AS & A Level Biology - 9700 Paper 4 2020 Summer Zone 2

(a) The enzyme alanine transaminase (ALT) is found in the liver. The function of ALT is to convert the amino acid α-ketoglutarate into another amino acid, glutamate.
Suggest why the liver may need to convert one amino acid into another.
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(b) Tumours can form in the liver.
Explain how a liver tumour develops.
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(c) ALT can leak into the blood from liver tumour cells.
An increase in the concentration of ALT in the blood causes a decrease in the water potential of the blood.
State precisely the name and location of the cells where a change in the water potential of the blood would be detected.
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(d) Describe the homeostatic role of ADH when the water potential of the blood decreases.
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The summer squash plant, Cucurbita pepo, produces edible fruits that vary in shape. Fig. 2.1 shows the fruits of three different varieties of squash plants.

Fruit shape in squashes is controlled by two genes, $A/a$ and $B/b$, that are located on different chromosomes.

• A disc-shaped fruit is produced when both dominant alleles, $A$ and $B$, are present.
• A spherical fruit is produced when either allele $A$ or allele $B$ is present, but not if both $A$ and $B$ are present.
• A long fruit is produced when both allele $A$ and allele $B$ are absent.

(a) (i) Table 2.1 shows the possible genotypes of the Patty pan and Alfresco varieties.
Complete Table 2.1 to show the possible genotypes of the Di Nizza variety.

[Table_1]

[1]
(ii) A gardener used pollen from a male flower of Alfresco to pollinate a female flower of Di Nizza. The gardener grew the seeds produced from this cross and found that half the offspring produced spherical fruits and half produced long fruits.
Draw one genetic diagram to explain this result.

parent genotypes
gametes
offspring genotypes
offspring phenotypes
[4]
(iii) The offspring show genetic variation with respect to fruit shape alleles.
Name the process that occurred during meiosis in the parents that produced this variation and state the stage of meiosis at which it occurred.
process ................................................... stage of meiosis ..................................................................................
[2]
(b) (i) Genetically modified (GM) summer squash plants with resistance to viral diseases have been grown in the USA since 1995.
Scientists have been concerned that viral resistance genes pass easily from GM squash plants to their wild relative, the Texas gourd, Cucurbita texana.
Explain why the possibility of gene flow from GM squash plants to the Texas gourd is a cause of social and environmental concern.
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(ii) A study compared the survival of two different types of hybrids that were formed by cross-pollination between GM virus-resistant squash plants and wild Texas gourd plants:

• virus-resistant hybrids that had inherited the viral resistance gene
• non-resistant hybrids that had not inherited the viral resistance gene.

Viral disease outbreaks caused many of the non-resistant hybrids to die. Leaf-eating beetles then moved in larger numbers to the surviving healthy virus-resistant hybrids. The beetles carried a pathogenic bacterium Erwinia which was capable of killing the plants.
Fig. 2.2 compares infection with Erwinia in the virus-resistant hybrids and the non-resistant hybrids.

Discuss whether these results provide support for the use of genetically modified organisms (GMOs) in food production.
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(iii) Suggest why the GM virus-resistant squashes grown by farmers rarely suffer infection by Erwinia.
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(a) Suggest and explain how the three subspecies of tiger on the Sunda Islands formed.
[4]

(b) Explain why specific primers were used for the tiger mtDNA sections.
[2]

(c) Describe and explain one characteristic of mtDNA that makes it more useful than using nuclear DNA to provide evidence of evolution.
[2]

(d) Suggest two reasons why $P.\ t.\ balica$ and $P.\ t.\ sondaica$ became extinct.
[2]

(e) Suggest why $P.\ t.\ sumatrae$ is still considered to be a member of the species $Panthera\ tigris$.
[2]

Gene therapy can be used to treat some genetic disorders. An appropriate vector is chosen to carry the normal allele into the target cell. Three types of vectors commonly chosen are naked DNA, viruses and liposomes.

(a) A trial of gene therapy to treat cystic fibrosis used a viral vector. The viral vector caused a primary immune response with the production of memory cells.

Explain why the production of memory cells prevents the gene therapy from working in long-term chronic conditions such as cystic fibrosis.
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(b) With reference to the three types of vectors that are commonly used, discuss the challenges in choosing appropriate vectors for use in gene therapy.

Do not include problems associated with an immune response in your answer.
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(c) A trial was carried out to find a new vector for use in gene therapy.

The new vector was made from red blood cells taken from the person with the genetic disorder. The cells had most of their cytoplasmic content removed and were then broken up to make small spherical vectors.

Most of these vectors lacked the ability to bind to receptors on the target cells.

To solve this problem, genetically engineered stem cells taken from the person were used to form red blood cells. These red blood cells had membrane proteins that were complementary to the target cell receptors. The vectors that were produced were well-tolerated by the immune system.

