T8 The control of Gene expression

Question 1

Increased methylation of the promoter region of a tumour suppressor gene causes one type of human throat cancer.
In this type of throat cancer, cancer cells are able to pass on the increased methylation to
daughter cells. The methylation is caused by an enzyme called DNMT.
Scientists have found that a chemical in green tea, called EGCG, is a competitive inhibitor of
DNMT. EGCG enables daughter cells to produce messenger RNA (mRNA) from the tumour suppressor gene. Suggest how EGCG allows the production of mRNA in daughter cells. 3 marks

Answer 1

1. (EGCG) binds to active site of DNMT;
   Ignore active site changes shape
   Ignore ‘forms enzyme-substrate’ complex
2. (DNMT) cannot methylate (promoter region of tumour suppressor gene);
3. Transcription(al) factor(s) can bind (to promoter region);
4. RNA polymerase (stimulated/activated);
   Accept less methylation (of promoter region/tumour suppressor
   gene)

Question 2

Myelodysplastic syndromes (MDS) are a group of malignant cancers. In MDS, the bone marrow does not produce healthy blood cells.
Haematopoietic stem cell transplantation (HSCT) is one treatment for MDS. In HSCT, the patient receives stem cells from the bone marrow of a person who does not have MDS. Before the
treatment starts, the patient’s faulty bone marrow is destroyed.
2.
(a) For some patients, HSCT is an effective treatment for MDS.
Explain how. 3 marks

Answer 2

1. Produce healthy (blood) cells;
   Accept produce ‘normal’ /non-MDS cells.
2. No MDS/faulty/cancerous (blood) cells;
   Produce only healthy/normal (blood) cells = two marks.
   Accept no (cancerous) tumour.
3. Stem cells divide/replicate;
   Ignore reference to totipotent/pluripotent/ multipotent/unipotent
   Accept ‘clone’ for divide.

Question 3

MDS can develop from epigenetic changes to tumour suppressor genes. In some patients,
the drug AZA has reduced the effects of MDS. AZA is an inhibitor of DNA
methyltransferases. These enzymes add methyl groups to cytosine bases.
Suggest and explain how AZA can reduce the effects of MDS in some patients. 3 marks

Answer 3

1. (AZA) reduces methylation (of DNA/cytosine/gene);
   Reject any reference to mutation.
2. (Tumour suppressor) gene is transcribed/expressed;
   Accept mRNA produced for transcription/transcribed.
   Ignore gene is ‘switched on’ or activated but allow protein is formed.
3. Prevents rapid/uncontrollable cell division
   OR
   Cell division can be controlled/stopped/slowed;
   Ignore growth.

Question 4

The control patients of cancer experiments were treated with conventional drugs.
Give a reason why. 1 mark

Answer 4

2. Unethical not to treat (control group);

Question 5

Explain how the methylation of tumour suppressor genes can lead to cancer. 3 marks

Answer 5

1. Methylation prevents transcription of gene;
2. Protein not produced that prevents cell division / causes cell death / apoptosis;
3. No control of mitosis.

Question 6

The scientists used tumour area as an indicator of tumour size.
Explain why tumour area may not be the best indicator of tumour size and suggest a more
reliable measurement. 2 marks

Answer 6

1. Tumours may be different depths / area does not take depth into account /
   tumours are 3-D / are not 2-D;
   Neutral: different sizes
   Accept: height / thickness for depth
2. (Measure) tumour volume / mass / weight.

Question 7

Describe how alterations to tumour suppressor genes can lead to the development of tumours. 3 marks

Answer 7

1. (Increased) methylation (of tumour suppressor genes);
   Accept abnormal methylation or hypermethylation
   Ignore decreased acetylation of histones
2. Mutation (in tumour suppressor genes);
3. Tumour suppressor genes are not transcribed/expressed
   OR
   Amino acid sequence/primary structure altered;
   Accept mRNA for transcription/transcribed
   Accept tertiary structure altered
   Accept different amino acid
   Ignore reference to protein not being formed
4. (Results in) rapid/uncontrollable cell division;
   Accept cell division cannot be regulated
   Ignore growth

Question 8

MM is a skin cancer

MM is caused by a faulty receptor protein in cell-surface membranes.
Cells in MM tumours can be destroyed by the immune system.
Suggest why they can be destroyed by the immune system. 4 marks

Answer 8

1. Faulty protein recognised as an antigen / as a ‘foreign’ protein;
2. T cells will bind to faulty protein / to (this) ‘foreign’ protein;
3. (Sensitised) T cells will stimulate clonal selection of B cells;
4. (Resulting in) release of antibodies against faulty protein.

