription and then that message is decoded during translation to make amino acids.

Strings of amino acids are then put together in the right order to make proteins that express the right genes.

This is a very intricate process that happens really quickly, so there are bound to be mistakes. Most of these mistakes are caught before they are made into proteins, but some slip through the cracks. Some of these mutations are actually minor and do not change anything. These DNA mutations are called synonymous mutations. Others can change the gene that is expressed and the phenotype of the individual. Mutations that do change the amino acid, and usually the protein, are called nonsynonymous mutations.

Synonymous Mutations

Synonymous mutations are point mutations, meaning they are just a miscopied DNA nucleotide that only changes one base pair in the RNA copy of the DNA. A codon in RNA is a set of three nucleotides that encode a specific amino acid. Most amino acids have several RNA codons that translate into that particular amino acid.

Most of the time, if the third nucleotide is the one with the mutation, it will result in coding for the same amino acid. This is called a synonymous mutation because, like a synonym in grammar, the mutated codon has the same meaning as the original codon and therefore does not change the amino acid.

If the amino acid does not change, then the protein is also unaffected.

Synonymous mutations do not change anything and no changes are made. That means they have no real role in the evolution of species since the gene or protein is not changed in any way. Synonymous mutations are actually fairly common, but since they have no affect, then they are not noticed.

Nonsynonymous Mutations

Nonsynonymous mutations have a much greater affect on an individual than a synonymous mutation. In a nonsynonymous mutation, there is usually an insertion or deletion of a single nucleotide in the sequence during transcription when the messenger RNA is copying the DNA. This single missing or added nucleotide causes a frame shift mutation which throws off the entire reading frame of the amino acid sequence and mixes up the codons. This usually does affect the amino acids that are coded for and change the resulting protein that is expressed. The severity of this kind of mutation depends on how early in the amino acid sequence it happens. If it happens near the beginning and the entire protein is changed, this could become a lethal mutation.

Another way a nonsynonymous mutation can occur is if the point mutation changes the single nucleotide into a codon that does not translate into the same amino acid.

A lot of times, the single amino acid change does not affect the protein very much and is still viable. However, if it happens early in the sequence and the codon is changed to translate into a stop signal, then the protein will not be made and it could cause serious consequences.

Sometimes nonsynonymous mutations are actually positive changes. Natural selection may favor this new expression of the gene and the individual may have developed a favorable adaptation from the mutation. If that mutation occurs in the gametes, this adaptation will be passed down to the next generation of offspring. Nonsynonymous mutations increase the diversity in the gene pool for natural selection to work on and drive evolution on a microevolutionary level.