A-LevelBiologyThe mitotic cell cycle9700/23 · May/June 2023 · Q111 Marks

The cells in a tissue are often at different stages of the cell cycle.

(a)(i)Fig. 1.1 shows cells at different stages of the cell cycle. Identify the stages of mitosis occurring in the cells labelled B and C in Fig. 1.1. B C[2]
(a)(ii)Describe the behaviour of the chromosomes in the stage of mitosis shown in cell A in Fig. 1.1.[3]
(a)(iii)Cell D in Fig. 1.1 is in interphase. Describe the role of DNA ligase in interphase.[3]
(b)(i)Telomerase is an enzyme that is active during interphase in some cells. Telomerase helps to maintain telomeres present on chromosomes. State the location of telomeres on a chromosome.[1]
(b)(ii)Some people with a long lifespan have cells showing a higher than normal activity of telomerase. Suggest why a long lifespan could result from a higher telomerase activity.[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 2023). The mark or mark type is shown in brackets.

[2] B prophase; I early / mid / late C metaphase; A early anaphase
[3] any three from: A identical chromatids for sister chromatids assume points are related to anaphase, unless stated as telophase sister/identical, chromatids (of each chromosome) separate / AW ; A idea of spindle fibres pulling apart sister chromatids in context of end of metaphase / start of anaphase movement of / AW, daughter chromosomes / chromosomes / (sister) chromatids, to (opposite) poles (of cell) ; R if context is a chromosome composed of two chromatids R if context is movement of sister chromatids to same pole I ends of cell movement (to opposite poles) by spindle fibres, contracting / pulling; I moved by spindle fibres centromeres, qualified; e.g. centromeres divide at start of anaphase (to separate sister chromatids) centromeres leading (during movement so arms of chromosome lagging) if telophase stated, mark behaviour of chromosomes in telophase to max 2: (daughter) chromosomes at poles ; A sister chromatids at poles A ref. to two separate groups of (daughter) chromosomes (chromosomes) become diffuse / become long and thin / decondense / uncoil ; A become chromatin ref. to re-forming nucleolus ;
[3] any three from: (active during) DNA replication / S phase / synthesis phase ; A during synthesis of DNA / semi-conservative replication joins / AW, Okazaki fragments ; A description of Okazaki fragments on lagging strand; (by catalysing) formation of phosphodiester bonds (between adjacent nucleotides); to complete sugar phosphate backbone / AW ; e.g. produces continuous, DNA / polynucleotide, strand
[1] at both ends (of interphase chromosomes) ; A on the ends (of, a chromosome
[2] any three from: telomeres, maintained / AW, for, longer / greater length of time ; allows increased, number of cell cycles / DNA replication cycles / AW ; A allows, mitosis / cell division, to occur more times AW R speeds up, mitosis / cell division / cell cycle idea of prevents loss of genes (at ends of chromosomes, for a longer time); A loss of, genetic information / coding DNA A less, genes / AW, lost I loss of genetic material ref. greater ability to replace, damaged / worn out / old, cells; in context of active for a longer time idea of cell metabolism remains optimal / cell does not go through apoptosis / AW ;