A mutation is nothing dramatic to look at — it is one letter in a very long instruction manual being swapped, added or dropped. What makes it interesting is that the cell reads that manual in blocks of three, so where the change happens matters far more than how big it is.
📘 What you need to know
A gene mutation is a change in the base sequence of DNA. It can create a new allele.
There are three types to know: substitution (a base swapped), insertion (a base added) and deletion (a base removed).
A substitution only affects its own triplet. Insertions and deletions shift everything after them — a frameshift.
Mutations happen at random, most often as copying errors during DNA replication in the S phase of interphase.
Mutagens raise the mutation rate: UV light, ionising radiation, and certain chemicals such as those in tobacco smoke.
Some parts of the genome mutate more than others, and no cell can choose to mutate a gene on purpose.
What counts as a mutation
A gene mutation is a change in the sequence of base pairs in a DNA molecule. Because the base sequence decides the amino acid sequence, a mutation can change the polypeptide the gene codes for — but only sometimes, as you will see.
Two things to be clear about from the start:
Mutations happen all the time, in every organism. They are not rare events.
They happen at random. An organism cannot mutate a gene because it would be useful to.
They are most likely at particular moments in the cell cycle. DNA replication in the S phase of interphase is the obvious one — billions of bases being copied is billions of chances to make a mistake.
Watch your wording here. “Mutations occur randomly” is a mark. “Organisms mutate so they can adapt” is a lost mark, every time. Nothing steers a mutation towards being useful.
The three types
The boxes are the point. One base has changed in each row, but only in the bottom two rows does the reading frame move.
Substitution
One base is swapped for a different one. The triplet it sits in now reads differently, so the amino acid at that position may change — but every triplet after it is untouched. There is no knock-on effect.
Insertion
An extra nucleotide is squeezed into the sequence. That triplet changes, and so does every triplet after it, because all the later bases have been pushed along by one place. This is a frameshift.
Deletion
A nucleotide is missed out. Same story as an insertion: the triplet changes and everything downstream is regrouped, so it is also a frameshift.
Why frameshifts are worse: a substitution can change at most one amino acid. A frameshift can change every amino acid from the mutation onwards, so the polypeptide usually cannot function at all.
What causes mutations
Some mutations are simply mistakes. Others are caused by something in the environment. An environmental factor that raises the mutation rate is called a mutagen (or mutagenic agent).
Type of mutagen
Examples
What it does
High-energy radiation
UV light from the Sun or sunbeds
Causes chemical changes in the DNA that lead to errors in the base sequence.
Ionising radiation
X-rays, gamma rays, alpha particles
Damages DNA directly, which is why exposure is carefully limited in hospitals.
Chemicals
Compounds in tobacco smoke; mustard gas
React with DNA and alter bases, raising the rate of copying errors.
From inside the cell
Certain enzymes and their products
Not every mutagen comes from outside — some are made by the cell’s own chemistry.
Proofreading catches most errors
DNA replication is not careless. DNA polymerase proofreads as it goes: if it spots that the wrong nucleotide has been added, it removes it, replaces it with the correct one, and carries on. A mutation only survives when that proofreading misses something.
Two enzymes, two jobs, and students mix them up constantly. RNA polymerase builds mRNA in transcription. DNA polymerase copies DNA and proofreads it during replication. Say the full name every time.
Mutations are random, but not evenly spread
A mutation can happen anywhere in the genome, on any chromosome, in any organism. That is how new strains of bacteria and viruses appear. But “random” does not mean every site is equally likely:
Uncoiled DNA is more exposed than DNA wound tightly around histone proteins, so it picks up more mutations.
Many mutations land in non-coding regions, such as satellite DNA, where they change no protein at all.
Some places are hotspots. One well-known example is a CpG site, where a cytosine sits directly before a guanine. After methylation, that C can change into a T by substitution.
Where CpG sites cluster together (a CpG island), repeated mutation is linked to certain cancers, including colorectal cancer.
Random does not mean uniform. Where the DNA is open and being worked on, it is easier to damage.
Cells cannot mutate on purpose
This one catches people out, so learn it as a flat statement: there is no known mechanism by which a cell deliberately changes its own base sequence to alter a trait. Proofreading and repair systems exist, but they work in the opposite direction — they put mistakes back the way they were. Nothing in the cell can decide that a different allele would be handy.
Worked examples
WORKED EXAMPLE
Name the type of mutation
An original DNA sequence reads TAC GGA CTT. After a mutation it reads TAC GGA CTA. Name the type of mutation and state how many triplets are affected.
Step 1: Line the two sequences up
Same number of bases, so nothing has been added or removed.
Step 2: Find the differenceCTT has become CTA — one base swappedA substitution, affecting one tripletCounting the bases first is the quickest way to rule out insertion and deletion.
WORKED EXAMPLE
Compare two mutations
Explain why a deletion is usually more damaging to a protein than a substitution.
Point 1: What a substitution does
It changes only the triplet it occurs in, so at most one amino acid changes.
Point 2: What a deletion does
Every base after it moves along one place, so all the following triplets are regrouped.
Point 3: The consequence
This is a frameshift — the amino acid sequence after the mutation is changed completely, so the polypeptide usually cannot function.
Frameshift changes everything downstreamUse the word frameshift. It is the term the mark scheme is looking for.
WORKED EXAMPLE
Explain an observed pattern
Skin cells show a higher mutation rate than cells deep inside the body. Suggest why.
The exposure
Skin cells are exposed to UV light, which is a mutagen.
The effect
UV causes chemical changes in DNA, so more errors appear in the base sequence.
The link
Skin cells also divide often, and copying errors are most likely during DNA replication.
More mutagen exposure and more replicationA “suggest” command word means apply what you know — you are not expected to have learnt this exact case.
💡 Exam tip
Count the bases first when identifying a mutation type. Same number means substitution; one more means insertion; one fewer means deletion.
Define a mutation as a change in the base sequence, not “a change in the DNA”.
Always pair insertion and deletion with the word frameshift, and substitution with “affects one triplet only”.
Name specific mutagens — UV light, X-rays, gamma rays, chemicals in tobacco smoke. Vague answers like “radiation” score less.
Credit the cell’s defences: DNA polymerase proofreads during replication and corrects most errors.
If a question mentions the cell cycle, the answer is usually the S phase, when DNA is replicated.
⚠ Common mix-up
Saying organisms mutate in order to adapt. Mutations are random. Natural selection acts afterwards.
Thinking a substitution shifts the sequence. It does not. Only insertions and deletions move the reading frame.
Assuming every mutation changes a protein. Many fall in non-coding DNA, and many others are silent.
Mixing up DNA polymerase and RNA polymerase. Replication and proofreading versus transcription.
Believing “random” means evenly spread. Exposed DNA and hotspots such as CpG sites mutate more often.
Thinking cells can edit their own genes deliberately. No such mechanism exists — repair systems only restore the original sequence.
Up next: Consequences of Mutations — silent, missense and nonsense substitutions, what a frameshift does to a polypeptide, and why it matters whether the mutation happened in a body cell or a gamete.
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