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 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 4
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 5
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