Pearson Edexcel International A Level Biology: Meiosis, Gametes, Fertilisation and Mitosis (YBI11) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions follow the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2, statements 3.9 to 3.16 and Core Practical 6. This is Topic 3, part of the International AS (IAS). Explanations are in the study guide and revision notes. See also the Edexcel A Level Biology hub, the checklist and the free diagnostics.

Questions

1. Define the term locus and state one way in which two genes being linked affects their inheritance. [3]

2. A species has a diploid number of 14. Calculate the number of different chromosome combinations possible in the gametes of one individual from independent assortment alone, and the number of different zygote combinations when two such individuals mate. Ignore crossing over. [3]

3. A plant heterozygous at two loci (alleles D, d and F, f) was test-crossed with a plant that was ddff. The 800 offspring were: DF 356, df 344, Df 52, dF 48. (a) Explain what these results show about the two genes. [2] (b) Calculate the recombinant frequency as a percentage. [2] (c) Explain how the recombinant offspring arose. [1]

4. Explain how meiosis produces genetically different gametes. [6]

5. Compare the structure and function of the sperm and the egg cell in mammals. [4]

6. Describe the events of fertilisation in a mammal from contact between sperm and egg to formation of the zygote nucleus. [5]

7. In an experiment, eggs were treated with a drug that stops cortical granules fusing with the egg surface membrane. After exposure to sperm, 38% of treated eggs contained more than one sperm nucleus, compared with 2% of untreated eggs. (a) Explain these results. [2] (b) Calculate how many times greater the treated percentage is than the control. [1]

8. Describe the process of fertilisation in a flowering plant, beginning with the pollen grain on the stigma. [5]

9. Pollen was grown at four temperatures. The mean length of pollen tubes after 40 minutes was: 10 °C, 120 µm; 20 °C, 360 µm; 30 °C, 540 µm; 40 °C, 300 µm. (a) Calculate the rate of growth at 30 °C in µm per minute. [1] (b) Describe the trend and suggest an explanation for the change between 30 °C and 40 °C. [3] (c) State one variable that should be kept constant. [1]

10. A cell cycle lasts 24 hours: G1 10 hours, S 8 hours, G2 4 hours, mitosis 2 hours. (a) State what happens in S phase and why it is needed before mitosis. [2] (b) Calculate the percentage of the cycle spent in mitosis. [1] (c) Explain why mitosis gives two daughter cells with the same genotype as the parent. [2]

11. A student made a root tip squash and counted the cells in several fields of view: 346 interphase, 21 prophase, 9 metaphase, 6 anaphase and 8 telophase. (a) Describe how the student should prepare the squash and why each step is done. [4] (b) Calculate the mitotic index as a percentage to 3 significant figures. [2] (c) The cell cycle in this tissue lasts 16 hours. Calculate the time spent in prophase, in minutes. [2] (d) A root tip treated with a spindle poison had 130 cells in mitosis out of 520. Calculate its mitotic index and suggest why it differs from the untreated tip. [2]

Answers

1. A locus is the position of a gene on a chromosome [1]. Linked genes are on the same chromosome so are inherited together [1], and so do not assort independently, giving fewer recombinant offspring than expected [1]. Examiner insight: say “position”, not “place” or “gene”, for locus; the effect on inheritance needs a stated consequence, not just the word linked.

2. n = 7 [1]. Gametes: 2^7 = 128 [1]. Zygotes: 128 × 128 = 16 384 [1]. Examiner insight: show n = 7 before using it; giving 14^2 or 2^14 for the gamete number is a common slip that a clear working line makes easy to spot.

3. (a) Parental types (DF and df) are much commoner than expected 200 each [1]; so the genes are linked on the same chromosome [1]. (b) Recombinants = 52 + 48 = 100 [1]; 100 / 800 × 100 = 12.5% [1]. (c) Crossing over between non-sister chromatids of the homologous pair in prophase I of the heterozygote’s meiosis [1]. Examiner insight: quote the expected figure (200 each) when you argue from data; and say non-sister chromatids and homologous chromosomes when explaining crossing over.

