A-LevelBiologyThe mitotic cell cycle9700/21 · Oct/Nov 2021 · Q410 Marks

Fig. 4.1 is a drawing made by a student of a plant cell, showing one of the stages of mitosis. [Figure 4.1]

(a)(i)Describe the role of the spindle during this stage of mitosis.[2]
(a)(ii)The chromosomes are composed of DNA. Describe the structure of a monomer of a DNA molecule.[2]
(b)(i)Telomeres are located at the ends of each chromosome. Telomeres progressively shorten each time a healthy human cell divides, until the cell is no longer able to divide. Stem cells have an enzyme called telomerase that prevents the telomeres shortening during the cell cycle. Suggest why telomerase activity is important in stem cells.[2]
(b)(ii)Suggest why tumour cells produce high concentrations of telomerase.[1]
(b)(iii)Scientists have researched the possibility of treating cancer by inhibiting the action of telomerase. One method that has been studied involves inserting small, single-stranded DNA into cancer cells. These sections of DNA have a base sequence that is complementary to a section of the mRNA transcribed from the gene coding for telomerase. Explain how inserting small, single-stranded DNA could prevent translation of the mRNA to produce telomerase.[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 (Oct/Nov 2021). The mark or mark type is shown in brackets.

[2] any two from: microtubules / spindle (fibres), are attached to, centromere / kinetochore ;
(spindle) fibres, contract/shorten / retract ; A microtubules disassemble / AW
move / pull, (sister) chromatids / (daughter) chromosomes, to opposite poles ;
[2] deoxyribose and phosphate and base ;
further detail one from diagram / description, of phosphate group and base attached to deoxyribose ;
one correctly named base or A,T C and G;
[2] any two from: allow DNA replication to continue;
idea that stem cell to, self renew / keep dividing;
idea that stem cells differentiate into (named) specialised cells ;
idea that mitosis produces cells for, cell replacement / tissue repair;
(maintains telomeres, so) prevents the loss of genes ;
[1] any one from: (high level of telomerase activity) allows,continuous / uncontrolled, mitosis / cell division;
idea of allowing shorter cell cycle;
[3] any three from: DNA binds to (complementary section of) mRNA / hydrogen bonds form between the complementary bases (of DNA and RNA);
mRNA cannot bind to the ribosome ;
ribosome cannot, move along the mRNA ;
consequence e.g. no sites for tRNA to bind / tRNA cannot bring attached amino acid;
tRNA anticodons cannot form hydrogen bonds with mRNA codons ;
AVP; e.g. idea that DNA enters nucleus to bind and mRNA then too large to leave via nuclear pores

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