A-LevelBiologyNucleic acids and protein synthesis9700/23 · May/June 2019 · Q16 Marks
Fig. 1.1 is a diagram showing the structure of a section of a DNA molecule. [Figure 1.1]
- (a)Draw a circle around one monomer of DNA in Fig. 1.1.[1]
- (b)Name the two bases forming the base pair at X in Fig. 1.1 and give a reason for your answer. bases reason[2]
- (c)The statements 1–5 describe events that occur during DNA replication. 1 DNA polymerase forms a phosphodiester bond 2 DNA double helix forms 3 hydrogen bonds break 4 hydrogen bonds form 5 two strands of the double helix separate Write the numbers 1 to 5 in the spaces below to show the order in which these events occur. The first one has been done for you. 3[1]
- (d)The telomere is a region found at the end of a chromosome. Outline the function of telomeres.[2]
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 (May/June 2019). The mark or mark type is shown in brackets.
- [1] one nucleotide circled;
- [2] cytosine and guanine;
- three hydrogen bonds between the base pairs ;
- [1] (3) 5,4,1,2;
- [2] I maintains length of chromosomes / prevents chromosome shortening
- any two from:
- permit continued replication ;
- prevent loss of genes; I prevents gene damage
- A genetic / coded, information
- A protein coding regions of DNA
- protect ends of chromosomes from being, degraded / AW ;
- AVP; e.g. prevents ends of chromosomes from being attached to each other
- not mistaken for a break in DNA that needs repairing
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