(a) Fig. 4.1 shows part of a DNA molecule. [Figure 4.1] Use Fig. 4.1 to explain how the structure of mRNA differs from the structure of DNA. (b) Fig. 4.2 shows: • the first seven amino acids of the β chain of haemoglobin • the first amino acid in the sequence is valine (Val) • the 21 base pairs in the sequence of DNA that code for these seven amino acids. [Figure 4.2] Table 4.1 shows the triplets of bases that code for seven amino acids. Using Fig. 4.2 and Table 4.1, state what will happen to the sequence of amino acids in the first part of the β chain of haemoglobin:

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

1. 1 single-stranded ;
2. 2 no hydrogen bonding / only DNA has hydrogen bonding;
3. 3 no base pairs / only DNA has base pairs;
4. 4 uracil and not thymine / DNA has thymine instead of uracil ; treat as neutral T and U, look for complete term
5. 5 ribose not deoxyribose;
6. 6 detail, e.g. -H and not –OH on C2 ;
7. 7 short(er) / DNA is longer; A smaller / bigger
8. 8 not a helix;
9. third triplet is a stop codon so
10. only two amino acids are joined by peptide bonds / chain only 2 amino acids long;
11. A will still have Val-His as the first two amino acids
12. very short molecule is produced / chain stops after Histidine);
13. R frameshift / description of frameshift
14. a triplet is deleted so
15. (polypeptide / sequence / β chain) has one less amino acid;
16. polypeptide does not have Leu (as the third amino acid);
17. I Leu is not, produced / made / synthesised
18. feature DNA replication transcription
19. a single-stranded molecule is produced ✗ ✓
20. hydrogen bonds are broken ✓ ✓
21. both strands of DNA act as templates ✓ ✗
22. phosphodiester bonds are formed ✓ ✓
23. DNA polymerase is used ✓ ✗
24. I functions of telomerase
25. permits continued replication (in stem cells / meristematic cells); A ora
26. prevents loss of, genes / genetic material / DNA ; A ora
27. A prevents shortening of, chromosomes / DNA
28. length of telomere determines lifespan of, cells / cell lineage ;
29. AVP; e.g. prevents ends of chromosomes attaching to each other prevents apoptosis / cell death / cell destruction
30. 1 translation / construction of polypeptide(s) ;
31. 2 provide binding site for mRNA / mRNA attaches to ribosome / AW ; A entering ribosome
32. 3 provides binding sites for (two) tRNA molecules ; A entering ribosome
33. 4 two amino acids are held close together ;
34. 5 formation of peptide bond(s) ; R dipeptide / polypeptide, bond
35. 6 (allows) assembly of amino acids into, sequence / primary structure;
36. 7 AVP; e.g. P and A site (and E site) bond between amino acids catalysed by peptidyl transferase