A-level Biology (7401/7402)



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Feversham College



A-level Biology (7401/7402)

Gene Probes, PCR and Stem Cells



Name:

Class:





 

Author:

SJB

Date:




Time:

102 mins

Marks:

91

Comments:




 



Q1.          Plant physiologists attempted to produce papaya plants using tissue culture. They investigated the effects of different concentrations of two plant growth factors on small pieces of the stem tip from a papaya plant. Their results are shown in the table.

 


 

Concentration of
auxin / μmol dm
–3

Concentration of cytokinin / μmol dm–3

 

5

25

50

 

0

No effect

No effect

Leaves produced

 

1

No effect

Leaves produced

Leaves produced

 

5

No effect

Leaves produced

Leaves and some plantlets produced

 

10

Callus produced

Leaves and some plantlets produced

Plantlets produced

 

15

Callus produced

Callus and some leaves produced

Callus and some leaves produced

Callus is a mass of undifferentiated plant cells. Plantlets are small plants.

(a)     Explain the evidence from the table that cells from the stem tip are totipotent.

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(2)
(b)     Calculate the ratio of cytokinin : auxin that you would recommend to grow papaya plants by this method.

 

 



 

 

Answer .........................................................



(2)




(c)     (i)      Papaya plants reproduce sexually by means of seeds. Papaya plants grown from seeds are very variable in their yield. Explain why.

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(2)

(ii)     Explain the advantage of growing papaya plants from tissue culture rather than from seeds.

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(1)

(Total 7 marks)

 


Q2.          (a)     Agrobacterium is a bacterium used in genetic engineering of plants. The diagram shows stages in the transfer of a gene into a plant.


(i)      Name structure X in stage 1.

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(1)

(ii)     In stage 2, explain why the bacteria are cultured before the plant tissue is added.

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(1)

(iii)     In stage 4, explain why the growth medium contains antibiotic.

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(2)

(iv)    Suggest why stages 5 and 6 are necessary for the commercial production of genetically engineered plants.

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(2)

(b)     (i)      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.

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(1)

(ii)     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.

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(1)

(Total 8 marks)

 

Q3.          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.

1 ...................................................................................................................

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2 ...................................................................................................................

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(2)

The doctors mixed the stem cells with viruses. The viruses had been genetically modified to contain alleles of a gene producing full immunity. The doctors then injected this mixture into the child’s bone marrow.

The viruses that the doctors used had RNA as their genetic material. When these viruses infect cells, they pass their RNA and two viral enzymes into the host cells.

(b)     One of the viral enzymes makes a DNA copy of the virus RNA. Name this enzyme.

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(1)

The other viral enzyme 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.

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(2)

(ii)     Some of the children in the trial developed cancer. How might the insertion of the DNA have caused cancer?

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(2)
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