Both kinds of mutation are single-letter changes in a gene’s DNA. The difference is whether the change alters the protein the gene builds. Nonsynonymous changes swap one building block of the protein for another or cut it short, while synonymous changes leave the protein’s recipe of building blocks untouched. Synonymous changes were long called “silent,” and scientists now know that label is sometimes wrong.1Penn State University. Why synonymous mutations are not always silent Comparing how often each kind piles up over evolution is also one of biology’s main ways of spotting natural selection at work.2PubMed. A beginners guide to estimating the non-synonymous to synonymous rate ratio of all protein-coding genes in a genome
How One DNA Letter Can Change a Protein or Not
Genes are read three letters at a time. Each three-letter code word, called a codon, stands for one protein building block, known as an amino acid. There are 64 possible codons but only 20 amino acids, plus the stop signals that end a protein. So most amino acids have two to six codons that mean the same thing.3PubMed. Roles for Synonymous Codon Usage in Protein Biogenesis The same protein can therefore be spelled by many different DNA sequences.4PubMed. Roles for Synonymous Codon Usage in Protein Biogenesis
That is what makes the two kinds of mutation possible. A change that turns one codon into a synonym for the same amino acid is synonymous. A change that produces a codon for a different amino acid, or for a stop signal, is nonsynonymous.
Take GAA and GAG, which both code for glutamic acid. Swapping the last letter changes nothing in the protein. But change GAG to GTG and you get valine instead, the single swap behind sickle cell disease.
The third letter is forgiving because the genetic code is arranged so that, for many amino acids, all four possible third letters give the same result. Changes at the first or second position usually swap the amino acid, though not always: leucine and arginine each have codons that differ at the first letter.
Position isn’t the only factor. Methionine and tryptophan have just one codon each, so any single-letter change to them must be nonsynonymous. Leucine, serine and arginine have six codons, which gives many ways to change a letter and keep the amino acid. The chance that a change is silent depends on which amino acid is being coded, not just on where in the codon it falls.
The Different Kinds of Nonsynonymous Mutations
A missense mutation swaps one amino acid for another, while a nonsense mutation creates an early stop signal. But because a swap still yields a complete protein, it is usually less damaging than a stop signal. A cut-short protein often lacks whole working parts, so it folds badly and clumps more.5PubMed Central. A quantitative comparison of the deleteriousness of missense and nonsense mutations using the structurally resolved human protein interactome A swap that destabilizes the protein can still make it partly or fully unfold and clump together.6PubMed Central. A quantitative comparison of the deleteriousness of missense and nonsense mutations using the structurally resolved human protein interactome
Swaps themselves vary. Those between chemically similar amino acids are called conservative, and those between very different ones are radical.7PubMed. Ratios of radical to conservative amino acid replacement are affected by mutational and compositional factors and may not be indicative of positive Darwinian selection Radical swaps occur less often over evolution than conservative ones.8bioRxiv. Physicochemical amino acid properties better describe substitution rates in large populations They are more likely to damage a protein, so natural selection weeds them out, especially in large populations.9bioRxiv. Physicochemical amino acid properties better describe substitution rates in large populations In the COVID virus’s first year, the mild swaps that barely affected its proteins were the ones that spread most widely.10PubMed Central. A global analysis of conservative and non-conservative mutations in SARS-CoV-2 detected in the first year of the COVID-19 world-wide diffusion
On a genetic report, a missense change is often written like p.Gly12Asp, naming the old amino acid and the new one. A nonsense change may appear with an asterisk or “Ter” marking the stop. Labs judge risk by where the change sits and how different the new amino acid is. A label alone doesn’t tell you a change causes disease; that depends on the gene and the evidence.
Not every protein change is bad. Many swaps land in parts of the protein that tolerate change, and some protein changes are the raw material that lets species adapt.
Location matters a great deal for stop signals. A nonsense mutation near the beginning of a gene leaves only a small fragment of the protein, which usually has none of the working parts. One very close to the end may cut off only a short tail, so the protein can sometimes still work, at least partly.
Cells also have a quality-control system that can detect a message carrying an early stop signal and break it down before many faulty proteins are made. For some nonsense mutations, then, the main effect is simply less of the protein, rather than a pile of short broken ones. Stop signals in the last stretch of a gene may escape this check.
Why Synonymous Mutations Are Not Always Silent
Before a protein is made, the gene’s message is cut and pasted to remove sections that aren’t used, a step called splicing. Short stretches inside the kept sections act as signposts that help the cell splice correctly.11PubMed. Exonic splicing enhancer A synonymous change in a signpost can make the cell skip a whole section or keep one it should remove.12PubMed. Code inside the codon: The role of synonymous mutations in regulating splicing machinery and its impact on disease The cell can end up with a protein that doesn’t work at all.13PubMed. Exonic splicing enhancer
These signposts are written in the same letters that spell amino acids, so one stretch of DNA can carry two messages at once: the protein recipe and a splicing instruction. A synonymous change keeps the recipe but can still spoil the signpost, because the signpost doesn’t care about the amino acid, only about the letters.