(i) Explain why the vectors were well-tolerated by the immune system.
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(ii) Suggest why it is not possible to produce genetically engineered red blood cells, except by using genetically engineered stem cells.
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Yeast cells respond to changes in glucose concentration in their environment by using transcription factors to switch off genes.
When glucose is present:
• Mig1 transcription factors bind to the promoters of five genes
• Mig1 binding to the promoters stops (represses) transcription of these genes.
The genes that are repressed by Mig1 code for five enzymes that allow yeast cells to metabolise the sugar galactose when glucose is absent.
(a) Complete Table 5.1 to show three chemical differences between a transcription factor, such as Mig1, and a promoter.
[Table_5.1]
[3]
(b) Mig1 binds to promoter sites with these features:
• 17 base pairs long
• includes a region of five repeating adenine-thymine pairs
• includes a region of six repeating cytosine-guanine pairs.
Promoter sites to which Mig1 binds are known as Mig1-binding promoter sites.
Bioinformatic techniques were used to analyse the yeast genome to look for sections of DNA that match these features. The information obtained for four chromosomes is shown in Table 5.2.
[Table_5.2]
(i) Explain why bioinformatic techniques were used to obtain the information in Table 5.2.
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(ii) Identify, with a reason, the yeast cell chromosome that is most likely to include genes that code for enzymes that metabolise galactose.
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(iii) Mig1 binds to 27 promoters on these four chromosomes. Yeast cells also have other chromosomes where Mig1 binds to additional promoters.
Five different enzymes, coded by five genes, must be made for yeast cells to metabolise galactose.
Suggest reasons why an individual diploid yeast cell has a larger number of Mig1-binding promoter sites than the expected number of ten.
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[2]
(c) The repression of genes involved in galactose metabolism in yeast is similar to events at the lac operon in the bacterium Escherichia coli.
Explain how E. coli represses the production of proteins needed to metabolise lactose sugar.
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[3]

(a) Thyrotoxic myopathy (TM) is a neuromuscular disorder caused by overproduction of the thyroid hormone thyroxine. One of the main symptoms of TM is muscle fatigue.

Fig. 6.1 outlines the effects of overproduction of thyroxine on striated muscle.

(i) The concentration of thyroxine in the blood usually fluctuates around a set point.
Name the mechanism that keeps the concentration of thyroxine in the blood close to its set point.
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(ii) Name the part of the striated muscle fibre that releases $\text{Ca}^{2+}$.
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(iii) Describe the role of $\text{Ca}^{2+}$, troponin and tropomyosin in the contraction of striated muscle.
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(iv) Cyclic AMP (cAMP) is also involved in the response of liver cells to glucagon.
Describe the role of cAMP in the response of liver cells to glucagon.
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(b) An increase in the concentration of thyroxine in the blood can lead to a condition called insulin resistance (IR). IR decreases the sensitivity to insulin of target cells, such as muscle and liver cells.
Suggest how decreased sensitivity to insulin affects target cells.
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(a) Define the term respiratory quotient (RQ). ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................

(b) Fig. 7.1 summarises the respiration of an organic substance, A, in aerobic conditions.
(i) Complete Fig. 7.1 in the spaces provided.

$C_{18}H_{32}O_{2}$ + ..........$O_{2}$ $\rightarrow$ .......... $CO_{2}$ + $16H_{2}O$

[Image_Fig_7.1]
(ii) Calculate the RQ for substance A.

Write your answer to two decimal places.

RQ = ..............................................................
(iii) Use your calculated RQ value to suggest the group of compounds to which substance A belongs.
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(c) Dinitrophenol (DNP) is a compound used in the production of a number of products, including chemical dyes, insecticides and wood preservers. People who work in factories with DNP are at risk of exposure if they do not follow health and safety guidelines.

DNP is a molecule that can transport protons across the inner mitochondrial membrane, allowing protons to leak out of the intermembrane space.

(i) Suggest and explain the effects of DNP on aerobic respiration in human cells.
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(ii) Suggest two symptoms that may be experienced by people after exposure to DNP for several months.
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(a) Most organisms are classified according to a taxonomic hierarchy.
The hierarchy is shown in Fig. 8.1 but the group names are not in the correct order.
1 – kingdom
2 – order
3 – genus
4 – family
5 – domain
6 – class
7 – species
8 – phylum

Fig. 8.1
Complete Table 8.1 by writing the numbers in the correct order.

Table 8.1

5

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7

[2]

(b) Members of the Eukarya domain share similar features but will also have several differences.
Complete Table 8.2 by stating the differences between the kingdoms Fungi and Plantae.

Table 8.2

	Fungi	Plantae
type of nutrition	.....................................	.....................................
storage polysaccharide	.....................................	.....................................
main component of cell wall	.....................................	.....................................

[3]

(c) Name the domain that contains organisms with a peptidoglycan cell wall.
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(d) Viruses are not included in the three domain classification.
Outline how viruses are classified.
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(a) Describe how gel electrophoresis is used to distinguish between two alleles of a gene. [9]
(b) Outline the advantages of screening for the presence of mutations of the genes for breast cancer, BRCA1 and BRCA2. [6]

(a) Describe the role of chloroplast pigments in light absorption.
(b) Outline how the Calvin cycle produces triose phosphate and outline the conversion of triose phosphate into amino acids.