What mutations are & their types

The big idea: A mutation is a random change to the base sequence of DNA.

DNA is a code spelled out in four bases (A, T, C and G). If that spelling is changed — even by a single base — the gene now carries different information.

Mutations are random: they are not made on purpose and cannot be aimed at a useful result. They are also the original source of all new alleles (new versions of a gene), which is why they matter so much for variation and evolution.

Mutation

A random change in the base sequence of DNA (the genetic material).

Gene

A length of DNA that carries the code for a particular characteristic.

Allele

A particular version of a gene. A new allele is produced when a mutation changes the base sequence of an existing gene.

Base sequence

The order of the four bases (A, T, C, G) along a strand of DNA — this order is the genetic code.

Mutagen

Anything that increases the rate of mutation, such as ultraviolet (UV) light, X-rays or certain chemicals.

Mutations make new alleles: Where do the different versions of a gene come from in the first place?

They come from mutation. A change to the base sequence of an existing gene creates a new allele — a slightly different version of that gene.

So mutation is the ultimate source of all the genetic variation in living things.

Most gene mutations you need at SL involve a change to a single base. There are three kinds, and they differ in what happens to the bases.

The key word for telling them apart is whether a base is swapped, added or removed.

Substitution

One base is swapped for a different base. The total number of bases stays the same.

Insertion

One (or more) extra base is added into the sequence, increasing the total number of bases.

Deletion

One (or more) base is removed from the sequence, decreasing the total number of bases.

Frameshift

The shift in the reading frame caused by an insertion or a deletion, so that every codon after the change is read differently.

Why insertion and deletion hit harder: DNA is read in triplets (groups of three bases called codons).

A substitution changes only the one base it touches, so usually just one codon is affected.

An insertion or a deletion changes the total number of bases, so the cell reads every triplet after the change out of step. This is a frameshift, and it usually changes all of the codons downstream — a much bigger effect from a single base change.

Mutagens can cause these changes: Mutations happen by chance, but their rate goes up when DNA is exposed to a mutagen.

Ultraviolet (UV) light, X-rays and some chemicals can damage DNA and cause mutations such as a substitution (or an insertion/deletion).

If asked to name a mutation type a mutagen could cause, any of substitution, insertion or deletion is a valid answer.

A memory hook: Sub = substitute (swap one for another). Insertion = put one in. Deletion = take one out.

Only insertion and deletion change the number of bases — and changing the number is what causes a frameshift.

How this is tested: A favourite Paper 1B task gives you a DNA base sequence before and after a change and asks you to classify the mutation — read the two sequences, then decide whether a base was swapped (substitution), added (insertion) or removed (deletion).

Paper 1A multiple-choice items often ask you to identify a true general feature of mutations (random; a change to the base sequence; the source of new alleles), or to name a mutation type a mutagen such as UV light can cause.

On Paper 1B a 2-mark Compare and contrast can ask for a similarity and a difference between two named types — give one of each, clearly labelled.

✓ How to tell the types apart in data: Count the bases before and after.

Same number of bases, one letter different → substitution.

One more base → insertion. One fewer base → deletion — and in both of these, every triplet after the change is read differently (a frameshift).