A-LevelBiologyNucleic acids and protein synthesis9700/23 · Oct/Nov 2024 · Q39 Marks

The five bases found in nucleic acids are described as nitrogenous organic compounds. There are two types of base. Fig. 3.1 shows the structure of the five bases. [Figure 3.1]

(a)(i)State the name of the type of base that includes U, T and C.[1]
(a)(ii)State why it is not correct to say that all nucleic acids have five bases.[1]
(b)Fig. 3.2 shows a stage in the replication of DNA. The circled part is enlarged in Fig. 3.3 to show the elongation of the DNA strand that is being synthesised. [Figure 3.2] [Figure 3.3] Describe the sequence of events that occurs at the stage shown in Fig. 3.3 to extend the synthesised strand.[4]
(c)The two strands in a molecule of DNA are described as antiparallel. With reference to Fig. 3.2 and Fig. 3.3, state what is meant by antiparallel, and explain how the antiparallel arrangement of the strands determines how new strands are synthesised.[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.

[1] pyrimidine ; accept phonetic or close spellings, e.g. pyramidine / pyrimadine / pirimadine / primidine
any one from:
[1] each nucleic acid has 4 (types of) base; A has 4 bases
[1] DNA has A, T, C and G and RNA has A, U, C and G; A full names even if misspelt unless another molecule (e.g. thiamine)
[1] idea that in RNA U replaces T / DNA has T but RNA has U;
any four from: max 2 if answer is about transcription and mRNA penalise polypeptide once
[1] (triphosphate) activated / free, nucleotide (with thymine);
[1] thymine / T, pairs with, adenine / A ; I complementary base pairing unqualified
[1] adenine / A, on template strand ;
[1] formation of two hydrogen bonds between T and A; I H bonds between C and G
[1] ref to DNA polymerase in correct context (e.g. correct base pairing / bond formation / proof reading);
[1] forms phosphodiester bond (between -OH of deoxyribose and phosphate of nucleotide) ; R if DNA ligase
[1] release of, pyrophosphate / P-P / two phosphates ;
[1] nucleotide added to the 3´ end of, newly-synthesised strand / leading strand / daughter strand (of DNA) ; A extension is in the 5' to 3' direction
[1] strands (are parallel and) run in opposite directions / one strand in 5' to 3' direction and other strand in 3′ to 5' direction;
any two from:
[1] elongation / synthesis / extension / AW, of strand is, in 5' to 3' direction / not in 3′ to 5′ direction;
[1] DNA polymerase, moves in the 5′ to 3′ direction / can only add nucleotides to the 3´ end;
[1] idea that phosphate of each nucleotide is added to C3 of last nucleotide of growing strand ;
[1] leading strand is made continuously / lagging strand is made up of Okazaki fragments ;
[1] Okazaki fragments are attached together by DNA ligase ;