What Is a Mutation?
A mutation is a change in the DNA sequence of an organism. It can affect a single gene, part of a chromosome, or even whole chromosomes.
DNA is the instruction manual for every living cell. It is made up of a sequence of chemical bases — adenine (A), thymine (T), cytosine (C), and guanine (G) — arranged in a precise order. This order codes for proteins, which carry out almost every function in your body.
When this sequence is altered — whether a base is changed, added, or removed — the instruction changes too. Sometimes the change is harmless, sometimes it improves the organism, and sometimes it causes serious problems.
Imagine the DNA sequence as a sentence: "THE CAT SAT ON THE MAT." If a single letter is changed to give "THE BAT SAT ON THE MAT," the meaning shifts slightly. But if a letter is inserted — "THE CAT SAT TON THE MAT" — the rest of the sentence no longer makes sense. Mutations work in exactly the same way with genetic code.
Mutations can occur in body cells (somatic cells) or in sex cells (gametes). Only mutations in gametes can be passed on to the next generation.
It is also worth knowing that cells are not defenceless against mutations — they have DNA repair enzymes that continuously scan the DNA and correct errors made during replication. Most mistakes are caught and fixed before they become permanent mutations. It is only when these repair systems are overwhelmed or fail that a mutation persists.
Types of Gene Mutations
Gene mutations affect a single gene by changing the sequence of DNA bases within it. There are three main types:
- Substitution — one base is swapped for a different base.
- Insertion — one or more extra bases are added into the sequence.
- Deletion — one or more bases are removed from the sequence.
Insertions and deletions that shift the reading frame of the genetic code, potentially altering every codon — and therefore every amino acid — after the point of the mutation. These tend to have more severe effects than substitutions.
Original DNA sequence (codons read in threes):
ATG · CAT · GGA · TCA
After deletion of the base 'C' (first base of the second codon):
Original: A-T-G-C-A-T-G-G-A-T-C-A
After removing 'C': A-T-G-A-T-G-G-A-T-C-A → ATG · ATG · GAT · CA...
Every codon from the deletion point onwards is changed, meaning a completely different (and usually non-functional) protein is produced.
Because the genetic code is degenerate (multiple codons can code for the same amino acid), some substitution mutations do not change the amino acid produced. These are called silent mutations.
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