Protein synthesis

Question 1

Q1.(a) (i) A mutation of a tumour suppressor gene can result in the formation of a tumour.
Explain how. (2)

Answer 1

1. (Tumour suppressor) gene inactivated / not able to control / slow down
   cell division;
   Ignore: references to growth
2. Rate of cell division too fast / out of control.
   1 and 2 Accept: mitosis
   1 and 2 Reject: meiosis

Question 2

(ii) Not all mutations result in a change to the amino acid sequence of the
encoded polypeptide.
Explain why. (1)

Answer 2

1. (Genetic) code degenerate;
Accept: codon for triplet
Accept description of degenerate code, e.g. another triplet
codes for the same amino acid
2. Mutation in intron.
Accept: mutation in non-coding DNA

Question 3

(b) Some cancer cells have a receptor protein in their cell-surface membrane that binds
to a hormone called growth factor. This stimulates the cancer cells to divide.
Scientists have produced a monoclonal antibody that stops this stimulation.
Use your knowledge of monoclonal antibodies to suggest how this antibody stops
the growth of a tumour. (3)

Answer 3

1. Antibody has specific tertiary structure / binding site / variable region;
   Do not accept explanations involving undefined antigen
2. Complementary (shape / fit) to receptor protein / GF / binds to receptor
   protein / to GF;
   Ignore: same shape as receptor protein / GF
3. Prevents GF binding (to receptor).

Question 4

Q2.The diagram below represents one process that occurs during protein synthesis.

(a) Name the process shown.
(1)

(b) Identify the molecule labelled Q.

Answer 4

Translation

(b) Transfer RNA / tRNA.

Question 5

(c) In the diagram above, the first codon is AUG. Give the base sequence of:
the complementary DNA base sequence

the missing anticodon (2)

Answer 5

TAC

UAC

Question 6

(d) Aspartic acid and proline are both amino acids. Describe how two amino acids differ
from one another. You may use a diagram to help your description. (1)

Answer 6

Have different R group

Question 7

(e) 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. (3)

Answer 7

1. Substitution would change CCG to CCC CCA CCU
2. Codes for same amino acid
3. Deletion would cause frame shift / change in all following codons /
   change next codon from UAC to ACC.

Question 8

Q3.(a) Messenger RNA (mRNA) is used during translation to form polypeptides.
Describe how mRNA is produced in the nucleus of a cell. (6)

Answer 8

Transcription

1. 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 9

(b) Describe the structure of proteins. (5)

Answer 9

1. Polymer of amino acids;
2. Joined by peptide bonds;
3. Formed by condensation;
4. Primary structure is order of amino acids;
5. Secondary structure is folding of polypeptide chain due to hydrogen
   bonding;
   Accept alpha helix / pleated sheet
6. Tertiary structure is 3-D folding due to hydrogen bonding and ionic /
   disulfide bonds;
7. Quaternary structure is two or more polypeptide chains.