Oxbridge Biology interview questions

Biology interviews at Oxford and Cambridge

Biology interview questions to practise

Say your first thoughts out loud or type them, then compare with how a strong answer goes. These questions are our own, written in the style tutors use.

1. Question 1

   On a tall sunflower, the leaves near the top are smaller and thicker than the leaves near the ground. Why might that be?

   What it shows a tutor: Whether you can turn an observation into several testable explanations, rather than one guess.

   Your first thoughts, as you would say them
   Show a strong answer
   How a strong answer goes

   1. Start with what differs between the two places: the top leaves get more direct sun and more wind; the low leaves are shaded by the ones above.
   2. Strong light means a leaf can make plenty of sugar from a small area, but it also heats up and loses water. A smaller leaf loses less water by transpiration.
   3. Thicker suggests more layers of photosynthetic cells stacked up, which uses strong light that would otherwise pass straight through a thin leaf.
   4. Low leaves are in dim, scattered light. A large, thin leaf catches as much of it as possible for the least material.
   5. Then name a rival explanation: the top leaves are younger, so they may just not have finished growing. Age and light are tangled together on one plant.
   6. Finish with a test: grow identical plants in full sun and in shade and compare leaves of the same age. If the shaded plants make large thin leaves at the top too, light is the cause.

   A tutor might then ask: What would you expect to see in the leaves of a plant that lives on a forest floor all its life?

   The usual wrong turn: Giving one explanation as fact and stopping. Tutors want the rival explanations and a way to tell them apart.

2. Question 2

   A bacterium divides every 20 minutes. Starting from one cell with plenty of food, roughly how long before the colony would weigh as much as the Earth? Why does that never happen?

   What it shows a tutor: Comfort with orders of magnitude, and whether you link a calculation back to biology.

   Your first thoughts, as you would say them
   Show a strong answer
   How a strong answer goes

   1. I need two masses. A bacterium is about a picogram, 10⁻¹⁵ kg. The Earth is about 6 × 10²⁴ kg.
   2. The ratio is about 6 × 10³⁹. Each division doubles the mass, so I need 2ⁿ ≈ 6 × 10³⁹.
   3. 2¹⁰ is about 10³, so 10³⁹ is about 2¹³⁰, and the factor of 6 adds between 2 and 3 more doublings. Call it 133 doublings.
   4. 133 × 20 minutes is about 2,660 minutes, roughly 44 hours. Under two days.
   5. It never happens because the assumption of plenty of food fails almost at once: nutrients run out, waste builds up, and oxygen and space become limiting. Growth slows into the S-shaped curve and then stops.
   6. Check the answer makes sense: even an error of 1,000 in the bacterium's mass only changes the answer by about 10 doublings, a few hours. The conclusion stands.

   A tutor might then ask: If an antibiotic kills 99.9% of the cells every hour, does the colony shrink or grow?

   The usual wrong turn: Freezing because you do not know the exact mass of a bacterium. Say a sensible estimate out loud and carry on.

3. Question 3

   On islands, small mammals such as rodents often evolve to be larger than their mainland relatives, while large mammals such as elephants have evolved to be smaller. Why might both happen?

   What it shows a tutor: Reasoning about selection pressures, and seeing that one setting can push in opposite directions.

   Your first thoughts, as you would say them
   Show a strong answer
   How a strong answer goes

   1. Ask what is different about an island: less land and food in total, and often fewer predators and competitors.
   2. For a large animal, food is the limit. A smaller body needs less food, so on a small island smaller individuals survive and breed more. Over generations the population shrinks in size.
   3. For a small animal, being small often protects against predators and lets it live on scarce food in a crowded habitat. With fewer predators and fewer competing species, a larger body can win: it holds territory, survives cold or famine better and has more young.
   4. So the same island removes the pressure that kept small animals small and adds the pressure that makes large animals smaller.
   5. Say how you would check it: compare many island and mainland pairs and see if size change depends on starting size and island area, and on whether predators are present.

   A tutor might then ask: What would you predict for a bird species that cannot fly, compared with one that can, when both reach an island?

   The usual wrong turn: Saying the animals change size because they need to. Selection acts on variation that already exists; individuals do not choose their size.

4. Question 4

   Is a virus alive?

   What it shows a tutor: Whether you can test a definition against hard cases instead of reciting one.

   Your first thoughts, as you would say them
   Show a strong answer
   How a strong answer goes

   1. I would start from a working list of what living things do: take in energy and use it, grow, respond, reproduce and evolve.
   2. A virus outside a cell does none of these. It has no metabolism of its own. Inside a cell it is copied by the cell's machinery and, over generations, evolves.
   3. Then test the list on other cases. Some bacteria can only reproduce inside other cells too, and we call them alive. A mule cannot reproduce, and it is clearly alive. So reproduction on its own cannot be the dividing line.
   4. That suggests the boundary is not sharp. Life looks more like a cluster of features, and viruses have some of them.
   5. My answer: the question is partly about how we choose to use the word. For biology the useful point is that viruses evolve by natural selection, so they behave like living things in the way that matters for medicine and ecology.

   A tutor might then ask: Would a computer program that copies itself and changes over time count as alive on your definition?

   The usual wrong turn: Reciting MRS GREN and stopping. The interesting part is what the definition does with awkward cases.

Oxford interview dates, December 2026

All Oxford interviews are online. Your invitation gives your own times.

What tutors are listening for

• Thinking aloud

  Both universities say they want to hear how you work things out, not a memorised answer.

• Using a hint

  Tutors expect to help. What matters is what you do with a prompt or an objection.

• Checking

  Test your answer on a simple case, an example or a counterexample, and fix slips out loud.

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