Speed is another way spelling matters. The cell’s protein factories read messages at uneven speeds, and those speed patterns help proteins fold properly.14PubMed. Timing during translation matters: synonymous mutations in human pathologies influence protein folding and function Common codons are read using plentiful helper molecules, while rare ones wait for scarcer helpers.15PubMed Central. Synonymous Codon Usage Controls Various Molecular Aspects Organisms tend to use some synonymous codons more often than others, and the cell makes more helpers for the common ones. A rare codon means the factory may have to wait for a matching helper, which causes a short pause.
Swapping a codon for a synonym can therefore speed up or slow down building at that spot.16PubMed. Timing during translation matters: synonymous mutations in human pathologies influence protein folding and function In bacteria, synonymous changes in one essential enzyme slowed cell growth dramatically by making the enzyme fold differently and get broken down faster.17PubMed Central. Synonymous codon substitutions perturb cotranslational protein folding in vivo and impair cell fitness In an eye lens protein, synonymous changes produced proteins with the same building blocks but different shapes and stability.18PubMed Central. SYNONYMOUS CODONS DIRECT CO-TRANSLATIONAL FOLDING TOWARDS DIFFERENT PROTEIN CONFORMATIONS Rarer codons don’t always do harm, though; in one bacterial protein they actually improved folding.19PubMed. Effects of single synonymous substitutions on fold efficiency demonstrate the influence of rare codons and protein structure
A protein starts folding as it leaves the factory, not after. Pausing at the right moment gives one part time to fold before the next arrives. Rushing past a pause can let parts fold wrongly together, and the cell’s quality control may destroy a badly folded protein even if its sequence is perfect. The message molecule also folds into shapes of its own, and in mammals natural selection appears to favor synonymous spellings that keep those shapes stable.20PubMed Central. Evidence for selection on synonymous mutations affecting stability of mRNA secondary structure in mammals
Most synonymous changes still have little or no noticeable effect. Effects are most likely near splicing signposts or at key folding pauses.
Synonymous Mutations in Cancer and Cystic Fibrosis
Synonymous mutations frequently contribute to human cancers.21PubMed. Synonymous Mutations Frequently Act as Driver Mutations in Human Cancers In cancer-promoting genes they often disrupt splicing signposts, changing how those genes are spliced in tumors.22PubMed. Synonymous Mutations Frequently Act as Driver Mutations in Human Cancers In the tumor-protecting gene TP53, recurrent synonymous changes sit next to splice sites and switch them off.23PubMed. Synonymous Mutations Frequently Act as Driver Mutations in Human Cancers TP53 normally helps stop damaged cells from dividing, so switching off its proper splicing removes a brake and lets the cell keep growing. Between one in five and one in two synonymous changes in cancer-promoting genes appear to have been favored in tumors, about 6 to 8% of all selected single-letter changes in those genes.24PubMed. Synonymous Mutations Frequently Act as Driver Mutations in Human Cancers
Tumors collect many mutations, and most are passengers that don’t help the cancer grow. Researchers look for changes that turn up in the same place again and again across many patients, because repetition suggests the tumor benefited. That is how recurrent synonymous changes next to TP53 splice sites stood out from the background of random ones.
Cystic fibrosis shows the same pattern. In one section of the cystic fibrosis gene, about a quarter of synonymous changes made the cell skip that section, leaving an inactive protein.25PubMed Central. Synonymous mutations in CFTR exon 12 affect splicing and are not neutral in evolution In lab tests of five synonymous variants predicted to affect splicing in that gene, two did alter splicing.26PubMed Central. Functional analysis of synonymous substitutions predicted to affect splicing of the CFTR gene
In one reported case, a person diagnosed late with very mild cystic fibrosis had no known disease-causing mutation, but carried a synonymous change causing low-level faulty splicing.27PubMed Central. The Effect of Synonymous Single-Nucleotide Polymorphisms on an Atypical Cystic Fibrosis Clinical Presentation Two other synonymous changes in the same person stabilized the protein and partly made up for the faulty splicing.28PubMed Central. The Effect of Synonymous Single-Nucleotide Polymorphisms on an Atypical Cystic Fibrosis Clinical Presentation
A heart condition offers a similar lesson. In one family with thickened heart muscle, a synonymous change in a heart gene caused section skipping.
If a synonymous variant appears on your report, it is most likely listed as benign or likely benign, and that usually holds. One flagged as uncertain may sit near a splice site, which is worth asking about. A genetic counselor can explain whether lab tests of splicing exist for that variant. Classifications get updated as evidence grows, so ask whether it is worth rechecking later.
How Scientists Use the Two to Detect Natural Selection
Synonymous changes build up much faster than nonsynonymous ones in almost all genes.29Nature Education. Neutral Theory: The Null Hypothesis of Molecular Evolution Most protein changes are harmful and get removed, while most synonymous changes pass unnoticed. So scientists compute the dN/dS ratio, the rate of protein-changing changes divided by the rate of synonymous ones.30PubMed. A beginners guide to estimating the non-synonymous to synonymous rate ratio of all protein-coding genes in a genome A ratio below 1 means selection is resisting change, above 1 means it is favoring change, and a ratio near 1 is consistent with no selection.31Wikipedia. Ka/Ks ratio The ratio is very sensitive, falling near zero when even modest harm is attached to protein changes.32PubMed Central. The Population Genetics of dN/dS
To tell whether selection is acting, you need a comparison for how fast changes would pile up with none. Synonymous changes come close to that no-selection rate, so they act as a ruler within the same gene. Using the same gene for both counts also helps cancel out differences in how fast mutations arise in that part of the genome.
