DP Biology (first assessment 2025)

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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.

(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;
(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

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