A-LevelBiologyNucleic acids and protein synthesis9700/23 · Oct/Nov 2018 · Q311 Marks

(a) Fig. 3.1 is a diagram of a monomer of the nucleic acid, messenger RNA. [Figure 3.1]

(a)(i)Name D, E and F in Fig. 3.1.[3]
(a)(ii)State one way in which the structure of DNA differs from the structure of messenger RNA.[1]
(b)Telomeres are repeating sequences of bases located at the ends of DNA molecules. These repeating sequences do not code for proteins. The enzyme telomerase ensures that telomeres do not shorten each time DNA is replicated. Fig. 3.2A shows the end of a DNA molecule during replication. DNA polymerase cannot attach to the region labelled X, so it cannot complete the synthesis of the new strand without the action of telomerase. Telomerase synthesises additional lengths of DNA that are added to the telomere. These additional lengths are used by DNA polymerase to complete the process of replication. Telomerase contains a short length of RNA that acts as a template for the synthesis of DNA as shown in Fig. 3.2B. Explain how a molecule of telomerase synthesises additional lengths of DNA. [Figure 3.2][4]
(c)Telomerase is not present in prokaryotic cells. Suggest why prokaryotes do not have telomerase.[1]
(d)One of the ways to diagnose lung cancer is to determine the concentration of telomerase in cells from the lining of the bronchus. Explain why determining the activity of telomerase may be useful in the diagnosis of lung cancer.[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 (Oct/Nov 2018). The mark or mark type is shown in brackets.

[(a)(i)] D (nitrogenous organic / nucleotide) base ; A pyrimidine / uracil / cytosine R purine
[(a)(i)] E ribose; I pentose
[(a)(i)] F phosphate (group);
[(a)(ii) any one from]
two, strands / polynucleotides (not one); A two chains R polypeptides
deoxyribose (not ribose); A correct ref. to –OH (on C2) instead of –H group on the pentose / sugar
thymine (not uracil) ; R thiamine
have base pairs;
(double) helix / helical (not straight chain);
longer;
[(b) any four from]
[1] template RNA binds to (part of) region X ;
[2] in active site of, telomerase / enzyme ;
[3] DNA / free, nucleotides, pair with / bind to / align with, RNA ;
[4] ref. to complementary, bases / base pairs;
[5] A-T and C-G and U-A;
[6] phosphodiester bonds form, between (DNA) nucleotides / AW ; treat as neutral any refs. to likely enzymes, e.g. ligase, etc.
[7] telomerase moves, in the direction of the arrow / to the right;
[8] AVP; e.g. hydrogen bonds between bases telomerase acts as a reverse transcriptase
[(c)] prokaryotes
[(c)] have, circular DNA ; A 'loop of DNA' I plasmid(s)
[(c)] do not have telomeres ;
[(c)] do not have chromosomes ;
[(c)] AVP; e.g. 'DNA has no ends'
[(d) any two from]
cancer cells divide, uncontrollable / continuously; ORA A cancer cells divide but do not undergo cell death
telomerase is required to ensure, chromosomes, do not shorten ;
telomerase is, synthesised / produced / found, in cancer cells; ORA
high levels of telomerase indicate cells that are cancerous / AW ;
AVP; e.g. idea that gene for telomerase switched on