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Guillain–Barré syndrome is a rare disease in which the immune system damages the myelin sheath of neurones - AQA - A-Level Biology - Question 10 - 2019 - Paper 1
Question 10
Guillain–Barré syndrome is a rare disease in which the immune system damages the myelin sheath of neurones. Myelin sheath damage can cause a range of symptoms, for e... show full transcript
Worked Solution & Example Answer:Guillain–Barré syndrome is a rare disease in which the immune system damages the myelin sheath of neurones - AQA - A-Level Biology - Question 10 - 2019 - Paper 1
Step 1
Damage to the myelin sheath of neurones can cause muscular paralysis (lines 2–4). Explain how.
Answer
The damage to the myelin sheath interrupts the normal transmission of nerve impulses along the neurones. Myelin facilitates saltatory conduction, allowing impulses to pass rapidly from node to node (nodes of Ranvier). When the sheath is damaged, this process is compromised, resulting in slower or disrupted impulse transmission that can lead to muscular paralysis.
Step 2
Sometimes Guillain–Barré syndrome causes heart rate irregularities (lines 4–5). Suggest and explain why.
Answer
Guillain–Barré syndrome can affect the autonomic nervous system, which regulates involuntary functions including heart rate. Damage to the myelin sheath can impair the nerve fibres that control heart rate, leading to irregularities caused by disrupted signals that should regulate heart contractions.
Step 3
The first successful drug trial to reduce concentrations of huntingtin in the brain used single-stranded DNA molecules (lines 13–14). Suggest and explain how this drug could cause a reduction in the concentration of the protein huntingtin.
Answer
The single-stranded DNA molecules are complementary to the mRNA that codes for the huntingtin protein. When introduced into the patient's system, these molecules bind to the mRNA, inhibiting its translation into protein. This mechanism reduces the amount of huntingtin produced, thus lowering its concentration in the brain.
Step 4
Scientists from the first successful drug trial to reduce concentrations of huntingtin (lines 9–11) reported that the drug is not a cure for Huntington's disease. Suggest two reasons why the drug should not be considered a cure. Do not include repeats of the drug trial in your answer.
Answer
Firstly, the reduction of huntingtin does not reverse existing damage to the brain caused by the disease. Secondly, the drug may only provide temporary relief without completely stopping the progression of the disease, as the underlying genetic factors leading to the production of the mutant huntingtin protein remain.
Step 5
Suggest two reasons why people had the drug injected into the cerebrospinal fluid (lines 12–13) rather than taking a pill containing the drug.
Answer
Injecting the drug into the cerebrospinal fluid allows for direct delivery to the central nervous system, ensuring that it can bypass the blood-brain barrier, which a pill might not achieve effectively. Additionally, direct injection ensures higher localized concentrations of the drug at the site of action compared to systemic circulation via oral administration.
Step 6
Explain and suggest one way epigenetics may affect the age when symptoms of Huntington's disease start.
Answer
Epigenetics can influence the expression of genes without altering the DNA sequence. Methylation of the CAG repeat region in the gene for Huntington's can affect how early the gene is expressed and consequently how quickly symptoms manifest. Increased methylation may silence the gene, delaying the onset of symptoms, while decreased methylation may lead to earlier expression and symptom onset.