AQA A Level Biology Paper 3 November 2021 question paper 7402/3

In one species of squirrel, Sciurus carolinensis, fur colour is controlled by one gene, with two codominant alleles. C^G represents the allele for grey fur colour, and C^B represents the allele for black fur colour.

Table 1 shows the three possible phenotypes.

In a population of 34 S. carolinensis, 2 had black fur.

Use the Hardy–Weinberg equation to estimate how many squirrels in this population had brown-black fur. Show your working.

The actual number of squirrels in this population that had brown-black fur was 16.

Use all of the information to calculate the actual frequency of the C^G allele.

Do not use the Hardy–Weinberg equation in your calculation.

Give your answer to 2 decimal places.

S. carolinensis were first introduced to the UK from North America in the 1870s. They are now widely distributed across the UK.

S. carolinensis from both North America and the UK show exactly the same genotypic and phenotypic variation. An identical mutation causing black fur has also been found in several other species closely related to S. carolinensis.

Use this information to deduce which one of the following conclusions is most likely true.

Tick () one box.

The mutation that caused the C^B allele was due to a 24 base-pair deletion from the C^G allele.

The protein coded for by the C^B allele is 306 amino acids long.

Calculate the percentage reduction in size of the protein coded for by the C^B allele compared with the protein coded for by the C^G allele.

Give your answer to 3 significant figures and show your working.

In S. carolinensis, fur colour depends on the distribution and relative amounts of light pigments and dark pigments in the hairs of the fur. Figure 1 shows how the protein produced from the C^G allele can result in the production of a light pigment or a dark pigment.

The deletion mutation in the C^B allele results in the production of a receptor protein that does not have glutamic acid. The lack of glutamic acid in the receptor protein has the same effect as αMSH leaving the receptor protein.

Use Figure 1 and this information to suggest why S. carolinensis with the genotype C^BC^B have black fur rather than grey fur.

Describe how the human immunodeficiency virus (HIV) is replicated once inside helper T cells (T_H cells).

HIV-1 is the most common type of HIV. HIV-1 binds to a receptor on T_H cells called

CCR5.

Current treatment for HIV-1 involves the use of daily antiretroviral therapy (ART) to stop the virus being replicated. Only 59% of HIV-positive individuals have access to ART.

Scientists have found that two HIV-1-positive patients (P and Q) have gone into remission (have no detectable HIV-1). This happened after a blood stem cell transplant (BSCT).

• Patient P was given two BSCTs, and patient Q was given one BSCT.

• All BSCTs came from a donor with T_H cells without the CCR5 receptor.

• In addition, patient P had radiotherapy, and patient Q had chemotherapy. Both of these treatments are toxic.

• Both patients (P and Q) stopped receiving ART 16 months after BSCT.

18 months after stopping ART, both patients had no HIV-1 RNA in their plasma, no HIV-1 DNA in their T_H cells and no CCR5 on their T_H cells.

Use the information given to evaluate the use of BSCT to treat HIV infections.

Scientists investigated movement in adult pine beetles. Adult beetles emerge from cracks in tree bark.

The scientists released a newly emerged adult beetle, G, from the centre of a sample area that had a single light source coming from one direction. They made a drawing of the beetle’s path of walking. They repeated this with three more beetles, J, P and R.

Figure 2 shows the scientists’ results.

Name the type of behaviour shown by beetles G, J, P and R, and suggest one advantage to adult beetles of the type of behaviour shown.

At higher temperatures and higher light intensities, adult pine beetles normally

• move more

• fly rather than walk.

When preparing to fly, these adult beetles walk slowly. The scientists investigated the movement of adult beetles at different temperatures, and in the light and the dark. They created a box that was half in the light and half in the dark. They released an adult beetle at the midpoint of the central dividing line between light and dark areas. They recorded the path of the beetle’s movement and its location after 5 minutes. From this, they calculated the mean speed of movement. They repeated the experiment with many beetles and at several temperatures.

Figure 3 shows the scientists’ results.

After studying these experiments, a student concluded:

• there is a significant change in movement between 35 °C and 37.5 °C

• between 35 °C and 37.5 °C, more beetles move away from the light

• between 35 °C and 37.5 °C, more beetles have a slower walking speed.

