Question 1
Paper 1A style
During transcription, which base on the RNA strand pairs with adenine on the DNA template strand?
- Thymine
- Uracil
- Guanine
- Cytosine
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Answer: B ;
Question 2
Paper 1A style
The codons GAA and GAG both code for glutamic acid. Which feature of the genetic code does this show?
- Universality
- It is a triplet code
- Degeneracy
- It is non-overlapping
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Answer: C ;
Question 3
Paper 1A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanDuring elongation, which site of the ribosome holds the tRNA attached to the growing polypeptide?
- A site
- E site
- P site
- The small subunit binding site
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Answer: C ;
Question 4
Paper 1B style
The table shows part of the mRNA codon table. A short section of mRNA has the sequence AUG UUU CAU GAG UGG UAA. (a) Deduce the sequence of amino acids coded by this mRNA. [2] (b) Deduce the base sequence of the DNA template strand for the first two codons. [1] (c) A mutation changes the fourth codon from GAG to GUG. Using the table, state the effect on the polypeptide and suggest how this could affect the protein's function. [2]
| Codon | Amino acid |
|---|---|
| AUG | Met (start) |
| UUU | Phe |
| CAU | His |
| GAG | Glu |
| GUG | Val |
| UGG | Trp |
| UAA | stop |
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- (a) Met – Phe – His – Glu – Trp;
- (a) translation ends at UAA / stop codon (no amino acid added); Award [1] for four correct amino acids in order
- (b) TAC AAA; Accept 3'-TAC AAA-5' or 5'-AAA CAT-3'
- (c) glutamic acid replaced by valine (at that position) / one amino acid changed;
- (c) different R group / hydrophobic instead of charged, so the polypeptide folds differently / changes shape;
- (c) protein may lose function / function altered, e.g. as in haemoglobin S; OWTTE
- max 5
Question 5
Paper 2A style
Explain why the genetic code must be a triplet code.
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- there are 20 (types of) amino acids (to be coded for) and 4 bases;
- a doublet code gives only 4² = 16 combinations, which is not enough;
- a triplet code gives 4³ = 64 combinations, which is enough (with some to spare for start/stop or degeneracy);
- max 2
Question 6
Paper 2B style
Describe how a polypeptide is elongated during translation.
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- mRNA binds to the small subunit of the ribosome;
- mRNA is read in codons / triplets of bases;
- tRNA carries a specific amino acid;
- tRNA anticodon pairs with the complementary codon on mRNA;
- by hydrogen bonding / complementary base pairing (A–U, C–G);
- two tRNAs bind (to the large subunit) at the same time;
- a peptide bond forms between the amino acid and the growing polypeptide;
- the polypeptide is transferred to the (amino acid on the) second tRNA;
- the ribosome moves along the mRNA by one codon;
- the first tRNA (without an amino acid) is released;
- process repeats, adding one amino acid at a time in the order set by the codons;
- max 6
Question 7
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanOutline how pre-mRNA is modified in eukaryotic cells, and how this allows one gene to code for more than one polypeptide.
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- introns are removed;
- exons are spliced/joined together (to form mature mRNA);
- 5' cap added;
- 3' polyA tail added;
- cap and tail stabilise the mRNA;
- alternative splicing joins different combinations of exons;
- producing different polypeptides from the same gene;
- max 4
Question 8
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels mean(a) Outline the two-stage modification of pre-proinsulin to insulin. [3] (b) State why cells constantly break down proteins in proteasomes. [1]
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- pre-proinsulin is the polypeptide produced by translation (at the rough ER);
- stage 1: signal sequence/peptide removed to give proinsulin;
- proinsulin folds and disulfide bonds form;
- stage 2: C-peptide removed / cut out;
- leaving A and B chains linked by disulfide bonds (active insulin);
- (a) [max 3]
- (b) sustaining a functional proteome requires constant breakdown (and synthesis) of proteins / removes damaged, misfolded or unneeded proteins / amino acids are recycled for new protein synthesis;
- max 4