1. 1
   A student prepares a stained squash of an onion (Allium cepa) root tip, where cells are dividing by mitosis. Onion body cells have a diploid number of 16. She describes four cells under the microscope. In cell 1, the chromosomes are visible as thick, condensed threads scattered through the nucleus area, and no nuclear membrane can be seen. In cell 2, the chromosomes are lined up in a single row across the middle of the cell. In cell 3, two groups of V-shaped chromosomes are moving towards opposite ends of the cell. In cell 4, there are two groups of chromosomes at opposite ends of the cell, each beginning to be surrounded by a nuclear membrane, with a thin line forming across the middle of the cell between them.

   (a)

   Which cell is in anaphase?

   [1 mark]

   • ACell 1
   • BCell 2
   • CCell 3
   • DCell 4

   (b)

   How many chromatids are present in cell 2?

   [1 mark]

   • A64
   • B32
   • C16
   • D8

   (c)

   Explain how the cytoplasm of cell 4 will be divided and why this differs from cytokinesis in an animal cell.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In human oogenesis, the first division of meiosis produces one large secondary oocyte, which receives almost all of the cytoplasm, and one very small cell called a polar body, which later degenerates. Baker's yeast (Saccharomyces cerevisiae) reproduces asexually by budding: a small outgrowth, the bud, forms on the parent cell and grows. The nucleus divides by mitosis, one nucleus moves into the bud with some mitochondria, and the bud then separates while still much smaller than the parent cell.

   (a)

   Why must the bud receive at least one mitochondrion?

   [1 mark]

   • AMitochondria can only be produced by division of pre-existing mitochondria
   • BMitochondria contain the genes needed to make the nucleus
   • CMitochondria are needed to build the plasma membrane of the bud
   • DThe bud's ribosomes are made inside mitochondria

   (b)

   What is the advantage of unequal cytokinesis during oogenesis?

   [1 mark]

   • AIt produces four eggs of equal size
   • BIt halves the chromosome number of the egg
   • CThe polar body can develop into a sperm cell
   • DThe egg keeps most of the cytoplasm, with the nutrients and organelles needed by the early embryo

   (c)

   Suggest why the nucleus must divide before the bud separates from the parent cell.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Down syndrome is caused by having three copies of chromosome 21. A large survey recorded the number of babies born with Down syndrome per 1000 births for mothers of different ages: age 20, 0.7; age 30, 1.1; age 35, 2.8; age 40, 10; age 45, 33. Genetic analysis shows that in about 90% of cases the extra chromosome comes from the mother's egg rather than the father's sperm. Human egg cells begin meiosis before the mother is born, and the first division of meiosis is not completed until shortly before ovulation, which may be decades later.

   (a)

   Explain how non-disjunction during meiosis can result in a child with Down syndrome.

   [3 marks]

   (b)

   Analyse the data to describe the effect of maternal age on the incidence of Down syndrome, and suggest a reason for this effect.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Humans have a diploid number of 46 chromosomes, made up of 23 homologous pairs. Apart from identical twins, no two children of the same parents are genetically identical. Genetic studies show that during each meiosis in humans there are, on average, several dozen crossing-over events spread across the homologous pairs.

   (a)

   Explain how the two divisions of meiosis produce four haploid nuclei from one diploid nucleus, deducing the number of chromosomes and chromatids in each human nucleus at the end of meiosis I and at the end of meiosis II.

   [6 marks]

   (b)

   Discuss how meiosis and fertilisation together generate the genetic diversity shown by the children of the same parents, including a calculation of the number of chromosome combinations possible in a human gamete from random orientation alone.

   [6 marks]

   Total for question 4: 12 marks

End of questions