Question 9

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. 3 marks

Answer 9

1.   Change in (sequence of) amino acid(s)/primary structure;
Reject amino acids are formed.
Reject amino acids code.
2. Change in hydrogen/ionic/disulfide bonds;
3. Alters tertiary/30
structure;
Reject active site.
Ignore quaternary.
Ignore 3D.

Question 10

What is a substitution mutation? 1 mark

Answer 10

1. Replacement of a base by a different base (in DNA);

Question 11

SCD: Sickle cell disease

Haematopoietic stem cell transplantation (HSCT) is a long-term treatment for SCD. In HSCT, the
patient receives stem cells from the bone marrow of a person who does not have SCD. The
donor is often the patient’s brother or sister. Before the treatment starts, the patient’s faulty bone
marrow cells have to be destroyed.
(b)  Use this information to explain how HSCT is an effective long-term treatment for SCD. 3 marks

Answer 11

1.   Produce healthy (red blood) cells
OR
Produce (normal) polypeptide/haemoglobin;
Produce only healthy (red blood) cells is only equivalent
to mark point 1.
Accept produce ‘normal’/non-SCD cells.
Ignore type of stem cell e.g. pluripotent.
2.   No sickle/faulty/SCD (red blood) cells (produced)
OR
No defective polypeptide/haemoglobin;
3.   Stem/marrow cells (continuously) divide/replicate
OR
Less chance of rejection (from brother/sister);
Differentiate is not equivalent to divide/replicate.
Ignore type of stem cell e.g. pluripotent.

Question 12

Suggest how gel electrophoresis separated the proteins obtained from the synapses. 2 marks

Forget that they’re from the synapses. How do they separate

Answer 12

1. (Depends on) size / mass (of protein);
2. (Depends on) charge (of protein);
   Accept for 2 marks ‘Smaller / more highly charged move further’

Question 13

Each type of labelled antibody binds specifically to one of the proteins.
Explain why. 3 marks

Answer 13

1. Each protein has a different tertiary structure;
2. (Each) antibody has a specific antigen / binding / variable region / site;
3. So, (each antibody) forms different antigen-antibody complex
   OR
   (each antibody) only binds to complementary (protein);

Question 14

Each year, a few people with type I diabetes are given a pancreas transplant. Pancreas transplants are not used to treat people with type II diabetes.
Give two reasons why pancreas transplants are not used for the treatment of type II diabetes. 2 marks

Answer 14

1. (Usually)Type II produce insulin;
2. Cells / receptors less sensitive / responsive (to insulin)
   OR
   Faulty (insulin) receptors;
3. (Treated / controlled by) diet / exercise;
4. Accept: cells / receptors do not respond.
5. Accept: ‘fewer receptors’
6. Accept: (Treated / controlled by) weight loss / medication /
   drugs.
7. Ignore: diabetes is caused by diet / exercise.

Question 15

The pancreas produces the hormone insulin.
Put a tick (✔) in the box next to the statement which describes incorrectly the action of
insulin. 1 mark

Activates enzymes involved in the conversion of glucose to glycogen.

Controls the uptake of glucose by regulating the inclusion of channel proteins in the surface membranes of target cells.

Attaches to receptors on the surfaces of target cells.

Activates enzymes involved in the conversion of glycerol to glucose.

Answer 15

Box 4

Question 16

Scientists investigated the use of induced pluripotent stem cells (iPS cells) to treat type I diabetes in mice. The scientists used four transcription factors to reprogramme skin cells to form iPS cells. The scientists then stimulated the in vitro differentiation of iPS cells into
pancreatic cells.

Suggest how transcription factors can reprogramme cells to form iPS cells 2 marks

Answer 16

1. Attach to gene / DNA / promoter region;
2. Stimulate / inhibit transcription / RNA polymerase;
   Note: Genes being expressed / inhibited or switched on / off is not
   enough on its own.

