D1.3 Mutations and gene editing

The question asks why mutations are random even when a mutagen is present. The key distinction is between increasing the rate of mutation versus directing mutations to specific locations.

A mutagen is any agent that raises the mutation rate above background levels. However, mutagens do not 'aim' at particular nucleotides or genes — they interact with DNA in a non-specific way, meaning any nucleotide along the genome could be affected.

Randomness in mutation refers to the fact that which specific base is altered is unpredictable and not driven by the organism's needs. A mutagen raises the probability of mutation everywhere, but cannot selectively mutate, say, a gene for antibiotic resistance to make the bacterium better adapted.

The correct answer is: Mutagens increase the frequency of mutations across the genome but cannot control which specific nucleotide is altered. This accurately captures the distinction between mutation rate and mutation direction.

We need to find the option that correctly explains why mutations remain random even in the presence of a mutagen.

This is incorrect. Mutagens such as UV radiation and alkylating agents can damage DNA anywhere in the genome, including coding regions. This option is factually wrong.

This is incorrect. Proofreading reduces errors but does not eliminate all of them — that is precisely why mutagens increase mutation rates. The premise of this option is false.

This is incorrect. Mutagens act on DNA in all cell types, including somatic cells. This option is not supported by evidence.

The remaining option — mutagens increase the frequency of mutations across the genome but cannot control which specific nucleotide is altered — is correct. Randomness means the location of the mutation is not directed, even if the overall rate is elevated.

The mutation occurs in a skin cell, which is a somatic cell. UV radiation is a non-ionizing mutagen that causes thymine dimers, leading to replication errors.

Tumour suppressor genes normally act as brakes on the cell cycle or trigger apoptosis in damaged cells. A mutation that inactivates such a gene removes this checkpoint, allowing cells to divide uncontrollably.

Because this occurs in a somatic cell, it is not heritable. However, uncontrolled division in a somatic cell can lead to tumour formation — specifically melanoma in skin cells exposed to UV.

The correct answer is: The cell may lose control of division, potentially contributing to melanoma development. This correctly links somatic mutation → inactivated tumour suppressor → loss of cell cycle regulation → cancer.

We need to identify the most likely consequence of an unrepaired UV-induced mutation in a tumour suppressor gene in a somatic skin cell.

This is incorrect. The mutation occurs in a somatic skin cell, not a germ cell. Somatic mutations cannot be inherited by offspring.

This is incorrect. Mutations typically inactivate tumour suppressor genes, not activate them. Inactivation removes the brake on cell division; it does not increase apoptosis.

This is incorrect and scientifically unsupported. Mutations do not spontaneously generate new repair enzymes as a direct consequence.

The cell may lose control of division, potentially contributing to melanoma development is correct. Inactivation of a tumour suppressor in a somatic cell removes cell cycle regulation, a key step in cancer development.