Basic Neurosciences · Paper A
Mutation types
Exam mutation questions really test whether you can predict damage from a genetic change, because the SIZE and TYPE of the alteration matters far more than the fact a mutation occurred at all. Say you swap one letter in a sentence versus deleting one letter partway through: the swap might stay readable, but the deletion shifts every letter after it into nonsense. That single-letter-swap-versus-deletion contrast is exactly what separates a point mutation from a frameshift, and it predicts how severe the resulting disease will be.
A point substitution changes only one base. It may be SILENT (a different codon but the same amino acid, so no effect), MISSENSE (a different amino acid, which may or may not disrupt protein function) or NONSENSE (creates a premature stop codon, usually truncating and disabling the protein). Take a nonsense mutation sitting near the start of a gene: it truncates almost the whole protein and usually destroys its function completely. An insertion or deletion not divisible by three causes a FRAMESHIFT, which alters every downstream codon and often introduces an early stop. An insertion or deletion that IS a multiple of three, by contrast, stays in-frame and simply adds or removes whole amino acids, so it is usually far less damaging than a frameshift. E.g., deleting three consecutive bases removes one whole amino acid but keeps the reading frame intact, unlike a single-base deletion.
A vignette describes a specific genetic change: a single base swap, a small insertion or deletion, or a repeat sequence that expands and worsens across generations; and asks you to predict its effect. Sort it by how many bases changed and whether the reading frame shifted; an expanding repeat with earlier onset each generation is the anticipation clue.
Every exam question on mutations is really asking one thing: given a change in DNA, how much damage does it do? A mutation (any change in the sequence of bases that make up DNA) can be harmless or devastating, and the ONLY way to tell which is to look at its size and its position, not just the fact that a change happened. That is the whole logic behind the whole topic, so hold it in mind through every section below.
DNA is a long string of four letters: A, T, C and G (the four bases adenine, thymine, cytosine and guanine). Groups of three letters, called codons, each spell out one amino acid (a building block of protein), and proteins are built by reading codons in order, like reading a sentence three letters at a time. A mutation is simply a change to one or more of those letters. What matters for the exam is not that a change happened, but exactly how many letters changed and whether it knocked the three-letter reading frame out of step.
Most candidates assume a mutation always changes the protein, or assume a bigger mutation is always worse. Neither is reliably true. A single-base change can be completely silent (no effect at all), while a change of just one or two bases in the wrong place can wreck the entire protein. Size alone does not predict severity: POSITION and whether the reading frame shifts predict severity far better.
Imagine a sentence written in fixed three-letter words with no spaces: THE CAT ATE THE RAT becomes THECATATETHERAT. Reading three letters at a time gives THE-CAT-ATE-THE-RAT. Now change one letter without adding or removing any: THE COT ATE THE RAT. Only one word changed meaning. The rest of the sentence still reads fine. That is what a point mutation, a change to a single base, does: it can alter one codon and leave everything downstream untouched. Now instead DELETE one letter: THECATATETHERAT becomes THCATATETHERAT. Every three-letter group after the deletion is now wrong: THC-ATA-TET-HER-AT: pure nonsense from that point on. That is a frameshift: an insertion or deletion of bases that is NOT a multiple of three, which shifts every downstream codon and usually destroys the protein.
Silent mutation: a base changes, but because of redundancy in the genetic code (several different codons can spell the same amino acid) the resulting amino acid stays identical. No effect on the protein at all.
Missense mutation: a base change swaps in a DIFFERENT amino acid at that position. The effect ranges from nothing (if the new amino acid behaves similarly) to complete loss of function (if it sits in a critical part of the protein, for example the active site of an enzyme).
Nonsense mutation: a base change creates a premature STOP codon: a three-letter signal that tells the cell to stop building the protein right there. The result is a truncated (cut-short) protein, usually non-functional, and the earlier in the gene this happens, the more of the protein is lost.
Frameshift mutation: an insertion or deletion of bases NOT divisible by three shifts every codon downstream of the change, scrambling the amino-acid sequence from that point on and usually introducing an early, accidental stop codon. An insertion or deletion that IS a multiple of three simply adds or removes whole amino acids in-frame: far less damaging, because everything else downstream reads correctly.
Trinucleotide-repeat expansion: a special category where a three-base sequence (such as CAG) is repeated many times in a row, and the number of repeats can grow across generations. This causes anticipation, meaning the disease appears earlier and more severely in each successive generation. Huntington disease (a CAG-repeat expansion) and Fragile X syndrome (a CGG-repeat expansion) are the two classic examples the exam tests.
The exam loves to test the boundary between missense and nonsense: both are single-base substitutions, but a missense swaps ONE amino acid while a nonsense STOPS the protein early: the nonsense mutation is generally far more damaging. It also loves the boundary between an in-frame and an out-of-frame insertion/deletion: three bases lost is one missing amino acid (usually tolerated), while one or two bases lost derails everything downstream (usually catastrophic). And it loves testing whether a candidate remembers that trinucleotide-repeat disorders are inherited differently, showing anticipation, unlike an ordinary single-gene point mutation.
In real practice, a suspected genetic mutation is confirmed by DNA sequencing (reading out the actual base sequence of a gene), most often now by next-generation sequencing panels that read many genes at once. For trinucleotide-repeat disorders specifically, a technique called PCR-based repeat-length analysis directly counts how many repeats of the triplet are present, which is how Huntington disease and Fragile X are confirmed on a blood test. Family history and pattern of inheritance across generations (checking for anticipation) is a key clinical clue that should prompt this genetic testing.
There is no way to reverse a mutation once it exists, so management is about genetic counselling (structured discussion of risk, inheritance pattern and reproductive options), presymptomatic testing for at-risk relatives (with formal psychological support, per the Huntington's Disease Association and UK genetic testing guidelines), and disease-specific symptomatic treatment. For Huntington's disease, the Maudsley Prescribing Guidelines and NICE support tetrabenazine for chorea (involuntary jerky movements) and antidepressants for the common co-occurring depression, alongside multidisciplinary care from neurology, psychiatry and genetics. The diagnostic criteria for the genetic condition itself are separate from naming the mutation type: the laboratory result identifies whether the variant is missense, nonsense, frameshift, in-frame, or a repeat expansion, while clinical diagnostic criteria and investigations establish the syndrome the variant has caused. Source-wise, variant interpretation is grounded in ACMG/AMP standards, which classify variants as pathogenic, likely pathogenic, uncertain significance, likely benign, or benign; this is why a report should not be treated as a diagnosis until the variant class, inheritance pattern, phenotype, and family segregation evidence all fit together.
When a question describes a mutation, resist the urge to fixate on WHAT changed. Ask HOW MANY bases changed and whether three divides evenly into that number: that single check separates a probably-tolerated change from a probably-catastrophic one faster than memorising every named mutation type.
A mutation's damage is set less by what it is than by two numbers: how many bases changed, and whether that number shifts the reading frame.