AQA Biology A-Level - Gene mutations QP PhysicsAndMathsTutor.
com
Q1.
Sickle cell disease (SCD) is a group of inherited disorders. People with SCD
have sickle-shaped red blood cells. A single base substitution mutation can
cause one type of SCD. This mutation causes a change in the structure of the
beta polypeptide chains in haemoglobin.
(a) Explain how a single base substitution causes a change in the structure of
this polypeptide.
Do not include details of transcription and translation in your answer.
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Q2.
Mitochondrial DNA (mtDNA) is a small circular DNA molecule located in
mitochondria. It is 16 569 nucleotides long and contains 37 genes and a control
region.
Sports scientists investigated whether a mutation in the control region of mtDNA
in human males was related to an ability to exercise for longer.
• The males in Group T had thymine at nucleotide position 16 519
• The males in Group C had a mutation resulting in cytosine at nucleotide
position 16 519
(a) The control regions of Group T and Group C were the same length.
Name the type of gene mutation that is most likely to have occurred at
nucleotide position 16 519
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AQA Biology A-Level - Gene mutations QP PhysicsAndMathsTutor.com
Group T and Group C completed the same 8-week training programme. The
following measurements were taken at the start of the 8-week programme, and
again at the end.
1. VO2 max (a measure of maximal oxygen uptake).
2. Citrate synthase (CS) activity (CS is an enzyme involved in the Krebs cycle).
The scientists then calculated the percentage increase in each measurement in
both groups.
Figure 1 and Figure 2 show their results.
(b) A student concluded from Figure 1 and Figure 2 that training has a
positive effect on VO2 max and CS activity.
Evaluate the student’s conclusion.
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AQA Biology A-Level - Gene mutations QP PhysicsAndMathsTutor.com
(c) The mitochondrial DNA (mtDNA) control region is an area of mtDNA that is
non-coding. This region stimulates the synthesis of both mtDNA and
mitochondrial messenger RNA.
Use this information to suggest two reasons why the mutation at
nucleotide position 16 519 could lead to the differences seen in Figure 2.
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AQA Biology A-Level - Gene mutations QP PhysicsAndMathsTutor.com
Q3.
Some autism spectrum disorders (ASDs) are associated with a mutation
affecting the neuroligin-3 gene. This gene codes for a protein called NL3, that is
found in synapses.
Scientists investigated the effects of a mutation affecting NL3 in mice. They
obtained brains from mice with the mutation and from mice without the mutation.
For each type of mouse they:
• obtained a solution containing all of the proteins from synapses in one part
of the brain
• separated these proteins using gel electrophoresis
• identified and measured the amount of three proteins from the solution
using three different labelled antibodies.
The three proteins are parts of a postsynaptic membrane receptor.
The diagram below shows the scientists’ results. Each band shows the presence
of a protein. The size of a band shows the amount of the protein present.
(a) The mutation affecting NL3 in these mice was a substitution in the
neuroligin-3 gene.
What is a substitution mutation?
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(d) What do these data show about the effects of the mutation on the
proteins?
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AQA Biology A-Level - Gene mutations QP PhysicsAndMathsTutor.com
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(e) These proteins are part of a receptor found in synapses in the part of the
brain called the hippocampus. A high ratio of NR2B to NR2A protein in this
receptor has been associated with good memory.
Using all of the information, suggest how the mutation affecting the NL3
protein may affect a mouse.
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(Total 10 marks)
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