A-LevelBiologyNucleic acids and protein synthesis9700/22 · Feb/Mar 2022 · Q612 Marks

Fig. 6.1 is a diagram showing the structure of part of a DNA molecule. [Figure 6.1] (b) Fig. 6.2 shows the RNA base sequence of a short length of primary transcript. Complete Fig. 6.2 by writing the DNA base sequence of the template strand used to form the primary transcript. [Figure 6.2] (c) In eukaryotic cells, the primary transcript is modified to form mRNA. Explain how the primary transcript is modified to form mRNA. (d) The mRNA strand is translated at the ribosome to form a polypeptide. Describe how the process of translation results in the formation of a polypeptide.

(a)(i)Identify structure E and structure F in Fig. 6.1. [Figure 6.1][2]
(a)(ii)On Fig. 6.1 draw a circle around one nucleotide. [Figure 6.1][1]
(a)(iii)State the name of the covalent bond that links two nucleotides together.[1]
(b)Fig. 6.2 shows the RNA base sequence of a short length of primary transcript. Complete Fig. 6.2 by writing the DNA base sequence of the template strand used to form the primary transcript. [Figure 6.2][1]
(c)In eukaryotic cells, the primary transcript is modified to form mRNA. Explain how the primary transcript is modified to form mRNA.[2]
(d)The mRNA strand is translated at the ribosome to form a polypeptide. Describe how the process of translation results in the formation of a polypeptide.[5]

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Mark scheme

Marking points from the official mark scheme (Feb/Mar 2022). The mark or mark type is shown in brackets.

[1] 6(a)(i) E = deoxyribose ;
[1] 6(a)(i) F = cytosine ;
[1] 6(a)(ii) a ring drawn around 1 base, 1 deoxyribose and 1 phosphate on the same strand ;
[1] 6(a)(iii) phosphodiester ;
[1] 6(b) All 4 triplets must be correct; CCA CGA TTA GAT
[1] 6(c) removal of introns / non-coding sequences ;
[1] 6(c) leaving, exons / coding sequences (joined) ; ref. to splicing RNA transcript = 1 mark
[1] 6(c) AVP ; e.g. adding a 5' cap / 3' poly A tail
[1] 6(d) two codons at a time are exposed (to the large subunit) ;
[1] 6(d) (idea of) a specific tRNA brings a specific amino acid ;
[1] 6(d) a tRNA anticodon binds to the mRNA codon ;
[1] 6(d) complementary base pairing occurs (by hydrogen bonding) ;
[1] 6(d) a second tRNA brings another amino acid (next to the first amino acid) ;
[1] 6(d) peptide bond formation between the two amino acids ;
[1] 6(d) ribosome moves along the mRNA, one codon at a time / and next codon is 'read' ;
[1] 6(d) the first tRNA leaves the ribosome ;
[1] 6(d) AVP ; e.g. first codon is always AUG first anticodon is always UAC first amino acid is always methionine ribosome moves along mRNA in a 5' to 3' direction role of peptidyl transferase eventually a stop codon is reached (and translation stops)

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