Mitosis, meiosis and fertilisation

Sample questions

Original practice questions written in the style of the exam.

1. Question 14 marks

   Explain how meiosis produces genetically different gametes.

   Mark scheme

   • Homologous chromosomes pair up in meiosis I (1)
   • Crossing over: non-sister chromatids exchange sections at chiasmata, producing new combinations of alleles (1)
   • Independent assortment: homologous pairs line up randomly at the equator, so maternal and paternal chromosomes are distributed in different combinations (1)
   • Chromosome number is halved, so each gamete has a different combination of one chromosome from each pair (1)
2. Question 24 marks

   Describe the events of double fertilisation in a flowering plant, from the time a pollen grain lands on the stigma.

   Mark scheme

   • A pollen tube grows down the style, secreting digestive enzymes (1)
   • The tube nucleus leads, and the generative nucleus divides by mitosis to form two male nuclei (1)
   • One male nucleus fuses with the egg cell nucleus to form a diploid zygote (1)
   • The other male nucleus fuses with the two polar nuclei to form a triploid endosperm nucleus (1)