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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 results in impaired transmission of nerve impulses along the affected neurones. This occurs because the myelin sheath facilitates saltatory conduction, where the impulse jumps between the nodes of Ranvier. Without proper myelination, the conduction is slowed or disrupted, leading to insufficient stimulation of the muscles, resulting in 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 disrupt the conduction of nerve impulses to the cardiac muscle, resulting in irregular heart rhythms due to improper electrical signaling.
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 transcribed from the huntingtin gene. When injected, they bind to the mRNA, preventing its translation into the huntingtin protein. This reduces the overall concentration of huntingtin in the brain, helping to alleviate some symptoms of Huntington's disease.
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 drug only reduces levels of the huntingtin protein but does not address any underlying genetic mutation causing the disease. Secondly, the damage already done to the brain may be irreversible, meaning that while the drug can lower protein concentrations, it cannot repair any existing neurological damage.
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, bypassing the blood-brain barrier, which can limit drug efficacy if taken orally. Additionally, delivering the drug via injection can achieve faster and more concentrated therapeutic levels within the brain.
Step 6
Suggest and explain one way epigenetics may affect the age when symptoms of Huntington's disease start.
Answer
Epigenetics involves changes in gene expression without alterations to the underlying DNA sequence. Modifications such as methylation of DNA can influence the expression of the huntingtin gene. If epigenetic changes lead to increased expression of the huntingtin gene in earlier years, symptoms may manifest sooner, while decreased expression may delay symptom onset.