A-LevelBiologyThe mitotic cell cycle9700/21 · May/June 2023 · Q413 Marks

The gene for the enzyme catalase is on chromosome 11 in humans.

(a)Explain the meaning of the term gene.[2]
(b)(i)Describe the roles of DNA polymerase and DNA ligase in the replication of DNA.[5]
(b)(ii)State the name of the stage of interphase in the cell cycle when DNA replication occurs.[1]
(c)(i)State the names and functions of structures A and B.[2]
(c)(ii)Complete Fig. 4.3 to show what happens to chromosome 11 in anaphase, so that the daughter nuclei are genetically identical. [Figure 4.3][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 (May/June 2023). The mark or mark type is shown in brackets.

[2] sequence of, nucleotides / bases, that is part of DNA ; A sequence of DNA nucleotides (that) codes for a polypeptide ; A protein / enzyme / amino acid chain
[5] must attempt both DNA polymerase and DNA ligase to gain max any five from: DNA polymerase 1 addition of, activated / phosphorylated, nucleotides ; 2 ref. to complementary, nucleotides / bases / strands ; 3 forms phosphodiester bonds ; 4 (between adjacent nucleotides and) elongating / growing, polynucleotide / strand / AW ; 5 ref. to proofreading ability of DNA polymerase ; DNA ligase 6 joins / AW, Okazaki fragments ; 7 on lagging strand ; 8 forms phosphodiester bonds, between the fragments / to complete phosphate-sugar backbone ;
[1] S phase / synthesis phase ; I 1 as in S1 phase / S unqualified
[2] A - centromere and I kinetochore site of attachment of, chromatids / chromosome(s) to, spindle fibres / microtubules or holds, sister / identical, chromatids together ; R daughter chromatids B - spindle fibres / microtubules, and orientating chromosomes at the (spindle) equator or separating chromatids, at end of metaphase / at start of anaphase or movement of, chromatids / chromosomes, to (opposite) poles ; allow 1 mark max if functions incorrect but A and B named correctly allow 1 mark max if functions are correct but A named as kinetochore and B poorly misspelt attempt at microtubules
[3] two separate, chromatids / single-stranded chromosomes ; chromatids attached to spindle with fibres drawn to, poles / centrioles ; single chromatids drawn U-shaped or V-shaped pointing towards the poles ; centromeres drawn in both chromatids ;