1. 1
   A biologist studies a species of lizard that can regrow a lost tail after escaping a predator by autotomy (deliberately detaching the tail). Over the following eight weeks, cells at the wound site divide repeatedly, gradually rebuilding the bone, muscle and skin of a new, shorter tail.

   (a)

   What type of cell division allows the lizard's cells at the wound site to produce new tail tissue?

   [1 mark]

   • AMitosis
   • BMeiosis
   • CFertilisation
   • DDifferentiation without division

   (b)

   In terms of chromosome number, what is the relationship between the new tail cells produced and the lizard's original body cells?

   [1 mark]

   • AThey contain half the number of chromosomes (haploid)
   • BThey contain the same number of chromosomes as the original body cells (genetically identical diploid cells)
   • CThey contain a randomly varying number of chromosomes
   • DThey contain no chromosomes, only cytoplasm

   (c)

   Explain why the new tail cells produced by mitosis are genetically identical to the lizard's original cells, and why this matters for successfully rebuilding a functioning tail.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A veterinary nurse weighs a Labrador puppy every two weeks from birth and plots each measurement on a breed-specific growth chart, which shows the range of weights recorded for many Labrador puppies of the same age. At ten weeks old, the puppy's weight falls on the 10th percentile line, meaning it is lighter than most Labrador puppies of the same age on record.

   (a)

   What does it mean for the puppy's weight to fall on the 10th percentile line at ten weeks old?

   [1 mark]

   • AThe puppy weighs 10% of the average Labrador puppy's weight
   • BThe puppy has grown by 10% since birth
   • C90% of Labrador puppies of the same age weigh more than this puppy, and 10% weigh the same or less
   • DThe puppy will weigh 10% more by the next check-up

   (b)

   Why might the vet be concerned that the puppy consistently tracks along the 10th percentile line at every check-up, rather than following the 50th percentile line?

   [1 mark]

   • AIt shows the puppy is definitely unhealthy and requires immediate treatment
   • BPercentile charts cannot be used for animals, only humans
   • CIt means the puppy's measurements were recorded incorrectly
   • DIt suggests the puppy is growing more slowly than most puppies of the same age and breed, which could indicate an underlying health or feeding problem worth investigating

   (c)

   Explain why comparing a single puppy's weight against a percentile chart built from measurements of many Labrador puppies is more useful to the vet than simply knowing the puppy's weight in kilograms alone.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A hospital treats a patient with severe skin burns using a skin graft grown in the laboratory from a small sample of the patient's own healthy skin. Scientists isolate epidermal stem cells from the sample and culture them under controlled conditions until they have produced enough new skin tissue to graft onto the burned area.

   (a)

   Explain how the epidermal stem cells taken from the patient's healthy skin are able to produce enough new tissue to cover the burned area.

   [3 marks]

   (b)

   Using the patient's own stem cells for the skin graft, rather than stem cells from a donor, reduces the chance of the graft being rejected. Explain why, and evaluate one reason embryonic stem cells might still be considered for other medical treatments despite this advantage of using a patient's own adult stem cells.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A tumour is found in the lining of a patient's intestine. Doctors explain that the intestine lining normally contains stem cells that divide continuously to replace worn-out lining cells, but that in this patient some of these cells have begun to divide uncontrollably, forming a mass of extra tissue.

   (a)

   Explain how mitosis normally allows the stem cells in the intestine lining to keep replacing worn-out lining cells, and explain what has gone wrong in terms of cell division to cause the tumour to form.

   [6 marks]

   (b)

   Explain why the presence of dividing stem cells in the intestine lining, rather than only fully differentiated lining cells, makes a tumour more likely to arise here than in a tissue made up entirely of cells that no longer divide.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A student keeps a culture of Hydra, a small freshwater animal that can reproduce asexually. Over several days, a small bump called a bud grows on the side of a parent Hydra. Cells at the site of the bud divide repeatedly, and after about a week the bud detaches from the parent as a fully formed, independent young Hydra.

   (a)

   What type of cell division produces the new cells that form the bud?

   [1 mark]

   • AMitosis
   • BMeiosis
   • CBinary fission
   • DSporulation

   (b)

   How does the number of chromosomes in the cells of the new young Hydra compare with the number in the parent Hydra's cells?

   [1 mark]

   • AHalf the number of chromosomes
   • BThe same number of chromosomes
   • CDouble the number of chromosomes
   • DA random, varying number of chromosomes

   (c)

   The new young Hydra produced by budding is genetically identical to its parent. Explain why this means asexual reproduction, unlike sexual reproduction, produces no variation between parent and offspring.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A doctor taps just below a patient's kneecap with a small rubber hammer. Almost instantly, without the patient consciously thinking about it, the patient's lower leg kicks forward.

   (a)

   What type of neurone carries the electrical impulse from the receptor in the knee to the spinal cord?

   [1 mark]

   • AMotor neurone
   • BRelay neurone
   • CSensory neurone
   • DEffector neurone

   (b)

   What structure at the junction between two neurones allows the electrical impulse to be passed on using a chemical neurotransmitter?

   [1 mark]

   • AAxon
   • BDendron
   • CMyelin sheath
   • DSynapse

   (c)

   Explain why this knee-jerk response happens much faster than a response that involves conscious thought, such as choosing to kick a ball.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   An optician examines a patient who struggles to read small print held close to their face, but can see distant objects clearly. Tests show that light entering the patient's eye is focused at a point behind the retina rather than directly on it.

   (a)

   Name the eye defect described, and explain its underlying cause.

   [3 marks]

   (b)

   Explain, in terms of refraction, how wearing glasses containing a converging lens allows this patient to focus light from close objects correctly onto the retina.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A patient suffers a spinal cord injury in a car accident, severing many neurones in the spinal cord. Unlike skin or bone marrow cells, which are readily replaced after injury, doctors explain that the neurones that were severed will not be replaced by new cell division, so the patient is likely to experience permanent loss of some movement and sensation below the injury.

   (a)

   Explain, in terms of mitosis, how skin and bone marrow cells are able to replace damaged cells after an injury such as this car accident, and explain why relying on new cells produced by mitosis is important for repairing tissue in general.

   [6 marks]

   (b)

   Explain why the loss of movement and sensation caused by a spinal cord injury like this is often permanent, referring to the ability of mature neurones to divide, and describe one way in which scanning technology could be used to investigate the extent of the patient's nervous system damage.

   [6 marks]

   Total for question 8: 12 marks

End of questions