Mark Scheme Answers Flashcards
Suggest how scientists should have treated the control group (2)
- Given only in ….
- Otherwise treated exactly the same way
Suggest and explain two factors which should be considered when deciding the number of mice to be used in an investigation (3)
- Ethical consideration: leads to death/suffering of mice
- Large number to improve repeatability/ reduce sampling error
- Number of mice is related to cost/species availability
The scientists measured the volume of the tumours. Explain the advantage of using volume rather than length to measure the growth of tumours (1)
- Vary in shape/ do not grow uniformly
What information does standard deviation give about the volume of the tumours in an investigation (1)
- Degree of spread/ variation from the mean
Apart from sex and age suggest the thee factors that scientists should have considered when selecting volunteers for a trial (5)
- Lifestyle e.g smoking, diet
- Genetic factors
- Ethnicity
- Health
- Body mass
How can mutations lead to the production of a non-functional protein (6)
- Change in base/ triplet sequence of DNA/ gene
- Changes sequence of codons on mRNA
- Changes sequence of amino acids in primary structure of polypeptide
-Changes the position of hydrogen/ ionic/ disulphide bonds in protein tertiary structure - Changes tertiary structure/ shape of protein and in the case of enzymes, the active site will change shape
- In the case of enzymes, the substrate will be unable to bind to the active site and form an enzyme-substrate complex
What is the role of RNA polymerase in transcription (1)
- Joins adjacent RNA nucleotides via phosphodiester bonds
Describe how alterations to tumour suppressor genes can lead to the development of tumours (5)
- Increased methylation of tumour suppressor genes
- Mutation in tumour suppressor genes
- Tumour suppressor genes are not transcribed/ expressed
- Amino acid sequence/primary structure altered
- Rapid/uncontrollable cell division
Papaya plants reproduce sexually by means of seeds. Papaya plants grown from seeds are very variable in their yield. Explain why (2)
- Meiosis/ independent assortment/ crossing over
- Fusion of genetically identical gametes/ random fertilisation of gametes
Explain the advantage of growing papaya plants from tissue culture rather than from seeds (1) KA-topic
- Will be clones/produced by mitosis/ will be genetically identical/ less variation
Tester one is a steroid hormone that belongs to a group of male sex hormones called androgens.
Steroid hormones are hydrophobic
Explain why steroid hormones can rapidly enter a cell by passing through its cell-surface membrane (2)
- Lipid soluble
- Diffuses through the phospholipid bilayer
The binding of testosterone to an AR changes the shape of the AR. This molecule now enters the nucleus and stimulates gene expression.
Suggest how the AR could stimulate gene expression (3)
- AR is a transcription factor
- Binds to DNA/promoter
- Stimulates RNA polymerase
Explain how the methylation of tumour suppressor genes can lead to cancer (3)
- Methylation prevents transcription of a gene
- Protein not produced that prevents cell division/ causes cell death
- No control of mitosis
Scientists investigated the role of a protein called CENP-W in mitosis. Their method involved cell fractionation and ultracentrifugation
The scientists began by lysing (breaking open) cells and organelles using a detergent that dissolves lipids in water
Suggest how the detergent releases CENP-W from cells (2)
- Cell membranes made from phospholipid
- Detergent dissolves membranes/ phospholipid bilayer
Explain how ultracentrifugation separates CENP-W from other molecules (2)
- Spin (liquid/ supernatant) at very high speed
- Molecules/ CENP-W separates depending on molecular mass/size/ density
Give one way in which benign tumours differ from malignant tumours (1)
For beginning cells don’t break off and spread to other parts in f the body/metastasise, for malignant they do
Explain how siRNA affects gene expression (3)
- siRNA has a specific complementary base sequence to mRNA
- Enzyme associated with siRNA hydrolyses/ destroys mRNA
- Translation of mRNA cannot occur
Explain how the deacetylation of histone proteins can lead to cancer (3)
- Deacetylation of histone proteins causes DNA to condense
- So transcription factors cant bind to promotor on DNA
- RNA polymerase is not activated and transcription does not occur
- Turns off the tumour suppressor gene, so leads to uncontrolled cell division
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)
- Has a (specific) tertiary structure/shape;
Accept in context of AR or testosterone.
