DP Biology (first assessment 2025)

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Question 22M.2.HL.TZ1.8

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Date May 2022 Marks available [Maximum mark: 15] Reference code 22M.2.HL.TZ1.8
Level HL Paper 2 Time zone TZ1
Command term Describe, Draw, Explain Question number 8 Adapted from N/A
8.
[Maximum mark: 15]
22M.2.HL.TZ1.8
(a)

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

[3]

Markscheme

  1. ribose drawn as a pentagon and labelled;
  2. base linked correctly (to C1) of ribose and labelled;
  3. phosphate linked correctly (to C5) of ribose and labelled;
Examiners report

Many candidates were able to draw a nucleotide and label the subunits correctly. Weaker candidates did not know what a nucleotide was, so often drew a diagram of either a DNA or RNA polynucleotide.

(b)

Describe how DNA profiling can be used to establish paternity.

[5]

Markscheme
  1. DNA sample is collected from the child and its (potential) parents;
  2. from saliva/mouth swab/blood/other body cells;
  3. PCR used to amplify/produce more copies of the DNA;
  4. short tandem repeats/genes consisting of a repeating sequence of bases repeats copied/used;
  5. number of repeats varies between individuals;
  6. unlikely that two individuals have same number of repeats for every gene included;
  7. gel electrophoresis used to separate DNA fragments according to length/number of repeats;
  8. gel electrophoresis generates a unique pattern of bands
  9. DNA profile is the pattern of bands / diagram showing pattern of bands as in a DNA profile;
  10. all bands in the child’s profile must be in one of the parents’ profiles / OWTTE;
Examiners report

Answers to this question were very variable, with some very well-informed accounts including the use of genes containing of tandem repeats and details of the collection of DNA samples from both parents and the child. There were also many accounts with errors of understanding. A common misconception is that paternity is established by finding the male whose profile has most similarities to that of the child. This does not prove that a man is the father of the child and instead the father's profile must contain all bands in the child's profile that do not occur in that of the mother. 

(c)

Explain the reasons for variation in human height.

[7]

Markscheme
  1. environment affects height;
  2. nutrition/malnutrition affects growth rate / other example of environmental factor affecting height;
  3. genes/alleles affect height / height is partly heritable;
  4. polygenic / many genes influence height;
  5. continuous variation;
  6. normal/bell-shaped distribution of height;
  7. some alleles (of these genes) increase height and some reduce it;
  8. many possible combinations of alleles of these genes;
  9. specific gene mutations/alleles cause dwarfism/extreme height;
  10. meiosis generates variation (in height);
  11. mutations generate variation (in height);
  12. males tend to be/are on average taller than females;
  13. loss of height during aging;
Examiners report

This was a relatively high scoring question, with a mean mark of 2.4 (out of 7). Teachers expressed surprise in G2 forms that their students were being expected to make 7 valid points in their answer, but the wide-ranging mark scheme ensured that this was possible. There was some confusion between polygenic inheritance and multiple alleles. Weaker candidates tended to think that dominant alleles make us taller and recessive alleles cause shortness. Another fault in many answers was to focus on natural selection and evolution of height — if anything natural selection will reduce variation in human height rather than cause it.

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