19.10 End of Chapter Questions
Question 1 – 3
1. Different enzymes are used in the various steps involved in the production of bacteria capable of synthesising a human protein. Which step is catalysed by a restriction enzyme?
A cloning DNA
B cutting open a plasmid vector
C producing cDNA from mRNA
D reforming the DNA double helix [1]
B;
2. What describes a promoter?
A a length of DNA that controls the expression of a gene
B a piece of RNA that binds to DNA to switch off a gene
C a polypeptide that binds to DNA to switch on a gene
D a triplet code of three DNA nucleotides that codes for ‘stop’ [1]
A;
3. Which statement correctly describes the electrophoresis of DNA fragments?
A Larger fragments of DNA move more rapidly to the anode than smaller fragments.
B Positively charged fragments of DNA move to the anode.
C Small negatively charged fragments of DNA move rapidly to the cathode.
D Smaller fragments of DNA move more rapidly than larger fragments. [1]
D;
Question 4
The table shows enzymes that are used in gene technology. Copy and complete the table to show the role of each enzyme. [4]
| Enzyme | Role |
| DNA ligase | |
| DNA polymerase | |
| restriction enzymes | |
| reverse transcriptase |
| Feature: | Role: |
| DNA ligase | joining gaps in sugar–phosphate chains of DNA |
| DNA polymerase | replicates DNA |
| restriction enzymes | cut DNA at specific sites |
| reverse transcriptase | make cDNA from mRNA |
Question 5
Rearrange the statements below to produce a flow diagram showing the steps involved in producing bacteria capable of synthesising a human protein such as human growth hormone (hGH). [4]
1 Insert the plasmid into a host bacterium.
2 Isolate mRNA for hGH.
3 Insert the DNA into a plasmid and use ligase to seal the ‘nicks’ in the sugar–phosphate chains.
4 Use DNA polymerase to clone the DNA.
5 Clone the modified bacteria and harvest hGH.
6 Use reverse transcriptase to produce cDNA.
7 Use a restriction enzyme to cut a plasmid vector.
2, 6, 4, 7, 3, 1, 5
Question 6
a. Genetic fingerprinting reveals the diff erences in variable number tandem repeats (VNTRs) in the DNA of different individuals. Explain what is meant by a VNTR. [3]
VNTR: a short length of highly repetitive DNA;
number of repeats and hence lengths of repeats differ markedly in different individuals;
inherited: half VNTRs from father, half from mother;
only identical twins have the same VNTRs;
b. Examine the figure, which shows diagrammatic DNA profiles of a mother, her child and a possible father of the child.
Decide, giving your reasons, whether the possible father is the biological father of the child. [3]
four bands in child’s profile match four of the bands in the mother’s;
the other four bands match four bands in the father’s profile;
the possible father is the actual father;
Question 7
Explain what is meant by:
i gene therapy [1]
gene therapy: treatment of a genetic disorder by altering the patient’s genotype;
ii genetic screening. [1]
genetic screening: determination of a person’s genotype using karyotype analysis for chromosome mutations and probes for identifying particular alleles;
b. Explain why it is easier to devise a gene therapy for a condition caused by a recessive allele than for one caused by a dominant allele. [5]
when a genetic disorder is caused by a recessive allele, the ‘normal’ allele is dominant; adding a dominant allele allows
some correct product to be made;
the individual effectively becomes heterozygous;
the recessive allele may code for defective product or no product;
production of some correct product may cure the disorder;
adding a recessive allele cannot block a faulty dominant allele;
Question 8
a. Draw a genetic diagram to show how two heterozygous parents may produce a child with cystic fibrosis.
Use the symbols A/a in your diagram. [3]
| Gametes | A | a |
| A | AA normal | Aa normal, but with cystic fibrosis trait |
| a | Aa normal, but with cystic fibrosis trait | aa cystic fibrosis |
correct gametes;
correct genotypes;
correct phenotypes;
b. State the probability of one of the children of these parents suffering from cystic fibrosis. [1]
1 in 4 / 0.25 / 25%;
Question 9
The figure shows the CFTR (cystic fibrosis transmembrane conductance regulator) protein in a cell surface membrane.
a. i Describe the normal function of the CFTR protein. [2]
chloride channel;
chloride moves out of cell by active transport;
ii Use the letter E to indicate the external face of the membrane. State how you identified this face. [1]
upper face because of presence of carbohydrate chains;
b. Cystic fibrosis is caused by a recessive allele of the CFTR gene.
i Explain the meaning of the term recessive allele. [2]
allele: variant form of a gene;
recessive: only affects phenotype when dominant allele is not present;
ii Explain how cystic fibrosis affects the function of the lungs. [3]
thick, sticky mucus produced;
mucus accumulates;
reduced gas exchange;
more infections;
c. As cystic fibrosis is caused by a recessive allele of a single gene, it is a good candidate for gene therapy.
Trials were undertaken in the 1990s, attempting to deliver the normal allele of the CFTR gene into cells of
the respiratory tract, using viruses or liposomes as vectors. Explain how viruses deliver the allele into cells. [2]
normal dominant CFTR allele added to viral DNA;
virus inserts DNA into cell;
d. In some people with cystic fibrosis, the allele has a single-base mutation which produces a ‘nonsense’ (stop) codon
within the gene.
i Explain how this mutation would prevent normal CFTR protein being produced. [2]
translation stopped at ‘stop’ codon;
protein chain not completed;
ii A new type of drug, PTC124, enables translation to continue through the nonsense codon. Trials in mice homozygous for a CFTR allele containing the nonsense codon have found that animals treated with PTC124 produce normal CFTR protein in their cells. The drug is taken orally and is readily taken up into cells all over the body.
Using your knowledge of the progress towards successful gene therapy for cystic fibrosis, suggest why PTC124 could be a simpler and more reliable treatment for the disease. [3]
comparisons include: drug easily taken up by cells, whereas therapy is poorly taken up;
drug taken orally, whereas therapy must be inhaled into lungs;