The following diagram shows the arrangement of DNA in a eukaryotic chromosome.
Identify structure A in the diagram.
Identify structures B and C shown in part (a).
Chromosomes in diploid cells occur in homologous pairs.
Define the term homologous chromosomes.
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Define the term mitosis.
The drawings below show a cell during different stages of mitosis.
List the stages P, Q, R, S and T in the correct sequence.
Cell M contains 74 chromosomes. It divides by mitosis.
State the number of chromosomes that will be present in the new daughter cells.
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The diagram below shows two different stages of mitosis.
Identify structures M, N and O.
List three roles of mitosis in living organisms.
Describe the process that occurs during prophase that causes the chromosomes to become visible as separate structures.
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The diagram below shows some grasshopper cells.
Identify the stages of mitosis taking place in cells X and Y.
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State the role of meiosis in living organisms.
The table below contains a series of statements about meiosis in human cells.
Statement | True / false |
2 daughter cells are produced | False |
Homologous pairs of chromosomes are separated |
|
Daughter cells have the full number of chromosomes |
|
Two cycles of division take place |
|
DNA replication occurs before the process begins |
|
Complete the table by indicating whether each statement is true or false. The first row has been completed for you.
The diagram below shows a simplified life cycle of a fern plant.
n = haploid
2n = diploid
Identify the types of cell division taking place at the positions marked A and B.
Identify the process taking place at the position marked C in the diagram in part c).
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The image below shows two chromosomes.
(i) Identify the structures labelled X and Y.
[2]
(ii) Structures Y and Z are identical.
State why this is the case.
[1]
The two chromosomes shown in (b) can be described as homologous chromosomes.
Define the term homologous chromosome.
During meiosis the homologous chromosomes shown in (b) are separated.
Identify the stage of meiosis during which the homologous chromosomes are separated.
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The image below shows a pair of chromosomes during meiosis.
Identify the process taking place in the image.
State the alleles that will be present on each of the grey shaded chromatids at the end of the process shown in (a).
The cells of domestic cats contain 38 chromosomes.
Calculate the number of possible chromosome combinations that can be generated in domestic cat gametes during meiosis.
Use the formula 2n, where n = the number of chromosome pairs.
Another source of variation during meiosis is a chromosome mutation that results from an event known as chromosome non-disjunction.
State the meaning of the term non-disjunction.
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Describe one way in which the process marked X in part (a) can increase genetic variation.
Describe the chromosome activity taking place at the stage marked Y in part (a).
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The diagram below shows two chromosomes during meiosis.
Identify structures X, Y and Z.
Outline the event shown in part (a).
At the end of meiosis the chromosomes shown in (a) form four new chromosomes, as illustrated in the diagram below.
Annotate the four new chromosomes below to show the results of the event in part (a). The first chromosome has been annotated for you.
Outline the importance of the event shown in (a) to living organisms.
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The diagram below shows a cell in anaphase of meiosis I.
State how it is possible to know the following:
(i) That the cell is in anaphase.
[1]
(ii) That the cell is in meiosis I.
[1]
Meiosis I is described as reduction division.
State why this is the case.
Meiosis I generates genetic variation due to the process of crossing over.
Outline one other process during meiosis I that generates genetic variation.
Rice, Oryza sativa, has a chromosome number of 24.
Use the formula 2n to calculate the number of different chromosome combinations that can be generated when rice cells undergo meiosis I.
Note that the term n here denotes the number of pairs of chromosomes.
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The diagram below shows chromosomes from a garden pea plant, Pisum sativum, during meiosis.
Identify, with a reason, the meiotic division that is occurring in the diagram.
At the end of meiosis the chromosomes were distributed to the pea plant pollen grains as shown below.
Explain how the chromosomes in (a) gave rise to the new allele combinations shown above.
The process in (b) and random orientation both contribute to genetic variation.
State one other process that contributes to genetic variation.
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Draw an annotated diagram to show how crossing over takes place during meiosis.
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Distinguish between the terms chromosome and chromatid.
The following statements contain information about the stages of mitosis, with some details missing.
Stage 1: | Chromosomes become shorter and thicker, and the nuclear membrane breaks down. |
Stage 2: | Chromosomes line up along the equator of the cell and (i) , |
Stage 3: | (ii) , causing the chromatids to separate and move towards (iii) . |
Stage 4: | A new nuclear membrane forms around each group of chromosomes. |
State how Stage 2 and Stage 3 should be completed.
