#165 Summary of Genetic technology

 1 Gene technology involves altering the genes in an organism, which is then said to be genetically modifi ed.

 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.

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]

[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

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]