Studyledge

The Mitotic Cell Cycle: Question 9

Syllabus 5.1, 5.2

Multiple choice AS 1 mark

Genes that normally help to control the cell cycle can, following mutation, contribute to the uncontrolled cell division seen in tumours. Two categories of such genes are proto-oncogenes (which can become oncogenes) and tumour-suppressor genes.

Which statement correctly distinguishes the typical effect of a mutation in each type of gene?

Choose an answer to check it, then compare with the worked solution below.
Show worked solution Hide worked solution

Worked solution

Step 1: Recall the normal roles of each gene type

A proto-oncogene is a normal gene whose product typically promotes or stimulates cell division (for example, as part of a signal that tells a cell to progress through the cell cycle). A tumour-suppressor gene is a normal gene whose product typically does the opposite: it restrains or slows cell division, for example by helping to enforce a cell cycle checkpoint.

Step 2: Consider the effect of mutation on each

A mutation can convert a proto-oncogene into an oncogene. This mutation typically makes the gene’s product more active than normal, active all the time, or produced in excessive amounts, so it continuously stimulates the cell to divide even when it should not. A mutation in a tumour-suppressor gene, by contrast, typically damages or destroys its product’s function, so it can no longer restrain the cell cycle. The cell then loses this normal “brake” on division. In both cases the overall outcome is the same, cell division proceeds when it should not, but the underlying mechanism is opposite: one is a gain of an activating function, the other is a loss of a restraining function.

Step 3: Evaluate each option

  • A: correct, an oncogene mutation strengthens or sustains a stimulus to divide, while a tumour-suppressor gene mutation removes a restraint on dividing.
  • B: incorrect. This reverses the two roles: it is the tumour-suppressor gene that normally restrains division, and the oncogene that normally stimulates it.
  • C: incorrect, although both contribute to uncontrolled division, their normal roles and the nature of the mutation (gain of function versus loss of function) are different, not identical.
  • D: incorrect, neither type of mutation acts by preventing DNA replication; blocking DNA replication would stop the cell cycle, not drive uncontrolled division.

Final answer

  • A mutated oncogene over-stimulates cell division, while a mutated tumour-suppressor gene loses its ability to restrain cell division, option A.