DP Biology (last assessment 2024)

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Question 23M.2.HL.TZ1.a

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Date May 2023 Marks available [Maximum mark: 4] Reference code 23M.2.HL.TZ1.a
Level HL Paper 2 Time zone TZ1
Command term Outline Question number a Adapted from N/A
a.
[Maximum mark: 4]
23M.2.HL.TZ1.a

Outline the changes to chromosomes that occur during prophase in the first division of meiosis.

[4]

Markscheme
  1. pairing/synapsis of homologous chromosomes / homologous chromosomes form bivalents;
  2. crossing over / chromatid breaks then rejoins to non-sister chromatid;
  3. exchange of DNA/alleles/genetic information between chromatids/chromosomes;
  4. recombination / new combinations of alleles/genes generated;
  5. condensation/shortening/thickening/supercoiling of chromatids/chromosomes;
  6. formation of a chiasma where crossing over occurred;
Examiners report

Answers were very varied, with some excellent accounts of first prophase in meiosis and some muddled ones. A common error was to think that a chiasma forms and then crossing over takes place, whereas the actual sequence of events is that crossing over leads to chiasma formation.

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