From Mutation to Protein and Phenotype

Explained

Following a mutation all the way through

A mutation is a random change in the base sequence of DNA. Whether anything comes of it depends on a chain of steps, and each link in that chain can break.

The chain, one step at a time

1. The base sequence of the DNA changes.
2. Transcription copies that sequence, so the mRNA codons change.
3. A changed codon may call for a different amino acid, so the amino acid sequence of the protein changes.
4. A different amino acid sequence may fold into a different shape.
5. A different shape may mean the protein no longer works, and the characteristic it controls changes.

Notice how many of those steps say may. That is the substance of the topic, not hedging.

Where the chain breaks

The mutation may fall in non coding DNA, which does not code for any protein. Nothing is transcribed differently and nothing follows.

Several different codons code for the same amino acid, so a changed codon may still call for the same one. The protein is built exactly as before.

The amino acid may change in a part of the protein that does not affect its shape where it matters. An enzyme whose active site is unaltered still works.

That is why most mutations have no observable effect, and a question asking why wants one of those three reasons rather than the general statement.

When it does matter

If the changed amino acid falls at a position that determines the folding, the shape changes.

In an enzyme, an altered active site no longer fits its substrate, so no enzyme substrate complexes form and the reaction it catalyses does not happen. In a structural protein, the tissue it builds is altered.

A few changes are beneficial, producing a protein that works better in that environment, and those are the raw material of natural selection. Whether a mutation is harmful or helpful depends on the surroundings, not on the mutation alone.

Inherited or not

This is the distinction that decides who is affected, and it depends entirely on which cell the mutation occurred in.

A mutation in an ordinary body cell is passed only to that cell's descendants. It can cause a tumour if it affects a gene controlling cell division, but it cannot be inherited, because it is not in a gamete.

A mutation in a cell that produces gametes can be passed to offspring. If that gamete is involved in fertilisation, the mutation is in the zygote, and therefore in every cell of the new organism, and it can then be passed on again.

So an inherited condition traces back to a mutation in a gamete, at some point, in some ancestor. A cancer usually traces back to a mutation in a body cell during that person's own lifetime.

Writing the answer

If asked how a mutation changes a characteristic, give the chain in order and name each stage: base sequence, codon, amino acid, protein shape, function, characteristic.

If asked why a mutation had no effect, name the specific reason rather than saying it was harmless. Non coding DNA, the same amino acid, or a position that does not affect the shape are three different answers, and any one of them earns the mark.