Meiosis

The diagram shows four consecutive phases of meiosis in an animal cell.

Phase 1
Phase 2
Phase 3
Phase 4

Which of the following statements is/are correct?

1 The cell is undergoing the first meiotic division.

2 Chiasmata formation starts taking place during phase 2.

3 Independent assortment of chromosomes is taking place during phase 1.

A 1, 2 and 3 are correct

B Only 1 and 2 are correct

C Only 2 and 3 are correct

D Only 1 is correct

Your answer

The hormones that control gametogenesis have different roles in males and females.

State the role of luteinising hormone (LH) in males and in females.

Explain why follicle stimulating hormone (FSH) is often used to treat infertility in women.

The light microscope image shows a section through part of a human ovary.

Identify the structures labelled H, J and K.

Outline the formation of secondary oocytes from oogonia.

The most common cause of Down’s syndrome is trisomy 21, which results in three copies of chromosome 21 being present in the cells of an affected individual.

Describe how trisomy 21 occurs.

Describe one other way in which Down’s syndrome can be caused.

The risk of having a child with Down’s syndrome increases with the mother’s age.

Suggest and explain a reason for this.

Describe two ways in which an older couple can reduce the risk of having a child with Down’s syndrome.

Identify one ethical issue associated with either of your answers to part (ii).

Regulation of the menstrual cycle involves interaction between several hormones.

Complete the sentences using the most appropriate words or phrases.

FSH is secreted by the........ and transported in

the blood to the ovary, where it stimulates follicle development. The mature follicle

releases oestrogen that stimulates a surge in secretion of........,

which causes........ and development of the follicle into the

During each cycle, FSH stimulates the development of multiple follicles.

One of these, known as the dominant follicle, grows faster than the others and produces more oestrogen.

Suggest how oestrogen produced by the dominant follicle leads to a reduction in the development of the other follicles.

Oestrogen also thickens the lining of the uterus in preparation for implantation of the blastocyst after fertilisation by a sperm cell.

Describe two functions of the acrosome in fertilisation.

The diagrams show three stages of meiosis in a model cell with two pairs of chromosomes.

Identify the stage of meiosis shown in each diagram. For each diagram, explain the evidence that indicates which stage is shown.

* Explain the importance of meiosis and fertilisation in increasing genetic variation.

Extra answer space if required.

Spermatogenesis in human males produces haploid gametes, called spermatozoa (sperm cells).

The structure of a mature human spermatozoan consists of three sections: a head, a midpiece and a tail.

For each of these sections, describe how its internal structure relates to the function of the spermatozoan.

During spermatogenesis, the chromatin in the nuclei of sperm cells undergoes a complex transition where histone is replaced by another protein, protamine, which binds to the DNA.

Protamine contains large quantities of the polar amino acid, arginine.

Suggest why having large quantities of arginine enables protamine to bind effectively to DNA molecules.

Substituting histone with protamine allows DNA to be more densely packaged.

Suggest one advantage of having densely packaged DNA in a sperm nucleus.

It has been found that protamine deficiency can lead to a severe disruption of spermatogenesis affecting male infertility.

An investigation was carried out into the effect of protamine deficiency on the production of healthy sperm cells.

Forty semen samples were collected from four groups of men as shown in the table.

Microscope slides were prepared by fixing and staining a smear from each semen sample. The staining technique used distinguished between protamine deficient and normal sperm cells.

The researchers concluded that the ages of the men in the groups would not affect the validity of the results of this investigation.

Explain how the researchers could have justified this conclusion.

What is the role of Group 1 in this investigation?

The results of the investigation are shown in the graph.

Using the information from the table and the graph, what conclusions can be made about the results of the investigation?

* Female gametes, or ova, are produced during oogenesis.

Compare and contrast spermatogenesis and oogenesis in the production of gametes. Include reference to the role of hormones.

Additional answer space if required.

Meiosis II pauses in the secondary oocyte and is only completed if fertilisation occurs.

Which of the options, A to D, is a description of the last meiotic stage prior to fertilisation?

A Homologous chromosomes align to the equator of the oocyte.

B Homologous chromosomes migrate to opposite poles of the oocyte.

C Single chromosomes align to the equator of the oocyte.

D Sister chromatids migrate to opposite poles of the oocyte.

Oogenesis occurs in the ovaries of female mammals, resulting in the production of gametes.

Name the type of nuclear division that results in the production of secondary oocytes from primary oocytes during oogenesis.

Complete the table below to indicate the stage and type of nuclear division in which the events being described occur.

Fig. 2.1 shows how oestrogen production varies throughout life in human females.

Explain how the rapid increase in oestrogen production towards the end of the growth phase results in the release of secondary oocytes.

Changes occur to the cells within ovaries over the course of a woman’s lifetime.

Use the data in Fig. 2.1 to explain the role of oestrogen production in the changes to ovarian cells that occur during adulthood.

In your answer you should refer to the stages of nuclear division that occur.

Fig. 2.2 shows how the number of ovarian follicles changes from birth to 50 years of age.

Using the data in Fig. 2.2, describe and explain the relationship between the number of follicles and age.

Name the biological process that occurs as a result of changes in the number of follicles between 45 and 50 years of age.

Describe two symptoms that women may experience during this process.