3.2 Meiosis

Polyploidy is a condition in which cells have a chromosome number that is greater than the normal diploid (2n) number. Polyploidy is considered to be a useful characteristic in crop plants as it gives rise to bigger plant organs and provides cells which contain a larger variety of alleles for breeding programmes. While polyploidy can occur naturally in plants, it can also be induced artificially using a chemical called colchicine. Colchicine works by preventing the formation of the microtubules that make up the spindle fibres inside cells.

Suggest how colchicine gives rise to tetraploid (4n) cells after mitosis in plants.

Plant scientists have been able to breed seedless watermelons by crossing tetraploid watermelons with regular, diploid plants to produce infertile, triploid (3n) plants. The diagram below illustrates this process. Note that diploid watermelons contain 22 chromosomes.

Suggest why the offspring of the tetraploid-diploid cross are infertile.

For the triploid offspring shown in part b) to start producing fruits, the triploid plants must be pollinated with pollen from diploid plants. Pollination involves the transfer of a plant gamete, pollen, from the flower of one plant to the flower of another. While the diploid pollen does not fertilise the triploid plants (they are infertile), it does trigger fruit production, and the resulting watermelon fruits are seedless. The graphic below shows a recommended planting plan for farmers that will ensure pollination of the triploid plants with pollen from diploid plants.

State why diploid plants need to be used for pollination rather than other triploid plants.

[1]

Suggest two disadvantages to farmers of the method described here and in part b) for growing seedless watermelons.

[2]

A watermelon variety that naturally produces fewer seeds has been identified by scientists, and observation of its cells indicates that an event known as reciprocal translocation of chromosomes occurs in the cells of the watermelon variety. Reciprocal translocation of chromosomes involves the exchange of entire sections of chromosomes between non-homologous chromosomes during meiosis.

Contrast reciprocal translocation of chromosomes and crossing over with each other.

[1]

Suggest how reciprocal translocation of chromosomes could result in a watermelon plant that produces fruits containing fewer seeds.

[2]

The graph below shows how the mass of DNA changes over time during two different types of cell division in a diploid cell.

State what is happening at the following stages in the graph:

Stage X

[1]

[1]

Use the information provided in the graph in part a) to state, with a reason, which of the division types represents meiosis.

The image below shows a diploid cell containing several chromosomes.

Draw the possible appearance of four different gametes that could be produced from this diploid cell during meiosis.

A diploid cell contains 26 chromosomes.

Calculate the number of different combinations of chromosomes that could be generated when this cell divides by meiosis.

[1]

Explain why the number calculated in part i) is not a true representation of the amount of genetic variation that can be generated from this cell during sexual reproduction.

[2]

Edwards syndrome is a rare but serious condition that influences birth weight and development. Death rates during infancy are high. The image below shows the karyogram of an individual with Edwards syndrome.

Use the karyogram to suggest the cause of Edwards syndrome.

[1]

Describe the events that have led to the feature noted in part i).

[2]

Edwards syndrome affects every cell in the body.

Explain why this is the case.

One of the life-threatening complications that babies with Edwards syndrome can be born with is a structural abnormality in the heart. The image below shows a heart abnormality that can be seen in babies born with Edwards syndrome.

Explain why the abnormality shown in the image can be life-threatening.

The graph below shows the number of pregnancies affected by Edwards syndrome between 1985 and 2008.

In the year marked X there were 1 100 000 pregnancies, and in the year marked Y there were 700 000 pregnancies.

Calculate the number of affected pregnancies at the times marked X and Y.

[2]

Suggest one reason for the difference in affected pregnancies between times X and Y.

[1]

One mark is available for clarity of communication throughout this question.

Describe the process of meiosis.

Explain the link between meiosis and evolution.

Complete the table using ‘yes’ / ‘no’ or numbers to compare mitosis and meiosis.

Explain how the diagram below illustrates that the resulting daughter cells will be genetically different. Give evidence from the diagram to support your answer

The diagram below shows a pair of chromosomes during meiosis in a cell in a Drosophila melanogaster (fruit fly) testis. The position of the alleles of some genes is indicated.

Explain whether the chromosomes are homologous or non-homologous.  

At the end of meiosis, each of the chromosomes shown in the diagram from part (c) will be in a different haploid cell.

Label the diagram above to show the combinations of alleles that would be present on each chromosome inside the haploid daughter cells.

The diagram shows the chromosomes found in a parent cell and the daughter cells produced after meiosis

Identify the daughter cell(s) that contain a chromosome mutation by circling the cell(s).

Explain how the spontaneous chromosome mutation shown in part (a) arose in the daughter cells during meiosis. 

The risk of a non-disjunction mutation increases with age. The table shows how the Down syndrome risk increases with age.

Calculate how much more likely it is that a child is conceived with Down Syndrome for a mother who is 41 compared to a mother who is 26.

Describe how the process of amniocentesis can be used to obtain cells which can be used for chromosome analysis.

The diagram below shows the life cycle of Calvatia gigantea (giant puffball). In this life cycle, only the zygote and mature puffball are diploid. All the cells in all the other stages of the life cycle of the puffball are haploid, including the spores.

Identify which letter in the diagram shows where meiosis occurs in the life cycle of Calvatia gigantea

The micrographs below show cells undergoing meiosis.

Identify, with a reason, the stages of meiosis shown in each micrograph.

A Pisum sativum (garden pea) pollen cell is undergoing meiosis. During the initial phase of meiosis a pair of homologous chromosomes located in these cells can be represented by the chromosomes shown below. The two different letters represent two different genes.

At the end of meiosis the chromosomes were distributed to the four pollen grains as shown in the gametes below.

Figure 2

Describe how the new allele combinations seen in these gametes were formed during meiosis. 

One mark is available for clarity of communication throughout this question.

Draw a labelled diagram to show a cell in the following stages of meiosis:

• Metaphase I
• Prophase I

Outline how developments in scientific methods facilitated the discovery of meiosis.

Explain how genetic variation may be introduced into a population.