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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 affects the transmission of nerve impulses along the affected neurones. When the myelin sheath is compromised, it disrupts the process of saltatory conduction, where nerve impulses jump from one node of Ranvier to another. This results in reduced speed and efficiency of impulse conduction. Consequently, the communication between the nervous system and muscles is impaired, leading 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 controls involuntary functions such as heart rate. Damage to the neurones can disrupt the signaling pathways involved in transmitting impulses from the cardiac center in the brain to the heart. This disruption may lead to improper heart rate regulation, resulting in irregularities.
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
Single-stranded DNA molecules can bind to complementary mRNA corresponding to the huntingtin protein. This binding can prevent the mRNA from being translated into the huntingtin protein, thus reducing its concentration. By inhibiting the translation process, the drug effectively reduces the levels of harmful huntingtin in the brain, which may alleviate the 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 targets the concentration of the huntingtin protein and does not address the underlying genetic mutation that causes Huntington's disease. Secondly, even if the drug reduces huntingtin levels temporarily, brain damage may already have occurred due to the presence of the toxic protein, meaning that the disease may continue to progress despite treatment.
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
One reason for injecting the drug directly into the cerebrospinal fluid is that it allows for immediate access to the brain and spinal cord, ensuring higher concentrations of the drug at the site of action. Secondly, injecting the drug bypasses the digestive system, preventing degradation by enzymes and acids, which may reduce the bioavailability of the drug if taken orally.
Step 6
Suggest and explain one way epigenetics may affect the age when symptoms of Huntington's disease start.
Answer
Epigenetics can influence gene expression without changing the DNA sequence. Factors such as DNA methylation could affect the expression of the CAG repeat in the Huntingtin gene. Increased methylation might lead to reduced expression of the toxic huntingtin protein, potentially delaying the onset of symptoms, while decreased methylation might result in earlier symptoms due to higher protein levels.