4. Crossing over in prophase I: homologous chromosomes pair [1] and non-sister chromatids exchange sections at chiasmata [1], so chromatids carry new allele combinations [1]. Independent assortment in metaphase I: pairs line up at the equator [1], each pair orientating randomly towards either pole [1], so many combinations of maternal and paternal chromosomes pass into each gamete [1]. Examiner insight: tie each mechanism to its stage and to the result (new combinations); listing “crossing over and independent assortment” alone gives little.

5. Sperm: acrosome with enzymes to digest the zona pellucida [1]; flagellum and many mitochondria in the mid-piece for ATP and movement [1]. Egg: large cytoplasm with food stores [1]; zona pellucida and cortical granules involved in sperm binding and blocking polyspermy [1]. Examiner insight: “compare” needs both gametes covered for each point, with structure linked to function.

6. Sperm binds to the zona pellucida [1]. Acrosome reaction: enzymes released by exocytosis digest the zona [1]. Sperm membrane fuses with the egg surface membrane [1]. Cortical reaction: cortical granules release contents, changing the zona so no more sperm bind [1]. Sperm nucleus enters and fuses with the egg nucleus to form the diploid zygote nucleus [1]. Examiner insight: follow the order of events; the two reactions each need their own location and effect.

7. (a) Without cortical granule release the zona is not changed [1], so more than one sperm can enter (polyspermy) [1]. (b) 38 / 2 = 19 times [1]. Examiner insight: link the drug’s effect to the missing cortical reaction rather than just describing the data.

8. Pollen grain germinates and a pollen tube grows from it [1]. Tube grows down the style towards the ovule [1]. Generative nucleus divides by mitosis to give two male nuclei [1]. Tube enters through the micropyle [1]. One male nucleus fuses with the egg nucleus (zygote) and the other with the two polar nuclei (endosperm nucleus) [1]. Examiner insight: state which nucleus fuses with which; “the nuclei fuse” scores less than naming the products.

9. (a) 540 / 40 = 13.5 µm per minute [1]. (b) Rate rises from 10 °C to 30 °C [1], then falls at 40 °C [1]; high temperature may denature enzymes needed for growth [1]. (c) Sucrose concentration (accept pH, time, pollen source) [1]. Examiner insight: give units on a rate, and use “suggests” or “may” for an explanation the data cannot prove.

10. (a) DNA is replicated [1], so each chromosome has two identical chromatids to share between daughter cells [1]. (b) 2 / 24 × 100 = 8.3% [1]. (c) Replicated DNA is shared equally: each chromatid goes to a different pole [1], so both nuclei have identical DNA, hence the same genotype [1]. Examiner insight: “genetically identical” needs the reason (semi-conservative copying, then sister chromatids separating) rather than a restatement.

11. (a) Cut the root tip, where cells divide [1]; warm acid softens tissue and separates cells [1]; stain to make chromosomes visible [1]; squash under a cover slip to form a single layer that light passes through [1]. (b) Total = 346 + 21 + 9 + 6 + 8 = 390; mitotic = 44 [1]; 44 / 390 × 100 = 11.3% [1]. (c) 21 / 390 × 16 = 0.862 hours [1]; × 60 = 51.7 minutes (about 52) [1]. (d) 130 / 520 × 100 = 25.0% [1]; the spindle cannot form, so cells are held in mitosis (at metaphase) and accumulate [1]. Examiner insight: the denominator must be all cells counted; each method step needs its reason.

Where marks are usually lost

• Saying “gene” when the answer is locus, and “chromosome” when it is chromatid.
• Placing crossing over in the wrong stage or between sister chromatids.
• Using n = 14 instead of n = 7 in 2^n.
• Leaving out the purpose of the acrosome reaction or cortical reaction.
• Giving rate answers without units, or using the wrong time.
• Using only dividing cells as the denominator of the mitotic index.
• Describing method steps without saying why they are done.

Next steps

• Revision notes
• Study guide
• Edexcel A Level Biology hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology specification, Issue 2, February 2021 (Pearson Education Limited), statements 3.9 to 3.16 and Core Practical 6.