IB Biology HL D2.1 Cell and nuclear division questions

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Question 1

Multiple choiceEasy1 mark

A child with Down syndrome has three copies of chromosome 21. Which meiotic event could produce a gamete with two copies of chromosome 21, which then fuses with a gamete containing one copy?

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Answer: D. Nondisjunction is the failure of chromosomes or chromatids to separate during meiosis. This can produce a gamete with two copies of chromosome 21; fusion with a gamete containing one copy gives a zygote with three copies, associated with Down syndrome. A: crossing over exchanges chromosome segments but does not produce an extra copy of chromosome 21. B: a mutation changes DNA sequence, not the number of chromosome 21 copies. C: normal separation gives each gamete one copy of chromosome 21. Remember: nondisjunction can change the number of chromosomes in a gamete.

Question 2

Paper 2Medium16 marks

Figure 1 shows possible chromosome outcomes in a eukaryotic species with a diploid chromosome number of 2n = 4. The parent nucleus has completed DNA replication before either pathway begins. Chromosomes of the same length are homologous; solid and dashed lines distinguish the two homologues in each pair. The central dot on each chromosome represents a centromere. Crossing-over is not shown.

replicated parentnucleusMitosisMeiosisafter meiosis Iafter meiosis II
(a)

State the number of chromosomes and the number of chromatids in the replicated parent nucleus in Figure 1.

[2]
(b)

Using Figure 1, compare the number of nuclei formed and the chromosome number per nucleus after mitosis and meiosis.

[4]
(c)

Explain the role of DNA replication before mitosis and meiosis, referring to chromatids and their separation during these processes.

[4]
(d)

Explain two processes by which meiosis can generate genetically different nuclei. Explain why nuclear division should be completed before cytokinesis.

[6]

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