Question 17

Scientists have investigated the use of different types of stem cell to treat damage to the heart
after a myocardial infarction. During a myocardial infarction, a number of different cell types in the
heart die. This includes cardiomyocytes which are heart-muscle cells.
Embryonic pluripotent stem cells (ESCs) can divide and differentiate into a wide range of different
cell types.
3.
(a) Using the information given, suggest one reason why ESCs might be suitable to treat damage to the heart. 1 mark

Answer 17

1. (ESCs) can replace any type of (heart) cell;
   Accept named type of cell, e.g. heart muscle cell

Question 18

The scientists used an optical microscope to measure the number of capillaries in thin
sections cut from samples of heart muscle.
Describe the method they would have used to find the mean number of capillaries
per mm2. 4 marks

Answer 18

1. Measure diameter of field of view and calculate area;
2. Using micrometer slide and eyepiece graticule;
   Accept descriptions
3. Count number of capillaries in large number of fields of view and calculate mean;
4. Select fields of view randomly

Question 19

SCID is a severe inherited disease. People who are affected have no immunity. Doctors carried out a trial using gene therapy to treat children with SCID. The doctors who carried out the trial obtained stem cells from each child’s umbilical cord.
(a) Give two characteristic features of stem cells. 2 marks

Answer 19

Will replace themselves / keep dividing / replicate;
Undifferentiated / can differentiate / develop into other cells / totipotent / multipotent /
pluripotent;
Accept tissues

Question 20

One of the viral enzymes makes a DNA copy of the virus RNA. Name this enzyme. 1 mark

Answer 20

Reverse transcriptase;
Allow phonetic spelling

Question 21

The other viral enzyme apart from reverse transcriptase is called integrase. Integrase inserts the DNA copy anywhere in the DNA
of the host cell. It may even insert the DNA copy in one of the host cell’s genes.
(c) (i) The insertion of the DNA copy in one of the host cell’s genes may cause the cell to make a non-functional protein. Explain how. 2 marks

Answer 21

Alters base / nucleotide sequence / causes frame shift;
Different sequence of amino acids in polypeptide / protein / primary structure
alters the tertiary structure;
Accept any reference, such as adding bases, to changing the base
sequence of the gene. Reject deletion / substitution.
Idea of sequence essential so not makes different amino acids.
Accept answers involving stop / start codons and effect on protein.

Question 22

One method of transferring RNAi molecules into cells involves combining these molecules with a lipid. Suggest why this increases uptake of RNAi molecules into cells. 1 mark

Answer 22

(Cell/membrane has a) phospholipid bilayer
OR
No channel/carrier protein (for uptake)
OR
No need for channel/carrier protein (for uptake);

Question 23

Define what is meant by epigenetics. 2 marks

Answer 23

1. Heritable changes in gene function;
2. Without changes to the base sequence of DNA;

Question 24

Give one way in which benign tumours differ from malignant tumours. 1 mark

Answer 24

Cells of benign tumours cannot spread to other parts of the body / metastasise;
OR
Cells of benign tumours cannot invade neighbouring tissues.
Accept answers clearly in the context of malignant tumours.

Question 25

Oestrogen is a hormone that affects transcription. It forms a complex with a receptor in the cytoplasm of target cells. Explain how an activated oestrogen receptor affects the target cell. 2 marks

Answer 25

(Receptor / transcription factor) binds to promoter which stimulates RNA polymerase / enzyme X; Transcribes gene / increase transcription;

Question 26

Oestrogen only affects target cells. Explain why oestrogen does not affect other cells in the body. 1 mark

Answer 26

Other cells do not have the / oestrogen / ERα receptors; But do not accept receptors in general.

Question 27

Testosterone is a steroid hormone that belongs to a group of male sex hormones called androgens. (a) Steroid hormones are hydrophobic. Explain why steroid hormones can rapidly enter a cell by passing through its cell-surface membrane. 2 marks

Answer 27

1. Lipid soluble; Ignore ‘not water soluble’ or ‘fat soluble’. 2. (Diffuse through) phospholipid (bilayer); Ignore reference to joining to receptors/channels/carriers but reject passage through protein channels/carriers

Question 28

In the cytoplasm, testosterone binds to a specific androgen receptor (AR). An AR is a protein. Suggest and explain why testosterone binds to a specific AR. 2 marks

Answer 28

1. Has a (specific) tertiary structure/shape; Accept in context of AR or testosterone. Ignore 3D. 2. (Structures are) complementary; Reject reference to antigen. Reject reference to active site, enzyme, substrate or induced fit.