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Melanoma is a type of skin cancer that develops from pigment-producing cells in the skin known as melanocytes. Sun exposure is a risk factor in the development of melanoma.
Explain why sun exposure increases the risk of melanoma.
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The diagram below shows two chromosomes in a cell that is undergoing mitosis.
Identify structure Q and state what happens to it during anaphase.
After looking at the diagram in part (a) a scientist concluded that the two chromosomes are homologous.
Explain why the scientist reached this conclusion.
A group of researchers investigated the relationship between the average number of cigarettes smoked by men per day and the number of men dying from colon cancer in 14 different countries. The data from the study is shown in the graph.
A website reported the results of this investigation using the headline ‘Smoking causes colon cancer’.
Explain why the researchers could not support this view based on the results of their investigation alone.
Explain why the death rate from colon cancer in (c) is given per 100 000 men and not as the total number of deaths.
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The drawing below shows a micrograph of actively dividing cells in tissue taken from the tip of a plant root.
Complete the table to identify the stage of cell division visible in cells W, V and Z.
Cell | Stage of cell division |
W |
|
V |
|
Z |
|
In which of the cells (U, V, W, X, Y or Z) in the micrograph from (a) will the next event involve vesicles fusing to form new cell membranes.
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Describe cytokinesis in animal cells and in plant cells.
Describe the events that take place during mitosis, including the name of the stage of mitosis during which each event occurs.
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The cell in the diagram is undergoing cell division.
Identify, with a reason, the type of cell division shown in the diagram.
Pisum sativum (garden pea) has a diploid chromosome number of 14.
Calculate the number of different chromosome combinations can result during meiosis, assuming no crossing over occurs.
Use the formula 2n, where n = the number of pairs of chromosomes in a cell.
Discuss the significance of meiosis in the life cycle of Pisum sativum (garden pea).
Chlamydosaurus kingii (Australian Frillneck lizard) is a diurnal lizard with a distribution that extends across northern Australia and into Papua New Guinea. If the lizard is startled it opens its mouth and flexes the muscles in its frill (a large fold of skin surrounding its throat), causing it to be raised. Scientists believe that the frill is used to deter predators and to attract females.
The graphs below show the variation in frill length of 164 male lizards from two different sites 150 km apart.
Explain how meiosis may have caused the variation shown in these graphs.
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Compare meiosis and mitosis by completing the table using yes / no or numbers.
Feature | Meiosis | Mitosis |
Number of nuclear divisions |
|
|
Number of daughter cells produced |
|
|
Genetically different daughter cells are produced |
|
|
Crossing over occurs |
|
|
Homologous chromosomes pair up |
|
|
The diagram below shows a cell undergoing cell division.
Explain how the process shown in the diagram will generate genetically different daughter cells.
The diagram below shows two chromosomes during meiosis in a cell in the testis of Drosophila melanogaster (fruit fly). The position of the alleles of some genes is indicated.
Identify evidence that shows that the chromosomes are 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.
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The diagram shows the chromosomes found in a parent cell and in 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.
An example of a condition that can result from an error during meiosis is Down syndrome. The table shows how the chance of having a baby with Down syndrome changes with maternal age.
Maternal age (years) | Chance of conceiving a baby with Down syndrome |
25-29 | 1 in 1250 |
30-34 | 1 in 1000 |
35-39 | 1 in 400 |
40-44 | 1 in 100 |
45+ | 1 in 30 |
Calculate how many times higher the chance of conceiving a baby with Down syndrome is for a 41-year-old mother compared with a 26-year-old mother.
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Explain how genetic variation can be generated.
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The image below illustrates the formation of sperm cells, also known as spermatozoa.
(i) Describe the change in chromosome number taking place during division I.
(1)
(ii) Explain the importance of the change described in (i).
(1)
A sperm-producing cell in the testes has 46 chromosomes in its nucleus.
Calculate the number of chromatids that would be in the nucleus of this cell after it has undergone meiosis I.
Outline the first steps in the process of meiosis, known as prophase I.
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The diagram below shows a homologous pair of chromosomes from a parent cell (top) and two gamete nuclei that form at the end of meiosis (bottom).