Suggest reasons why these conclusions might not be valid.

Freshwater marshes have one of the highest rates of gross primary production (GPP and net primary production (NPP) of all ecosystems.

Carbon use efficiency (CUE) is the ratio of NPP:GPP. Freshwater marshes have a high CUE.

Use your knowledge of NPP to explain why freshwater marshes have a high CUE and the advantage of this.

Do not refer to abiotic factors in your answer.

Freshwater marsh soils are normally waterlogged. This creates anaerobic conditions.

Use your knowledge of the nitrogen cycle to suggest why these soils contain relatively high concentrations of ammonium compounds and low concentrations of nitrite ions and nitrate ions.

A student investigated the growth rate of a freshwater marsh plant.

The growth rate (R) of a plant can be determined using this equation.

Where ln = natural logarithm t = duration of the investigation in days W₁ = plant biomass at the start of the investigation W₂ = plant biomass at the end of the investigation

The student used the equation above; however, she substituted height for biomass. This was because she did not want to destroy the plants to measure their biomass.

State the assumption the student has made and suggest why this assumption might not be valid.

At the end of the investigation, the student noted the freshwater marsh plant had grown 268 mm in height, and now measured 387 mm. She calculated the rate of

Use this information and, substituting height for biomass, use the equation to calculate the duration of the student’s investigation.

Give your answer to the nearest full day. Show your working.

The action of endopeptidases and exopeptidases can increase the rate of protein digestion. Describe how.

As humans age, there is a decrease in body protein.

Give the name of one body protein that could have resulted in:

reduced muscle power

Scientists investigated the effect of two types of dietary protein on the ability of old men to produce body proteins.

Table 2 shows information about the two types of dietary protein investigated.

Figure 4 shows the percentage of protein absorbed that becomes body protein in old men following a meal of casein or whey.

A statistical test confirmed that the difference between the results shown in Figure 4 was significant.

Suggest which type of dietary protein would be better for old men to eat to cause a net gain of body proteins. Use the information provided to explain your answer.

Plants transport sucrose from leaves to other tissues for growth and storage. SUT1 is a sucrose co-transporter protein.

Scientists investigated whether the cells of tobacco plant leaves used SUT1 to transport sucrose to other tissues.

The scientists used a radioactively labelled DNA probe to show that the cells of tobacco plant leaves contained the SUT1 gene.

Describe how they would do this.

Do not include PCR in your answer.

To study the role of SUT1 in tobacco plants, scientists reduced the expression of the SUT1 gene.

When the SUT1 gene is transcribed, the SUT1 mRNA produced is called ‘sense’ SUT1 mRNA. The scientists genetically modified plants by inserting an extra gene so that this also allowed the production of ‘antisense’ SUT1 mRNA.

The scientists had two types of tobacco plants:

• type A – plants that were genetically modified

• type B – plants that were not genetically modified.

Suggest how the production of ‘antisense’ SUT1 mRNA in type A plants would reduce the expression of the SUT1 gene.

The scientists hypothesised that lower rates of sucrose transport from leaves would cause reduced growth.

To test this hypothesis, the scientists provided leaves of type A and type B plants with labelled carbon dioxide (¹⁴CO₂). To estimate sucrose transport out of leaves, they measured the percentage of¹⁴C remaining in the leaves for 16 hours.

Figure 5 shows their results.

Calculate the ratio of percentage of¹⁴C remaining in leaves of type B to type A plants 16 hours after providing¹⁴CO₂

In type B plants, the percentage of¹⁴C remaining in the leaves does not reach zero per cent, as shown in Figure 5.

Suggest two reasons why.

1

2

The scientists measured physiological differences between type A plants and

type B plants.

Table 3 shows the scientists’ results as they presented them.

Sucrose is able to inhibit the production and activity of rubisco in leaves of a plant. Type A plants have decreased dry mass compared with type B plants.

Use all the information to suggest and explain how the physiological factors in Table 3 would contribute to the decreased dry mass observed in type A plants.

Turn over for Section B

Section B

one question.

You are advised to spend no more than 45 minutes on this section.

Write an essay on one of the topics below.

Either

The importance of complementary shapes of molecules in organisms

Or

The importance of ions in metabolic processes