JUNE 2011

Question 1

Give one way in which the structure of an mRNA molecule is different from the structue of a tRNA molecule (1 mark)

Answer 1

1. tRNA molecules contain hydrogen bonds

Question 2

Explain the difference between pre-mRNA and mRNA (1 mark)

Answer 2

1. pre-mRNA contains introns whereas mRNA does not

Question 3

Explain why the percentages of bases from the middle part of the chromosome and the end part are different (2 marks)

Answer 3

1. Different genes present
2. Different base sequences i
3. pre-mRNA transcribed from different DNA base sequence

Question 4

Describe how hormones are different in the cells that they affect (1 mark)

Answer 4

1. Affect cells with only the right receptor

Question 5

Describe how hormones and local chemical mediators reach the cells they affect (2 marks)

Answer 5

1. Hormones in the blood

2. Local chemical mediators are spread by diffusion

Question 6

Synapses are unidirectional. Explain how acetylcholine contributes to the synapse being unidirectional. (2 marks)

Answer 6

1. Ach released from the presynaptic side
2. Diffusion from higher to lower concentration
3. Receptors in the postsynaptic side

Question 7

When a young shoot is illuminated from one side, IAA stimulates growth on the shades side. Explain why growth on the shaded side helps to maintain the leaves in a favourable environment (2 marks)

Answer 7

1. Leaves bend towards the light

2. Light used in photosynthesis

Question 8

Scientists can separate DNA fragments using electrophoresis. Suggest how they can use electrophoresis to estimate the number of base pairs in the separated fragments. (2 marks)

Answer 8

1. Smaller fragments of DNA will move faster

2. Compare the distance moved by each fragment to see how the different fragments contain a different number of bases.

Question 9

Scienists need to take precautions when they carry out restriction mapping. They need to make sure that the enzyme they used has completely digested the DNA. One change they may carry out is to add the sizes of fragment together. How could the scientists use this information to show that the DNA has not been completely digested? Explain your answer. (2 marks)

Answer 9

1. Large pieces of DNA present
2. Add up to more than total length of original DNA
3. Because this would add undigested to total original DNA

Question 10

Papaya plants reproduce sexually by means of seeds. Papaya plants grown from seeds are very variable in their yield. Explain why. (2 marks)

Answer 10

1. Meiosis occurs
2. Crossing over/ Independent segregation occurs
3. Fusion of genetically different gametes/ random fertilisation

Question 11

Explain the advantage of growing papaya plants from tissue culture rather than from seeds (1 mark)

Answer 11

1. Will be genetically identical so all plants will have the same desirable characteristics

Question 12

Explain how an increase in the iguana’s body temperature affects its oxygen consumption when it is at rest (3 marks)

Answer 12

1. Increase in temperature increases the rate of reactions
2. More ATP needed
3. Oxygen consumption increases so more aerobic respiration can occur

Question 13

The units in the table allowed the scientists to compare the oxygen consumption’s of different iguanas. Explain how. (1 mark)

Answer 13

1. Units given per gram

Question 14

The behaviour of the desert iguanas keeps their body temperatures within a narrow range. Explain how. (2 marks)

Answer 14

1. Outside the optimum temperature the iguana moves between sun and shade.
2. To either warm up or cool down

Question 15

At high temperatures, a desert iguana keeps its mouth wide open and breathes in and out rapidly. This is called panting. Explain how panting helps to reduce the body temperature of an iguana. (2 marks)

Answer 15

1. Panting allows evaporation of water from the lining of the mouth
2. Heat transferred from the blood to the outside environment

Question 16

Transcriptional factors are important in the synthesis of particular proteins. Describe how. (2 marks)

Answer 16

1. Bind to the DNA
2. At the promoter sequence
3. Stimulates transcription

Question 17

Explain what causes the siRNA to attach to only one sort of mRNA molecule. (1 mark)

Answer 17

1. siRNA has a complementary base sequence to only one molecule of mRNA

Question 18

Describe and explain how expression of the target gene is affected by siRNA (2 marks)

Answer 18

1. Target gene no longer able to make the specific protein

2. Because the mRNA has been cut into smaller pieces

Question 19

Scientists have suggested that siRNA may be useful in treating some diseases. Suggest why siRNA may be useful in treating disease. (2 marks)

Answer 19

1. Some diseases are genetic/ caused by mutations

2. siRNA will stop the production of proteins coded for by this gene

Question 20

Hydrolysis and condensation are important in the formation of new proteins. Explain how. (2 marks)

Answer 20

1. Hydrolysis of proteins produces amino acids
2. Protein synthesis involves condensation
3. Hydrolysis of polysaccharides to provide energy for protein synthesis

Question 21

Most of the protein stored in the body of a fly larva is a protein called calliphorin. Explain why different adult proteins can be made using calliphorin (1 mark)

Answer 21

1. Amino acids can be rearranged into different sequences