Exam questions

Question 1

Compare and contrast the structure and function of nicotinic receptors and voltage-gated sodium channels (5 marks)

Answer 1

Both permeable to sodium, cause depolarization and action potential firing (1 mark),
nACHrs made up of 5 protein subunits, each subunit 4 TMs (1 mark),
VGNA one protein 4 domains, each domain has 6 TMs (1 mark), nicotinic gated by ligand (1 mark), VGNA gated by change in voltage / depolarization (1 mark).

Question 2

How does agonist binding to a G protein-coupled receptor lead to increased production of cAMP? (4 marks)

Answer 2

Ligand binding to the receptor causes the affinity of the G protein alpha subunit for GDP to decrease (1 mark). This brings about the exchange of GDP for GTP (1 mark). GTP bound alpha subunit dissociates from beta gamma subunits (1 mark), gtp-bound alpha subunit and beta-gamma subunit now free to engage downstream effectors, change levels of second messenger (1 mark).

Question 3

Briefly describe the function of SNARE proteins? (3 marks)

Answer 3

SNARE proteins are involved in vesicle fusion (1 mark). They form a coiled-coil structure (1) between vesicles and target membranes forcing membrane merger (1 mark).

Question 4

4. What is telomerase and why is it essential for cell viability? (5 marks)

Answer 4

Telomerase is a specialised DNA polymerase (1 mark) which specifically synthesises multiple copies of a simple sequence hexameric repeat (1 mark), which are called telomeres, found at the ends of linear eukaryotic chromosomes (1 marks). Telomeres prevent the gradual loss of DNA sequences at the ends of chromosomes in proliferating cell populations (1 mark), which would otherwise occur due to the removal of RNA primer sequences from the Okazaki fragment at each end of a recently replicated chromosome (1 mark).

Question 5

5. Explain how pyrimidine dimers are removed from DNA and how the damaged DNA is then repaired, naming the four main enzymes involved. (4 marks)

Answer 5

An Excision Nuclease cleaves the damaged DNA strand on either side of the pyrimidine dimer, several nucleotides away from the dimer (1 mark). DNA Helicase removes the fragment of single-stranded DNA containing the pyrimidine dimer, leaving an approximately 12 nucleotide gap (1 mark). DNA Polymerase then repairs the gap with new DNA synthesis from the primer:template junction (1 mark). DNA Ligase seals the nick (1 mark).

Question 6

How does the kinetochore maintain genetic stability? (5 marks)

Answer 6

Define what kinetochores are (1 mark)

Kinetochores are proteins that interact with centromeres of chromosomes

Describe mechanism (2 marks)

The inner plate of the kinetochore recognises and binds to the repeat sequences of the centromere. The outer plate then binds to these and forms a bridge between the centromere and the microtubules within the mitotic spindle

Describe how they maintain stability (2 marks)

This allows accurate segregation of chromosomes during mitosis ensuring that each chromosome is segregated into each daughter cell maintaining genetic stability

Question 7

Deamination of a cytosine base leads to a transversion mutation. True or false. Explain your answer

Answer 7

1 mark = false
1 mark = deamination of cytosine into uracil
1 mark = because uracil is recognised as a T
1 mark = When DNA is being replicated it leads to a G being turned into an A which is a transition mutation