Question 1
Paper 1A style
What achieves the splitting of the cytoplasm during cytokinesis in a plant cell?
- A ring of contractile actin and myosin proteins pinching the cell membrane inwards
- Vesicles that assemble sections of membrane and cell wall across the cell
- Spindle microtubules pulling the two halves of the cell apart
- The nuclear membrane re-forming across the middle of the cell
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Answer: B
Question 2
Paper 1A style
In a micrograph of a root tip, a cell shows two groups of V-shaped chromosomes moving towards opposite poles. Which phase of mitosis is the cell in?
- Prophase
- Metaphase
- Anaphase
- Telophase
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Answer: C
Question 3
Paper 1A style
An organism has a diploid number of 8. How many different combinations of chromosomes can be produced in its gametes by random orientation of bivalents alone?
- 4
- 8
- 16
- 256
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Answer: C (2⁴ = 16, since n = 4)
Question 4
Paper 2A style
Explain why cytokinesis is usually equal, and outline one example of unequal cytokinesis.
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- each daughter cell must receive at least one mitochondrion / chloroplast / organelles that can only be made by division of pre-existing ones [1]
- each daughter cell receives a nucleus / enough cytoplasm to survive OWTTE [1]
- oogenesis: almost all cytoplasm passes to the (secondary) oocyte / ovum, small polar bodies formed [1]
- OR budding in yeast: a small bud receives a nucleus and pinches off from the larger parent cell [1]
- max 3
Question 5
Paper 2A style
Explain how non-disjunction can lead to a child with Down syndrome.
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- non-disjunction is failure of chromosomes / chromatids to separate in meiosis [1]
- homologous chromosomes fail to separate in meiosis I / sister chromatids fail to separate in meiosis II [1]
- a gamete receives two copies of chromosome 21 / 24 chromosomes [1]
- fertilised by a normal gamete with 23 chromosomes [1]
- zygote has three copies of chromosome 21 / trisomy 21 / 47 chromosomes [1]
- max 4
Question 6
Paper 2B style
Explain how meiosis halves the chromosome number and produces genetically different haploid nuclei.
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- DNA is replicated before meiosis so each chromosome has two (sister) chromatids [1]
- chromosomes condense by supercoiling (around histones) [1]
- homologous chromosomes pair to form bivalents in prophase I [1]
- crossing over between non-sister chromatids at chiasmata [1]
- produces new combinations of alleles / recombinant chromatids [1]
- bivalents line up at the equator in metaphase I in random orientation [1]
- orientation of each bivalent is independent of the others, giving 2ⁿ combinations [1]
- spindle microtubules / motor proteins move chromosomes to the poles [1]
- homologous chromosomes separate in meiosis I / first segregation, halving the chromosome number [1]
- sister chromatids separate in meiosis II / second segregation [1]
- four haploid nuclei are produced from one diploid nucleus [1]
- max 7