Ignore 3D. - (Structures are) complementary;
Reject reference to antigen.
Reject reference to active site, enzyme, substrate
or induced fit
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 (3)
- (AR is) a transcription factor;
Ignore ‘binds to bases’ or ‘binds to gene’. - Binds to DNA/promoter;
Reject reference to active site, enzyme, substrate
or induced fit. - (Stimulates) RNA polymerase;
Explain how increased methylation could lead to cancer (3)
- Methyl groups (could be) added to (both copies of) a tumour
suppressor gene; - The transcription of tumour suppressor genes is inhibited;
- Leading to uncontrolled cell division.
A mutation of a tumour suppressor gene can result in the formation of a tumour.
Explain how. (2)
- (Tumour suppressor) gene inactivated / not able to control / slow
down cell division;
Ignore: references to growth - Rate of cell division too fast / out of control.
Some cancer cells have a receptor protein in their cell-surface membrane that binds to a
hormone called growth factor. This stimulates the cancer cells to divide.
Scientists have produced a monoclonal antibody that stops this stimulation.
Use your knowledge of monoclonal antibodies to suggest how this antibody stops the
growth of a tumour. (3)
- Antibody has specific tertiary structure / binding site / variable
region;
Do not accept explanations involving undefined
antigen - Complementary (shape / fit) to receptor protein / GF / binds to
receptor protein / to GF;
Ignore: same shape as receptor protein / GF - Prevents GF binding (to receptor).
Some human DNA was cut into separate pieces using a restriction enzyme
which produced a staggered cut. A scientist wanted to insert these pieces of DNA
into plasmids and used the same restriction enzyme to cut the plasmids. Explain
why the pieces of human DNA would be able to join to the cut DNA of the plasmids. (2)
Sticky ends / description;
Reference to complementary base-pairing
Which other enzyme must the scientist have added to the mixture to form recombinant plasmids? (1)
Ligase;
A plasmid may be used as a vector. Explain what is meant by a vector.(2)
Carrier of DNA / gene; (context of foreign DNA)
Into cell / other organism / host;
Molecular biologists often use plasmids which contain antibiotic resistance genes.
Explain the reason for this. (2)
Act as marker gene;
Allows detection of cells containing plasmid / DNA;
BRCA 1 and BRCA 2 are human genes that code for tumor suppressor proteins. Mutations in BRCA 1 and BRCA 2 can cause cancer. Explain how? (3)
- Change in DNA base sequence
- Change in amino acid sequence
- Leads to uncontrolled cell division
Blood tests can be used to test for cancers. Men with prostate cancer have a high concentration of prostate-specific antigen (PSA) in their blood. Urinary infections and a naturally enlarged prostate can also increase concentrations of PSA (2)
- High/ increased concentration of PSA could be a false positive
- Could be due to urinary infections
Treatment with drugs might be able to reverse the epigenetic changes that cause cancers. Suggest and explain how? (3)
- Drugs could increase methylation of oncogenes
- Inhibits the transcription of genes
- Drugs could decrease methylation of tumour suppressor genes
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)
- Restriction endonucleases cut plasmid
- Ligase joins gene/ DNA and plasmid
Microinjection of DNA into fertilised egg cells is a frequent method of
producing transgenic fish. However, the insertion of the transferred gene
into nuclear DNA may be delayed. Consequently, the offspring of
transgenic fish may not possess the desired characteristic.
