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Date	May 2022	Marks available	7	Reference code	22M.2.SL.TZ1.7
Level	Standard level	Paper	Paper 2	Time zone	Time zone 1
Command term	Explain	Question number	7	Adapted from	N/A

Question

Draw a labelled diagram to show the structure of a single nucleotide of RNA.

[3]

Distinguish between the processes of meiosis and mitosis.

[5]

Explain the development of antibiotic resistance in terms of natural selection.

[7]

Markscheme

ribose drawn as a pentagon and labelled;
base linked correctly (to C1) of ribose and labelled;
phosphate linked correctly (to C5) of ribose and labelled;

Award [2 max] if more than one nucleotide drawn.

“Sugar” alone is insufficient.

antibiotics can (generally) kill/destroy bacteria;
some bacteria show variation/antibiotic resistance;
variation/resistance is due to a random mutation;
resistant bacteria are not killed/destroyed by the antibiotic
OR
bacteria without the mutation die;
(resistant) bacteria have a selective advantage / unequal success;
the bacteria with this variation/resistance reproduces/multiplies;
mutation/gene is passed on to the offspring / the offspring will be resistant to the antibiotic;
resistant bacteria become more common;
bacteria have evolved to be resistant to the antibiotic;

Award [6 max] if pathogen is used instead of bacteria throughout the answer with no mention of bacteria.

Examiners report

Occasionally, deoxyribose was seen rather ribose; sometimes more than one nucleotide was shown limiting maximum marks.

The best answers had clear comparative statements or were in a comparison table. The worst answers were continuous prose descriptions of the processes, as these all too often failed to be able to be pieced together to make full marking points.

About half of the candidates did not mention cell divisions, but simply mentioned "two cycles" which was too vague and instead of writing haploid/diploid, many said 23/46, which only applies to human or certain organisms.

This natural selection question had better responses than in previous years. There seems to be a widespread misconception that bacteria can mutate after exposure to an antibiotic rather than surviving because of a genetic variation. Some confusion was demonstrated concerning the use of antibiotics: some candidates did not understand that antibiotics are used against bacteria (not viruses). There was lots of confusion in the use of different but similar words — antibiotics / antibody / antigen

Syllabus sections

Core » Topic 5: Evolution and biodiversity » 5.2 Natural selection

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