Question 29

The binding of testosterone to an AR changes the shape of the AR. This AR molecule now enters the nucleus and stimulates gene expression. Suggest how the AR could stimulate gene expression. 2 marks | AR: Androgen receptor

Answer 29

1. (AR is) a transcription factor; Ignore ‘binds to bases’ or ‘binds to gene’. 2. Binds to DNA/promoter; Reject reference to active site, enzyme, substrate or induced fit. 3. (Stimulates) RNA polymerase;

Question 30

Describe and explain how the polymerase chain reaction (PCR) is used to amplify a DNA fragment. 4 marks

Answer 30

1. (Requires DNA fragment) DNA polymerase, (DNA) nucleotides and primers; 2. Heat to 95 °C to break hydrogen bonds (and separate strands); Accept temperature in range 90 to 95 °C. 3. Reduce temperature so primers bind to DNA/strands; Accept temperature in range 40 to 65 °C. 4. Increase temperature, DNA polymerase joins nucleotides (and repeat method); Accept Taq polymerase for DNA polymerase. Accept temperature in range 70 to 75 °C.

Question 31

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. 2. (a) 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. 5 marks

Answer 31

1. Extract DNA and add restriction endonucleases/restriction enzymes; 2. Separate fragments using electrophoresis; 3. (Treat DNA to) form single strands OR (Treat DNA to) expose bases; Ignore method used to separate strands 4. The probe will bind to/hybridise/base pair with the SUT1/gene; 5. Use autoradiography (to show the bound probe); Accept use photographic or X ray film (to show the bound probe) X rays alone is not sufficient

Question 32

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. 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.

Answer 32

1. Antisense mRNA is complementary to 'sense' mRNA; 2. Antisense mRNA would bind/base pair to (sense) mRNA; OR Double stranded (m)RNA forms; 3. Ribosomes would not be able to bind; 4. Preventing/less translation (of mRNA) OR Preventing/less production of SUT1 (protein); Accept descriptions of translation

Question 33

What is a DNA probe? 2 marks

Answer 33

1.   (Short) single strand of DNA; 2.   Bases complementary (with DNA/allele/gene);

Question 34

Describe how the DNA is broken down into smaller fragments. 2 marks

Answer 34

1. Restriction endonuclease/enzyme; 2. (Cuts DNA at specific) base sequence OR (Breaks) phosphodiester bonds OR (Cuts DNA) at recognition/restriction site; Accept palindromic sequence.

Question 35

Explain the role of reverse transcriptase in RT-PCR. 1 mark

Answer 35

Produces (c)DNA using (m)RNA; Accept: ‘converts’ (m)RNA to (c)DNA. Reject: tRNA

Question 36

Explain the role of DNA polymerase in RT-PCR. 1 mark

Answer 36

Joins nucleotides to produce (complementary strand/s of) DNA; Accept: ‘joins DNA nucleotides’.

Question 37

Suggest one reason why DNA replication stops in the polymerase chain reaction. 1 mark

Answer 37

Limited number of primers / nucleotides; Accept: DNA polymerase (eventually) denatures Accept: primers / nucleotides ‘used up’.

Question 38

Scientists have used the RT-PCR method to detect the presence of different RNA viruses in patients suffering from respiratory diseases. The scientists produced a variety of primers for this procedure. Explain why. 2 marks

Answer 38

1. Base sequences differ; 2. (Different) complementary primers required; 1. Accept: reference to either RNA or DNA base sequences but reject reference to DNA base sequence in viruses.

Question 39

# ``` A scientist produced transgenic zebrafish. She obtained a gene from silverside fish. The gene codes for a growth hormone (GH). She inserted copies of this GH gene into plasmids. She then microinjected these recombinant plasmids into fertilised egg cells of zebrafish. (a) Describe how enzymes could be used to insert the GH gene into a plasmid. 2 marks

Answer 39

1. Restriction endonucleases/enzymes cuts plasmid; OR Restriction endonucleases/enzymes produces ‘sticky ends’; Ignore restriction enzymes cuts out the gene. Reject restriction enzymes cuts the gene. 2. Ligase joins gene/DNA and plasmid OR Ligase joins ‘sticky ends’;