Suggest and explain how delayed insertion of the GH gene could produce
offspring of transgenic fish without the desired characteristic. (2)
- Cell division has occurred (before gene added);
Accept mitosis but reject meiosis. - (Cells producing) gametes do not receive the gene
The scientists used a radioactively labelled DNA probe to show that the cells of the tobacco plant leaves contained the SUT1 gene. Describe how they would do this (5)
- Extract DNA and add restriction endonuclease
- Separate fragments using gel electrophoresis
- Treat DNA to form single strands
- The probe will bind to the SUT1 gene
- Use autoradiography to show the bound probe
Suggest how the production of ‘antisense’ SUT1 mRNA in type A plants
would reduce the expression of the SUT1 gene (4)
- Antisense mRNA is complimentary to ‘sense’ mRNA
- Antisense mRNA would bind to ‘sense’ mRNA
- Ribosomes would not be able to bind
- Preventing/ less transcription of mRNA
explain why the bacteria are cultured before the plant tissue is added (1)
the bacteria divide / grow, producing many copies of desired
gene / plasmid;
OR
the bacteria divide / grow to cover the agar
explain why the growth medium contains antibiotic (2)
- plant tissue that has antibiotic resistance survives;
- identifies plant tissue which has desired gene / plasmid
A toxin that kills insects can be sprayed directly onto the leaves of crop plants. A
gene has now been transferred into crop plants that makes their leaves produce this
toxin.
Explain one advantage to farmers of growing the genetically engineered crop
plants, rather than spraying leaves with the toxin. (1)
e.g. toxin present all the time;
save costs of buying / application of spray;
no spray drift onto other fields / insects;
Suggest one reason why some people are concerned that the toxin gene might get
transferred to wild plants that are related to the crop plants. (1)
e.g. killing of harmless / useful insects that feed on wild plants;
damage to food chains starting with wild plants
Suggest why two different primers are required (1)
because the sequences at the ends of the target sequence
are different / one is at the beginning and one at the end;
Explain why radioactive DNA probes are used to locate specific DNA fragments (2)
- DNA invisible on gel / membrane;
- Allows detection;
(b) Suggest why the scientists used a marker gene and why they used the EGFP gene. (1)
- Not all eggs will successfully take up the plasmid;
- Silkworms that have taken up gene will glow
Lane 1 of Figure 2 enabled the size of the different viral fragments to be
determined.
Suggest and explain how (2)
- (Lane 1 has DNA fragments) of known sizes/lengths;
- Compare (position of viral fragment/s)
Describe how enzymes could be used to insert the GH gene into a
plasmid. (2)
- Restriction endonucleases/enzymes cuts plasmid;
OR
Restriction endonucleases/enzymes produces ‘sticky ends’; - Ligase joins gene/DNA and plasmid
OR
Ligase joins ‘sticky ends’;
Suggest and explain how delayed insertion of the GH gene could produce
offspring of transgenic fish without the desired characteristic. (2)
- Cell division has occurred (before gene added);
Accept mitosis but reject meiosis. - (Cells producing) gametes do not receive the gene;
Accept DNA replication has occurred
Suggest two features of the structure of different proteins that enable them
to be separated by gel electrophoresis. (2)
- Mass/number of amino acids/polypeptides;
Accept weight for mass
Ignore density/size
Accept length of polypeptide/amino acid chain
Accept primary structure /sequence of amino acids.
Accept tertiary structure - Charge;
- R groups (differ);
Doctors can use the method with DNA probes to identify the specific strain
of M. tuberculosis infecting a patient. This is very important when there is
an outbreak of a number of cases of tuberculosis in a city.
Suggest and explain why it is important to be able to identify the specific
strain of M. tuberculosis infecting a patient (6)
- To see if strain is resistant to any antibiotics;
- So can prescribe effective/right antibiotic;
OR
AQA Biology A-Level - DNA probe and Gel Electrophoresis MS PhysicsAndMathsTutor.com - To see whether (any) vaccine works against
this strain/ see which vaccine to use/ to
produce specific vaccine; - (So) can vaccinate potential contacts/to stop
spread;
OR - Can test other people to see if they have the
same strain/ to trace where people caught
TB; - Allowing control of spread of
disease/vaccinate/treat contacts (of people
with same strain) before they get TB;
Mycobacterium tuberculosis causes tuberculosis. The DNA of M. tuberculosis
contains a direct repeat (DR) region. The DR region consists of 43 different,
non-coding base sequences called spacers. Each spacer is found in a specific
place in the DR region.
