DP Biology (last assessment 2024)

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Question 23M.2.SL.TZ1.6

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Date May 2023 Marks available [Maximum mark: 15] Reference code 23M.2.SL.TZ1.6
Level SL Paper 2 Time zone TZ1
Command term Describe, Explain, Outline Question number 6 Adapted from N/A
6.
[Maximum mark: 15]
23M.2.SL.TZ1.6

In eukaryotes, the chromosomes are located in the nucleus, and the nucleus can divide by mitosis or meiosis.

(a)

Outline the sequence of events that occurs during mitosis.

[5]

Markscheme
  1. supercoiling/condensation of chromosomes;
  2. breakup of nuclear membrane;
  3. growth of spindle/microtubules;
  4. attachment of spindle/microtubules to chromosomes/centromeres;
  5. chromosomes line up at the equator/middle
  6. division of centromeres;
  7. separation of sister chromatids/chromosomes
    OR
    sister chromatids/chromosomes move to opposite poles;
  8. reformation of nuclear membranes around chromosomes at each pole;

 

If events are not in the correct order, then award a maximum of 4 marks.

Names of phases of mitosis not required.

Award a max of 1 mark for listing the phases in the correct order when the list constitutes the entire response.

Examiners report

The greatest success occurred with (a) as most candidates were able to describe the events of mitosis even though the names of the stages might be incorrect. As names of stages were not required, the sequencing of events was successful.

(b)

Describe the processes that occur in the nucleus of a cell during interphase.

[7]

Markscheme
  1. DNA is replicated;
  2. unwinding of the double helix/DNA (by helicase)
    OR
    supercoiled DNA is relaxed (by gyrase);
  3. hydrogen bonds broken
    OR
    strands separate;
  4. synthesis of new strands of DNA (by DNA polymerase);
  5. DNA replication occurs semi-conservatively
    OR
    existing strand used as a template (for a new strand of DNA);
    OR
    DNA replication is continuous on the leading strand, but discontinuous on the lagging strand;
  6. copying base sequences occurs through complementary base pairing (in both transcription and replication);
  7. DNA is transcribed;
  8. synthesis of RNA/mRNA (by RNA polymerase);
  9. copy of a gene produced/only one gene transcribed;
  10. introns are removed;
  11. RNA/mRNA exported/moves to the cytoplasm;
Examiners report

The two most unsuccessful questions on the examination were (b) and (c). In question (b), many candidates talked about the G1, S and G2 phases rather than the processes in the nucleus. Those who did understand the question earned their marks by accurately describing replication; transcription was seldom included. There did not seem to be any fundamental misunderstandings in the responses.

(c)

Explain how the presence of a Y chromosome in the cells of a human embryo causes it to develop as a male.

[3]

Markscheme
  1. gene/genetic information on Y chromosome causes embryo to develop testes;
  2. (developing) testes in embryo secrete testosterone;
  3. testosterone causes male genitalia/penis to develop;

 

Accept SRY gene in mp-a

Examiners report

(c) was missed by most candidates as they did not explain that a gene on the Y chromosome causes the development of testes which then secrete testosterone. Many candidates stated that the Y chromosome stimulated the production of testosterone for no marks.

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