Sexual reproduction in mammalsEdexcel A-Level Biology B: Subtopic test

1. 1
   A fertility clinic is investigating why a couple have been unable to conceive. Analysis of the man's semen sample shows that the sperm swim normally, but in laboratory tests most of them fail to penetrate the layer surrounding an egg.

   (a)

   Which structure in the head of a sperm contains the enzymes that digest the layer surrounding the egg?

   [1 mark]

   • AMidpiece
   • BFlagellum
   • CNucleus
   • DAcrosome

   (b)

   The midpiece of a sperm is packed with mitochondria. What is the function of these mitochondria?

   [1 mark]

   • ATo supply ATP for the movement of the flagellum
   • BTo digest the zona pellucida
   • CTo store the haploid set of chromosomes
   • DTo make testosterone

   (c)

   Explain why sperm that cannot release the contents of the acrosome are unlikely to fertilise an egg.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Oogenesis begins before a girl is born. By birth her ovaries contain about one million primary oocytes, each surrounded by follicle cells. From puberty, roughly once every 28 days, one follicle matures and an oocyte is released from the ovary.

   (a)

   Which cell is released from the ovary at ovulation?

   [1 mark]

   • AA primary oocyte arrested in prophase I
   • BA secondary oocyte arrested in metaphase II
   • CA mature ovum with a haploid nucleus
   • DAn oogonium

   (b)

   During oogenesis the cytoplasm divides unequally, producing very small polar bodies. What is the advantage of this?

   [1 mark]

   • AIt increases the number of eggs produced from each primary oocyte
   • BIt makes the egg diploid
   • CThe egg keeps most of the cytoplasm and food reserves while the spare chromosomes are discarded
   • DIt allows the polar bodies to be fertilised instead

   (c)

   Explain why the cell released at ovulation is not yet a mature ovum.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Researchers study fertilisation in the laboratory by adding sperm to secondary oocytes that are surrounded by a layer called the zona pellucida. Many sperm bind to the zona pellucida, but only one sperm nucleus normally enters each egg.

   (a)

   Describe how a sperm penetrates the layer surrounding the egg and enters it.

   [3 marks]

   (b)

   Explain how the egg ensures that only one sperm nucleus enters, and why this is important.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In humans, gametes are made by different processes in the ovaries and the testes, but both are needed to produce an embryo. After fertilisation the embryo travels along the oviduct for about five days before it reaches the uterus.

   (a)

   Compare oogenesis with spermatogenesis.

   [6 marks]

   (b)

   Describe the development of the embryo from fertilisation to the blastocyst stage.

   [6 marks]

   Total for question 4: 12 marks

End of questions