Topic 8A: The Control of Gene Expression

Question 1

Describe how mRNA is formed by transcription. [5]

Answer 1

1. Hydrogen bonds (between DNA bases) break;
2. (Only) one DNA strand acts as a template;
3. (Free) RNA nucleotides align by complementary base pairing;
4. (In RNA) uracil is used instead of thymine;
5. RNA polymerase joins (adjacent RNA) nucleotides;
6. (By) phosphodiester bonds (between adjacent nucleotides);
7. Pre-mRNA is spliced (to form mRNA)
   OR
   Introns are removed (to form mRNA)

Question 2

Describe how a polypeptide is formed by translation of mRNA. [6]

Answer 2

1. (mRNA attaches) to ribosome;
2. (tRNA) anticodons (bind to) complementary (mRNA) codons;
3. tRNA brings a specific amino acid;
4. Amino acids joined by peptide bonds;
5. with the use of ATP;
6. tRNA released (after amino acid joined to polypeptide);
7. The ribosome moves along the mRNA to form the polypeptide

Question 3

Explain how a gene mutation can have no effect on an individual. [2]

Answer 3

1. Genetic code is degenerate (so amino acids sequence may not change)
   OR
   Mutation is in an intron (so amino acid sequence may not change)
2. Does change amino acid but no effect in tertiary structure;
3. (New allele) is recessive so does not influence phenotype

Question 4

Explain how a gene mutation can have a positive effect on an individual. [2]

Answer 4

1. Results in change in polypeptide that positively changes the properties (of the protein);
2. May result in increased survival (chances)

Question 5

Explain how a gene mutation can have a negative effect in an individual. [2]

Answer 5

1. Results in change in polypeptide that negatively changes the properties (of the protein);
2. May result in reduced survival (chances)

Question 6

Suggest how a mutation can lead to the production of a protein that has one amino acid missing. [2]

Answer 6

Loss of 3 bases/triplet = 2 marks

Question 7

Suggest how the production of a protein with one amino acid missing may lead to a genetic disorder. [2]

Answer 7

1. Change in tertiary structure;
2. (So) non-functioning protein

Question 8

A mutation in a gene coding for an enzyme could lead to the production of a non-functional enzyme. Explain how. [3]

Answer 8

1. Change in base sequence (of DNA) leading to change in primary structure (of enzyme);
2. Change in hydrogen/ionic/disulphide bonds leading to change in tertiary structure (of enzyme);
3. No enzyme-substrate complexes form