Questions on gametes, fertilisation and early development

Question 1

Describe the events that take place from the moment a sperm reaches the zona pellucida to the moment the two nuclei fuse.

Accepted Answer

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

Question 2

Compare the production of sperm in the testis with the production of eggs in the ovary.

Accepted Answer

B1 sperm production is continuous from puberty, whereas egg production begins before birth, is then arrested, and one cell usually resumes meiosis and is released 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

Question 3

Describe the function of each of the acrosome, the mid-piece, the tail and the haploid nucleus of a sperm cell.

Accepted Answer

B1 the acrosome holds hydrolytic enzymes that are released to digest a path through the zona pellucida around the egg B1 the mid-piece is packed with mitochondria, which release ATP by aerobic respiration B1 the tail uses that ATP to beat, and is the only means by which the sperm moves B1 the haploid nucleus carries 23 chromosomes, tightly condensed with very little cytoplasm around it, keeping the cell small

Question 4

Explain how the cortical reaction prevents more than one sperm from fertilising an egg.

Accepted Answer

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

Question 5

Explain why an embryo formed when two sperm fuse with the same egg does not develop.

Accepted Answer

B1 each gamete carries one haploid set of chromosomes, so the cell formed contains three sets, 69 chromosomes in a human, instead of two sets B1 every gene is then present in three copies rather than two, so the balance of gene products is disturbed throughout development; mitosis itself can still copy and divide three sets A1 the embryo and the placenta develop abnormally and development stops at an early stage

Question 6

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.

Accepted Answer

B1 the mitochondria are the site of aerobic respiration, which releases ATP B1 ATP is required for the movement of the tail, which is the only means by which the sperm moves B1 a greater distance requires movement for longer, so more ATP is needed and more mitochondria supply it

Question 7

Outline what happens to a human zygote between fertilisation and implantation.

Accepted Answer

B1 the zygote divides by mitosis while travelling along the oviduct, in divisions called cleavage, with no growth between them, so the total size does not increase B1 a solid ball of cells called a morula forms by about day four, and by about day five a fluid-filled cavity has formed and it is a blastocyst with an inner cell mass B1 the blastocyst leaves the zona pellucida and implants in the endometrium, the lining of the uterus, about six or seven days after fertilisation

Question 8

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.

Accepted Answer

M1 radius = 110 divided by 2 = 55 µm M1 V = 4/3 × π × 55³ A1 7.0 × 10⁵ µm³; accept 696 910 µm³ correctly rounded

Question 9

Explain why mitochondrial DNA in a person is inherited from their mother and not from their father.

Accepted Answer

B1 the sperm contributes very little cytoplasm to the zygote, so its few mitochondria are vastly outnumbered by those in the egg's cytoplasm B1 the paternal mitochondria that enter with the sperm's mid-piece are recognised and destroyed inside the egg A1 all the mitochondria in the zygote, and so in every cell descended from it, therefore came from the egg's cytoplasm, which is maternal

Question 10

A healthy adult male produces sperm at a rate of about 100 million cells a day. Calculate the number of sperm produced in 90 days, and give your answer in standard form to two significant figures.

Accepted Answer

M1 number produced = rate per day × number of days M1 100 000 000 × 90 A1 9.0 × 10⁹ sperm, to two significant figures

Question 11

A man's sperm count is found to be much lower than normal, and investigation shows that his Sertoli cells are damaged. Suggest why this would reduce his sperm production.

Accepted Answer

B1 Sertoli cells nourish the developing spermatids as they mature into spermatozoa, supplying them with nutrients B1 if Sertoli cells are damaged, the spermatids they support cannot obtain what they need to complete maturation A1 fewer sperm cells would complete development successfully, so the number of mature, functional sperm falls

Question 12

Explain why fertilisation between two different species usually fails, even if sperm from one species reaches an egg of the other.

Accepted Answer

B1 a sperm can only bind to the zona pellucida if its surface molecules are complementary in shape to the receptors on that egg's zona pellucida B1 the receptor and its complementary molecule have evolved together within a species, so a sperm from a different species is unlikely to have a matching shape A1 without binding, the acrosome reaction is not triggered and the sperm cannot digest a path through the zona pellucida, so fertilisation does not proceed

Question 13

Occasionally a blastocyst implants in the oviduct instead of the uterus, which is called an ectopic pregnancy. Suggest why this pregnancy cannot continue safely and is dangerous to the mother.

Accepted Answer

B1 the oviduct is a narrow tube not built to support the growth of a placenta or the size of a growing embryo B1 as the embryo grows it can rupture the wall of the oviduct, causing internal bleeding A1 this bleeding is a direct danger to the mother's life, so the pregnancy has to be ended rather than allowed to continue

Question 14

State where in the human reproductive system fertilisation takes place, and where implantation takes place.

Accepted Answer

B1 fertilisation takes place in the oviduct, also accepted as the fallopian tube or egg tube B1 implantation takes place in the endometrium, the lining of the uterus

Question 15

State the function of the zona pellucida of an egg, and state what the egg's large cytoplasmic volume contains.

Accepted Answer

B1 the zona pellucida is a glycoprotein coat that carries the receptors a sperm binds to B1 the cytoplasm contains organelles, maternal RNAs and nutrients that support the early stages of development after fertilisation