protein synthesis and mutation

Question 1

A change from Glu to Lys at amino acid 300 had no effect on the rate of reaction catalysed by the enzyme. The same change at amino acid 279 significantly reduced the rate of reaction catalysed by the enzyme.

Use all the information and your knowledge of protein structure to suggest reasons for the differences between the effects of these two changes.

Answer 1

(Both) negatively charged to positively charged change in amino acid;

2. Change at amino acid 300 does not change the shape of the active site

OR

Change at amino acid 300 does not change the tertiary structure OR Change at amino acid 300 results in a similar tertiary structure;

Reference to ‘shape’ of active site only needed once.

3. Amino acid 279 may have been involved in a (ionic, disulfide or hydrogen) bond and so the shape of the active site changes

OR

Amino acid 279 may have been involved in a (ionic, disulfide or hydrogen) bond and so the tertiary structure changed;

OR

Question 2

Describe the role of a ribosome in the production of a polypeptide. Do not include transcription in your answer.

Answer 2

mRNA binds to ribosome;

2. Idea of two codons / binding sites;
3. (Allows) tRNA with anticodons to bind / associate;
4. (Catalyses) formation of peptide bond between amino acids (held by tRNA molecules);
5. Moves along (mRNA to the next codon) / translocation described;

Question 3

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

Answer 3

Helicase;

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

Question 5

Explain how the structure of DNA is related to its functions.

Answer 5

Sugar-phosphate (backbone) / double stranded / helix so provides strength / stability / protects bases / protects hydrogen bonds;

Must be a direct link / obvious to get the mark

Neutral: reference to histones

2. Long / large molecule so can store lots of information;
3. Helix / coiled so compact;

Accept: can store in a small amount of space for ‘compact’

4. Base sequence allows information to be stored / base sequence codes for amino acids / protein;

Accept: base sequence allows transcription

5. Double stranded so replication can occur semi-conservatively / strands can act as templates / complementary base pairing / A-T and G-C so accurate replication / identical copies can be made;
6. (Weak) hydrogen bonds for replication / unzipping / strand separation / many hydrogen bonds so stable / strong;

Question 6

Give two differences between the structure of mRNA and the structure of tRNA.

Answer 6

mRNA longer

OR

Has more nucleotides than tRNA;

2. mRNA is a straight molecule but tRNA is a folded molecule / clover-leaf shaped molecule;
3. mRNA contains no paired bases / hydrogen bonds but tRNA has some paired bases / hydrogen bonds.

Question 7

In the diagram above, the first codon is AUG. Give the base sequence of:

the complementary DNA base sequence __________________________________

the missing anticodon ____

Answer 7

TAC;

UAC.

Question 8

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 8

Substitution would result in CCA / CCC / CCU;

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