DNA and protein synthesis

Question 1

A mutation of a tumour suppressor gene can result in the formation of a tumour. Explain how.

Answer 1

(Tumour suppressor) gene inactivated / not able to control / slow down cell division;

Rate of cell division too fast / out of control.

Question 2

Not all mutations result in a change to the amino acid sequence of the encoded polypeptide.
Explain why.

Answer 2

(Genetic) code degenerate;

Mutation in intron.

Question 3

Not all mutations result in a change to the amino acid sequence of the encoded polypeptide.
Explain why.

Answer 3

Antibody has specific tertiary structure / binding site / variable region;
Complementary (shape / fit) to receptor protein / GF / binds to receptor protein / to GF;

Question 4

Deletion of the sixth base (G) in the sequence shown in the diagram above would change the nature of the protein produced but substitution of the same base would not. Use the information in the table and your own knowledge to explain why.

Answer 4

Substitution would result in CCA / CCC / CCU;
(All) code for same amino acid / proline;
Deletion would cause frame shift / change in all following codons / change next codon from UAC to ACC.

Question 5

Messenger RNA (mRNA) is used during translation to form polypeptides. Describe how mRNA is produced in the nucleus of a cell.

Answer 5

Helicase;
Breaks hydrogen bonds;
Only one DNA strand acts as template;
RNA nucleotides attracted to exposed bases; (Attraction) according to base pairing rule;
RNA polymerase joins (RNA) nucleotides together; Pre-mRNA spliced to remove introns.

Question 6

Describe the structure of proteins.

Answer 6

Polymer of amino acids;
Joined by peptide bonds;
Formed by condensation;
Primary structure is order of amino acids;
Secondary structure is folding of polypeptide chain due to hydrogen bonding;

Tertiary structure is 3-D folding due to hydrogen bonding and ionic / disulfide bonds;
Quaternary structure is two or more polypeptide chains.

Question 7

Describe how proteins are digested in the human gut.

Answer 7

Hydrolysis of peptide bonds;
Endopeptidases break polypeptides into smaller peptide chains; Exopeptidases remove terminal amino acids;
Dipeptidases hydrolyse / break down dipeptides into amino acids.

Question 8

Mitochondrial disease (MD) often causes muscle weakness (lines 1–3). Use your knowledge of respiration and muscle contraction to suggest explanations for this effect of MD.Suggest how the change in the anticodon of a tRNA leads to MD (lines 10–13).

Answer 8

Reduction in ATP production by aerobic respiration;
Less force generated because fewer actin and myosin interactions in muscle;
Fatigue caused by lactate from anaerobic respiration.

Question 9

Suggest how the change in the anticodon of a tRNA leads to MD (lines 10–13).

Answer 9

Change to tRNA leads to wrong amino acid being incorporated into protein;
Tertiary structure (of protein) changed;
Protein required for oxidative phosphorylation / the Krebs cycle, so less / no ATP made.

Question 10

If someone has MD, the concentration of lactate in their blood after exercise is usually much higher than normal (lines 15–17). Suggest why.

Answer 10

Mitochondria / aerobic respiration not producing much / any ATP; (With MD) increased use of ATP supplied by increase in anaerobic respiration;
More lactate produced and leaves muscle by (facilitated) diffusion.

Question 11

A small amount of DNA can be extracted from mitochondria and DNA sequencing used to try to find a mutation (lines 18–19).
From this sample:
• how would enough DNA be obtained for sequencing?
• how would sequencing allow the identification of a mutation?

Answer 11

Enough DNA using PCR;

Compare DNA sequence with ‘normal’ DNA.