A gene for a vital body protein might show a change-to-silent ratio like 0.1, strongly resisting change. Immune genes fighting fast-changing germs often show stretches above 1. Most genes in most species score well below 1.
A ratio above 1 is hard to find across a whole gene. Most positions in a protein are constrained, so a few positions favoring change can be hidden by many that resist it. Averaging across the gene can produce a ratio below 1 even when a few sites are under positive selection, which is why researchers often look at specific sites or short stretches.
The baseline has limits. Synonymous changes are not perfectly neutral, so their rates vary between genes and partly track DNA letter content.33PubMed. Synonymous nucleotide substitution rates in mammalian genes: implications for the molecular clock and the relationship of mammalian orders In fruit flies, weak selection on codon choice makes synonymous change rates especially uneven.34PubMed. The molecular clock revisited: the rate of synonymous vs. replacement change in Drosophila Over short time spans or in slowly mutating regions, the ratio can show extreme values by chance alone.35PubMed Central. Why Time Matters: Codon Evolution and the Temporal Dynamics of dN/dS
The steady pace of synonymous change also underlies the molecular clock idea, which lets scientists estimate how long ago species split. Uneven rates between genes and lineages make the clock less precise, so researchers correct for this by using many genes together.
Synonymous Changes in Viruses and Vaccines
Across nearly 400,000 COVID virus samples, synonymous changes that made viral codons more like human ones tended to stick over time.36PubMed Central. Large-scale analysis of SARS-CoV-2 synonymous mutations reveals the adaptation to the human codon usage during the virus evolution The virus seems to be tuning its spelling to use human cells’ helper molecules more efficiently.37PubMed Central. Large-scale analysis of SARS-CoV-2 synonymous mutations reveals the adaptation to the human codon usage during the virus evolution A virus has no protein factories of its own, so it depends on the host’s factories and helpers. If its codons match the ones human cells supply plentifully, it can be built faster, and over many generations better-matched copies can outgrow the rest. Changes defining several major variants also concentrated in parts of the genome under some constraint.38PubMed Central. Variation in synonymous evolutionary rates in the SARS-CoV-2 genome
This matters when reading variant news. Headlines focus on protein changes like those in the spike, which are nonsynonymous. Synonymous changes rarely make headlines but can quietly affect how well a virus grows.
Vaccine makers use the idea in reverse. Rewriting a virus’s genes with poorly suited synonyms weakens it without changing its proteins.39PubMed Central. CocoVax: a web server for codon-based deoptimization of viral genes in live attenuated vaccine design The immune system still sees the same proteins and can learn to fight them. This approach has produced vaccines for several diseases in people and livestock.40PubMed Central. CocoVax: a web server for codon-based deoptimization of viral genes in live attenuated vaccine design In lab work on the COVID Delta strain, breaking up favored codon pairs weakened the virus more than adding rare codons did.41PubMed Central. A Synthetic Biology Approach for Vaccine Candidate Design against Delta Strain of SARS-CoV-2 Revealed Disruption of Favored Codon Pair as a Better Strategy over Using Rare Codons
Such viruses are hard to revert. The weakening is spread across hundreds of synonymous changes, and undoing them all by chance is very unlikely. Older weakened vaccines relied on few changes that could flip back.
How Mutations Interact Within a Gene
In yeast, synonymous changes interacted with both synonymous and protein-changing changes in the same gene.42University of Michigan Deep Blue. Fitness and Epistatic Effects of Synonymous Mutations in Yeast Some of these interactions shifted depending on growing conditions.43University of Michigan Deep Blue. Fitness and Epistatic Effects of Synonymous Mutations in Yeast One transporter gene moved into yeast worked only when its protein-changing and synonymous changes were present together.44bioRxiv. Epistasis between synonymous and nonsynonymous mutations in Dictyostelium discoideum ammonium transporter amtA drives functional complementation in Saccharomyces cerevisiae In cucumbers, a single synonymous change in one gene, combined with a change in a partner gene, helped produce longer domesticated fruit.45PubMed. Recessive epistasis of a synonymous mutation confers cucumber domestication through epitranscriptomic regulation Judging each change alone can miss its real effect.
A gene works as a whole, so the effect of one change depends on the letters around it and on the rest of the protein. A synonymous change that slows building at one spot may only matter if another change alters the shape that needs that pause. Effects do not simply add up.
This makes prediction hard. Computer tools now try to predict which synonymous changes are harmful, and newer ones claim better accuracy than older ones, but none is definitive. Interactions between changes remain hard for any tool to capture, and lab evidence still carries more weight than a prediction score.