Name the process by which the base sequence of a spacer is lost
from a DR region (1)
- Deletion mutation
A mutation of a tumour suppressor gene can result in the formation of a tumour.
Explain how. (2)
- (Tumour suppressor) gene inactivated / not able to control / slow
down cell division;
Ignore: references to growth - Rate of cell division too fast / out of control.
Give two characteristic features of stem cells. (2)
- Will replace themselves / keep dividing / replicate;
- Undifferentiated / can differentiate / develop into other cells /
totipotent / multipotent / pluripotent;
Some of the children in the trial developed cancer. How might the insertion of the
DNA have caused cancer? (2)
- Affects tumour suppressor gene;
- Inactivates (tumour suppressor) gene;
- Rate of cell division increased / tumour cells continue to divide
Five out of the 20 children in the trial developed cancer. Although the cancer was treated
successfully, the doctors decided to stop the trial in its early stages. They then reviewed
the situation and decided to continue. Do you agree with their decision to continue?
Explain your answer. (2)
Yes
SCID patients unlikely to survive / quality of life poor unless treated;
Cancer that develops is treatable / only affects 25% / five children;
No
Risk of developing cancer is high / 25%;
Cancer may recur / may not be treated successfully in future / only
short time scale so more may develop cancer;
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)
- (Receptor / transcription factor) binds to promoter which
stimulates RNA polymerase / enzyme X; - Transcribes gene / increase transcription
Oestrogen only affects target cells. Explain why oestrogen does not affect other
cells in the body.(1)
Other cells do not have the / oestrogen / ERα receptor
The jellyfish gene attached to the human Factor IX gene (Stage 2) codes for a
protein that glows green under fluorescent light. Explain the purpose of attaching
this gene. (2)
- (Acts as a) marker gene to show that the (human) gene
has been taken up / expressed; - Accept: gene marker
- (Only) implant cells / embryos that show fluorescence /
contain the jellyfish gene;
The promoter DNA from sheep (Stage 3) causes transcription of genes coding for
proteins found in sheep milk.
Suggest the advantage of using this promoter DNA. (2)
- Factor IX present in / extracted from milk;
- Gene only expressed in mammary glands / udder / gene
not expressed elsewhere; - Ignore references to milk
The ‘only’ aspect is important here. - Do not need to kill sheep (to obtain Factor IX)
Many attempts to produce transgenic animals have failed. Very few live births result from
the many embryos that are implanted.
(i) Suggest one reason why very few live births result from the many embryos that are
implanted. (2)
- Mutation / nucleus / chromosomes / DNA may be
damaged / disrupts genes; - Neutral: cell may be damaged
- May interfere with proteins (produced) / gene expression /
translation;
Ignore references to hormone levels or time of
implantation
OR - Embryo / antigens foreign;
- Neutral: antigens change
- Embryo is rejected / attacked by immune system;
The DNA probe the geneticist used was for an exon in the DNA, not an intron. Explain
why (3)
- Introns not translated / not in mRNA / (exons) code for amino
acids / introns do not code for amino acids; - Accept not expressed
- Accept polypeptide / protein for amino acids
- Mutations of these (exons) affect amino acid sequences (that
produce) faulty protein / change tertiary structure of protein; - Accept deletion leads to frameshift
- In this context, accept affects protein made
- So important to know if parents’ exons affected, rather than any
other part of DNA / introns;
To make the DNA probe, the geneticist had to find the base sequence of the normal gene.
Once he had copies of the gene, what methods would he use to find the base sequence
of the gene? (2)
To make the DNA probe, the geneticist had to find the base sequence of the normal gene.
Once he had copies of the gene, what methods would he use to find the base sequence
of the gene? (2)
