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Gametes and fertilisation: from two cells to a blastocyst questions
How sperm and eggs are produced and the ways spermatogenesis and oogenesis differ, the structure of each gamete tied to the job it does, binding to the zona pellucida and the acrosome reaction, fusion of the two membranes and then of the two haploid nuclei, the cortical reaction and why polyspermy has to be prevented, cleavage from zygote to blastocyst, and where fertilisation and implantation each take place.
8 original questions · 25 marks · the gametes and fertilisation: from two cells to a blastocyst notes · Cell cycles, reproduction and development
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Describe the events that take place from the moment a sperm reaches the zona pellucida to the moment the two nuclei fuse.
Mark scheme
- B1 the sperm binds to a complementary receptor on the zona pellucida
- B1 the acrosome membrane fuses with the sperm cell surface membrane and hydrolytic enzymes are released by exocytosis, which digest a path through the zona pellucida
- B1 the sperm cell surface membrane fuses with the egg cell surface membrane and the sperm nucleus enters the cytoplasm of the egg
- B1 the egg completes meiosis II, releasing the second polar body, and the two haploid nuclei fuse to give a diploid zygote nucleus
Compare the production of sperm in the testis with the production of eggs in the ovary.
Mark scheme
- B1 sperm production is continuous from puberty, whereas egg production begins before birth, is then arrested, and one cell is usually completed in each cycle
- B1 cytokinesis is equal in spermatogenesis, whereas in oogenesis it is unequal, so one daughter cell keeps almost all the cytoplasm
- B1 one primary spermatocyte gives four functional gametes, whereas one primary oocyte gives one functional gamete and two or three polar bodies
- B1 meiosis is completed before a sperm is released, whereas in the egg the second meiotic division is completed only if a sperm fuses with it
Explain how the cortical reaction prevents more than one sperm from fertilising an egg.
Mark scheme
- B1 fusion of the sperm and egg membranes raises the calcium ion concentration in the cytoplasm of the egg, which causes the cortical granules to fuse with the cell surface membrane
- B1 their contents are released by exocytosis into the space between the cell surface membrane and the zona pellucida
- A1 these contents alter the sperm receptors on the zona pellucida and harden it, so no further sperm can bind to it or pass through it
Explain why an embryo formed when two sperm fuse with the same egg does not develop.
The mid-piece of a sperm cell contains many mitochondria. Suggest why a sperm cell that must travel a greater distance to reach an egg would be expected to contain more of them.
Outline what happens to a human zygote between fertilisation and implantation.
A human egg is a sphere of diameter 110 µm. Calculate its volume, using V = 4/3 πr3. Give your answer in µm3 to two significant figures.
State where in the human reproductive system fertilisation takes place, and where implantation takes place.
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