Draw the chromosomes present in each of the indicated gamete nuclei at the end of meiosis. Consider both shading and alleles in your answer.
Outline the events that occur during anaphase I of meiosis.
Explain how random orientation contributes to genetic variation in gametes.
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Compare and contrast meiosis II and mitosis.
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Compare and contrast the process of cytokinesis in plants and animals.
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The graph below shows changes in the mass of DNA over the course of a cell cycle.
Explain the role of the process represented by the graph, in living organisms.
Variation is introduced at the points labelled X and Y in the graph in part a).
Identify processes that introduce variation at point X and at point Y.
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The diagram below illustrates the life cycle of the pea aphid, Acyrthosiphon pisum. Note that the term autosome refers to any chromosome that is not a sex chromosome.
Identify the biological sex of the individuals labelled P and Q.
The diagram in part a) shows that aphids use a different type of reproduction in the spring and summer to the type used in the autumn.
(i) Identify the types of reproduction used by aphids in the spring and summer, and in the autumn.
[1]
(ii) Explain your answer to (i).
[2]
(iii) Suggest the advantage to the aphids of switching their method of reproduction in the autumn.
[1]
Most eukaryotic chromosomes are described as being monocentric. Aphids have unusual chromosomes known as holocentric chromosomes. A holocentric chromosome after DNA replication is shown in the diagram below.
Distinguish between holocentric chromosomes and the monocentric chromosomes found in humans.
While scientific understanding of aphid meiosis is still limited, the holocentric nature of their chromosomes means that aphids are thought to carry out a form of meiosis known as inverted meiosis. The possible behaviour of a homologous pair of aphid chromosomes during metaphase I is shown in the diagram below.
Suggest, with reasons, two ways in which meiosis in aphids might be different to conventional meiosis.
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The spider mite Eutetranychus africanus has very few chromosomes (2n = 4).
The diagram below shows a series of cells undergoing cell division.
Identify, with reasons, which of the cells in the diagram above belong to E. africanus.
A species of false spider mite, Brevipalpus phoenicis, is the only animal to have so far been identified as having exclusively haploid cells throughout its life cycle. B. phoenicis populations are entirely female, producing eggs which hatch into more females.
The discovery of the haploid nature of B. phoenicis was a surprise to scientists, who believed that being diploid was essential due to the evolutionary advantage that it provides.
(i) Identify the type of cell division by which B. pheonicis produces eggs.
[1]
(ii) Suggest why scientists might think that diploidy provides an evolutionary advantage.
[1]
(iii) B. phoenicis is a highly successful pest of citrus, tea, and palm plantations.
Suggest how B. phoenicis might have evolved to become such a successful pest despite the points covered in (i) and (ii) above.
[1]
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Describe the roles of mitosis and meiosis in living organisms. Include named examples for each role.
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Rotifers are multicellular, aquatic animals that range in size from 50 μm to 3 mm. Their reproduction can be either asexual, resulting in the production of genetically identical females, or sexual, resulting in the production of eggs that can remain dormant for many years. A representation of a rotifer life cycle is shown in the diagram below.
Annotate the diagram as follows:
(i) Use the letter A to indicate one location within the asexual phase where mitosis is occurring.
[1]
(ii) Use the letter B to indicate one location within the sexual phase where mitosis is occurring.
[1]
(iii) Use the letter C to indicate two locations where meiosis is occurring.
[1]
(iv) Use the letter D to indicate one location where fertilisation is occurring.
[1]
Explain why meiosis is essential for sexual reproduction.
Rotifers mainly reproduce asexually, switching to sexual reproduction for brief periods. The X in the diagram in part a) indicates the occurrence of a stimulus that shifts the rotifers from asexual to sexual reproduction. This stimulus could be a change in the environmental conditions, such as a drought that reduces the size of their habitat.
Use the information provided here and in part a) to explain why it is advantageous to rotifers to switch to sexual reproduction when environmental conditions change.
Rotifers can be observed using an optical microscope.
Explain why this is the case.
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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:
(i) Stage X
[1]
(ii) Stage Y
[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.
(i) Calculate the number of different combinations of chromosomes that could be generated when this cell divides by meiosis.
Use the formula 2n, where n represents the number of pairs of chromosomes.
[1]
(ii) 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]
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Describe the process of meiosis.
Explain the link between meiosis and evolution.
Note; details of the process of meiosis are not required.
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