2 The usual way of genetically modifying bacteria is to insert a plasmid containing the desired gene into them.
3 The steps involved in the production of bacteria capable of synthesising human insulin are:
identifying the human insulin gene;
isolating mRNA and making cDNA using reverse transcriptase;
cloning the DNA using DNA polymerase;
inserting the DNA into a plasmid vector using restriction enzymes and DNA ligase;
inserting the plasmid vector into the host bacterium;
identifying the genetically modifi ed bacteria;
cloning the bacteria; and harvesting the human insulin.
4 The main advantage of treating diabetics with human insulin produced by gene technology is that it is chemically identical to the insulin that they would have produced had they not been diabetic. It also avoids any ethical issues that may arise from the use of insulin derived from an animal.
5 A promoter must be inserted along with the gene, because bacteria will not express a gene unless a suitable promoter is present.
6 The bacteria that have taken up the gene can be identifi ed using resistance to antibiotics or the presence of a fluorescing protein as markers.
7 Gene technology can provide benefits in, for example, agriculture, medicine and industry, but has the associated risk of the escape of the gene concerned into organisms other than the intended host. The risk is seen to be particularly high for genetically modified crops that are released into the environment to grow.
8 The social implications of gene technology are the benefi cial or otherwise effects of the technology on human societies.
9 Ethics are sets of standards by which a particular group of people agree to regulate their behaviour, distinguishing an acceptable from an unacceptable activity. Each group must decide, first, whether research into gene technology is acceptable, and then whether or not it is acceptable to adopt the successful technologies.
10 Electrophoresis is a technique that can be used to separate lengths of DNA (or RNA or proteins) of different sizes by applying an electric current to them. Small fragments move faster and therefore further than large ones, and can be made visible using radioactive labels or fluorescent compounds. Electrophoresis is used in genetic profi ling (genetic fingerprinting) and in DNA sequencing.
11 Cystic fibrosis is a genetic disease caused by a recessive allele of the gene that codes for the production of a chloride transporter protein called CFTR. People with two copies of the recessive allele produces thick, sticky mucus in their lungs, pancreas and reproductive organs.
12 Several attempts have been made to insert normal alleles of the CFTR gene into people with cystic fibrosis, a process called gene therapy. So far, there has been only limited success, because it is difficult to get the alleles into the cells. Even when this is successful, it needs to be repeated at frequent intervals because the cells have a very short natural lifespan.
13 Genetic screening involves testing people to find out if they carry any faulty alleles for genes that can cause disease. Genetic counsellors may help people who find that they have a disease-causing allele, or that their unborn child has, to make a decision about how to act on this information.
9 Ethics are sets of standards by which a particular group of people agree to regulate their behaviour, distinguishing an acceptable from an unacceptable activity. Each group must decide, first, whether research into gene technology is acceptable, and then whether or not it is acceptable to adopt the successful technologies.
10 Electrophoresis is a technique that can be used to separate lengths of DNA (or RNA or proteins) of different sizes by applying an electric current to them. Small fragments move faster and therefore further than large ones, and can be made visible using radioactive labels or fluorescent compounds. Electrophoresis is used in genetic profi ling (genetic fingerprinting) and in DNA sequencing.
11 Cystic fibrosis is a genetic disease caused by a recessive allele of the gene that codes for the production of a chloride transporter protein called CFTR. People with two copies of the recessive allele produces thick, sticky mucus in their lungs, pancreas and reproductive organs.
12 Several attempts have been made to insert normal alleles of the CFTR gene into people with cystic fibrosis, a process called gene therapy. So far, there has been only limited success, because it is difficult to get the alleles into the cells. Even when this is successful, it needs to be repeated at frequent intervals because the cells have a very short natural lifespan.
13 Genetic screening involves testing people to find out if they carry any faulty alleles for genes that can cause disease. Genetic counsellors may help people who find that they have a disease-causing allele, or that their unborn child has, to make a decision about how to act on this information.
1. End-of-chapter questions
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
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'
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 Positivelycharged 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.
4 The table shows enzymes that are used in gene technology. Copy and complete the table to show the role of each enzyme.
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).
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.
6 a Genetic fingerprinting reveals the differences in variable number tandem repeats (VNTRs) in the DNA of different individuals. Explain what is meant by a VNTR. [3]
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 actual father of the child. [3]
b Explain what is meant by:
i gene therapy [1]
ii genetic screening [1]
iii genetic counselling. [1]
A cloning DNA
B cutting open a plasmid vector
C producing cDNA from mRNA
D reforming the DNA double helix
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'
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 Positivelycharged fragments of DNA move to the anode.
C Small negatively charged fragments of DNA move rapidly to the cathode.
4 The table shows enzymes that are used in gene technology. Copy and complete the table to show the role of each enzyme.
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).
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.
6 a Genetic fingerprinting reveals the differences in variable number tandem repeats (VNTRs) in the DNA of different individuals. Explain what is meant by a VNTR. [3]
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 actual father of the child. [3]
[Total: 6]
7 a Copy and complete the table to explain the roles of primers and dideoxynucleotides in DNA sequencing.
[3]
b Explain what is meant by:
i gene therapy [1]
ii genetic screening [1]
iii genetic counselling. [1]
c 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]
[Total: 11]
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]
b State the probability of one of the children of these parents suffering from cystic fibrosis. [1]
[Total: 4]
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] ii Use the letter E to indicate the external face of the membrane. State how you identified this face. [1]
b Cystic fibrosis is caused by a recessive allele of the CFTR gene.
i Explain the meaning of the term recessive allele. [2]
ii Explain how cystic fibrosis affects the function of the lungs. [3]
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]
d In some people with cystic fibrosis, the allele has a single-base mutation which produces a 'nonsense' (stop) codon within the gene.
i Expalin how this mutation would prevent normal CFTR protein being produced. [2]
ii A new type of drug, PTCI24, 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 allover 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]
[Total: 15]
[Cambridge International AS and A Level Biology 9700/04, Question 2, October/November 2008]
2. End-of-chapter answers
1 B
2 A
3 D
Exam-style questions
2 A
3 D
Exam-style questions
6 a VNTR: a short length of highly repetitive DNA;
number of repeats and hence lengths of repeats diff er markedly in diff erent individuals; inherited: half VNTRs from father, half from mother;
only identical twins have the same VNTRs; [max. 3]
b 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; [3]
[Total: 6]
b i gene therapy: treatment of a genetic disorder by altering the patient’s genotype; [1]
ii genetic screening: determination of a person’s genotype using karyotype analysis for chromosome mutations and probes for identifying particular alleles; [1]
iii genetic counselling: a service that seeks to explain the nature of genetic disorders and probability of their transmission; [1]
c 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; [max. 5]
[Total: 11]
8 a
correct gametes; correct genotypes; correct phenotypes; [3]
b 1 in 4/0.25/25%; [1]
[Total: 4]
9 a i chloride channel; chloride moves out of cell by active transport; [2]
ii upper face because of presence of carbohydrate chains; [1]
b i allele: variant form of a gene; recessive: only aff ects phenotype when dominant allele is not present; [2]
ii thick, sticky mucus produced; mucus accumulates; reduced gas exchange; more infections; [max. 3]
c normal dominant CFTR allele added to viral DNA;
virus inserts DNA into cell; [2]
d i translation stopped at ‘stop’ codon; protein chain not completed; [2]
ii comparisons include: drug easily taken up by cells, whereas therapy is poorly taken up;
drug taken orally, whereas therapy must be inhaled into lungs;
no vector needed for drug, whereas virus vector for therapy may cause side-eff ects; [3]
[Total: 15]
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