A-LevelBiologyThe mitotic cell cycle9700/21 · Oct/Nov 2023 · Q68 Marks

State the name of the phase of the mitotic cell cycle during which DNA replication occurs.

(a)(i)State the name of the phase of the mitotic cell cycle during which DNA replication occurs.[1]
(a)(ii)During research, scientists can use modified nucleotides to prevent elongation of a polynucleotide chain during DNA replication. Fig. 6.1 shows the structure of a DNA nucleotide found in the nucleus of a cell. [Fig. 6.1] Fig. 6.2 shows the structure of a modified nucleotide. [Fig. 6.2] Suggest how the structure of the modified nucleotide prevents DNA polymerase joining it to another nucleotide.[2]
(b)(i)RNA aptamers are short, single-stranded RNA molecules that can be used to study some infectious diseases. Scientists studying an infectious disease in animals investigated the effect of RNA aptamers on the activity of RNA polymerase that is produced by the pathogen. The aptamers bind to a specific region of the RNA polymerase. Table 6.1 shows the effect of two aptamers, F47 and F52, on the activity of RNA polymerase produced by the pathogen. [Table 6.1] With reference to Table 6.1, compare the effect of the aptamers on the affinity of RNA polymerase for its substrate.[2]
(b)(ii)With reference to Table 6.1, suggest explanations for the effect of the presence of an aptamer on the rate of transcription catalysed by the RNA polymerase.[3]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.

[1] S phase / synthesis phase / interphase;
[2] any two from:
does not have an OH group (on C3) so condensation reaction cannot occur ;
phosphodiester bond cannot form between deoxyribose and phosphate of adjacent nucleotide ;
is a different shape because it has H instead OH (on C3);
shape not complementary to the active site of DNA polymerase
or
no, enzyme substrate complexes / ESCs, can form ;
[2] any two from:
(both increase Km, therefore both) reduce the affinity (of RNA polymerase for its substrate);
(presence of) F52 reduces the affinity for the substrate more than F47; ora for F47
AVP; e.g. manipulation of data to support described trend
correct link stated between Km values and affinity
[3] any three from:
decreased, maximum rate of reactions/rate of transcription because
Vmax lower, qualified;
binding (of the aptamer) changes, the tertiary structure / shape of, active site (of RNA polymerase) ;
A binding changes shape of enzyme so active site less complementary to substrate
(so) substrate / (RNA) nucleotides, bind less easily to active site ;
A fewer enzyme-substrate complexes form
binding of aptamer prevents, substrate / (RNA) nucleotide, binding to active site;
binding prevents RNA polymerase from binding to template strand ;
binding prevents RNA polymerase from forming bond between RNA nucleotides ;
AVP;

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