Cell Division (AQA GCSE Biology: Combined Science)

Figure 1 below is a photomicrograph of cells in the root of an onion.

Figure 1

On the image above, label a cell that is actively dividing with the letter X.

Name the type of cell division that occurs in the meristem of plants for growth.

Before a cell divides, what must happen to its genetic material?

A group of students decided to investigate the cell cycle.

They prepared a microscope slide of a root tip from an onion.

They then visualized the slide under a light microscope to count the number of cells in each stage of the cell cycle in one field of view.

The table below shows their results.

Stages in the cell cycle

Different stages of the cell cycle take different amounts of time.

Which stage of the cell cycle is the slowest?

Give a reason for your answer.

It takes 18 hours for one complete cell cycle in a typical onion root tip cell.

Calculate the length of time Stage 3 lasts in a typical onion root tip cell.

Give your answer to 2 significant figures.

Read the information below about stem cells:

It may be possible to treat some human diseases, such as Parkinson’s and diabetes, with stem cells in the future.

Human early embryos produced in vitro are a source of stem cells.

The use of embryonic stem cells to treat human disease is new and patient trials have already begun.

Another source of stem cells is adult bone marrow.

The operation for collecting these cells is simple but it can be painful. It is already possible to use adult stem cells to treat certain blood diseases.

There is also hope that adult stem cells from bone marrow can also be used to treat other diseases such as heart disease; such trials have already begun.

Explain how an embryonic stem cell differs from an adult stem cell.

Evaluate the use of embryonic and adult stem cells to treat human diseases.

Your answer should contain a conclusion.

How many pairs of chromosomes are there in a body cell of a human?

For any organism to grow, it must be capable of making new cells.

Describe what must happen in a cell before it can divide.

Outline the process by which a cell divides by mitosis.

Why is cell specialisation (differentiation) an important process in the development and growth of a fertilised egg into a healthy baby?

After a baby is born, it is possible to use the umbilical cord as a source of stem cells.

What are stem cells?

In Vitro Fertilisation can be used to produce embryos for research purposes. Stem cells can be extracted from a 3-day-old embryo made in this way.

Suggest why it may be considered more ethically acceptable to take stem cells from an umbilical cord rather than an embryo.

Stem cells taken from an umbilical cord can be frozen and stored. They could be used later in that child’s life to treat a condition.

Suggest a reason why it would be better to use a child’s own umbilical cord stem cells instead of stem cells donated from another person.

Figure 2 below outlines the process of therapeutic cloning.

Figure 2

This technique could be used to produce cells to treat conditions such as paralysis or diabetes, or to produce a baby.

Scientists may be allowed to use therapeutic cloning to treat disease but not to produce a baby.

Using information from the diagram and your own knowledge, suggest an explanation for this.

The nucleus of a typical onion cell contains 16 chromosomes.

What is a chromosome?

An onion cell divides by mitosis.

How many chromosomes will each of its daughter cells contain?

Figure 3 shows a photograph of an onion cell at one point during the cell cycle.

Figure 3

Describe what is happening to the cell in Figure 3.

Suggest the events that would occur after those observed in the cell in Figure 3.

Onion plants contain meristem tissue.

Compare the meristem tissue found in an onion plant with bone marrow tissue found in humans.

Many plants are at risk of extinction as a result of human activity.

Stem cells from the meristem can be used to clone rare plant species.

Outline the advantages of cloning plants in this way.

Suggest one other useful application of cloning plants. 

Suggest one disadvantage of producing clones of plants from meristems.

Embryonic stem cells have the potential to be used to treat many human diseases.

Suggest how embryonic stem cells could be used to treat a person who has been paralysed.

Figure 1 shows the quantity of DNA found in the nucleus of a human cell throughout the cell cycle.

Identify what is happening during stage B on Figure 1. Explain your answer.

Figure 1 shows two complete cell cycles. 50% of the cell cycle is spent in stage A and 30% is spent moving through stage C. 

Calculate how long the cells spend in stage B of the cell cycle.

How many chromatids would be found in the nucleus of this human cell during stage B?

Describe the stages of the cell cycle.

When a cell divides by mitosis, it produces identical daughter cells, however, when a zygote divides by mitosis, it develops into an embryo and then a foetus.

Explain how the process of mitosis can produce a whole organism from a single cell.

Compare and contrast the processes of mitosis and meiosis.

Initially, before a cell can divide, it needs to prepare for division by replicating DNA within the nucleus. During this time, there are no visible chromosomes. 

Figure 2 shows a cell sample taken from plant tissue. The cell cycle in this plant tissue takes 20 hours.

Figure 2

Calculate how long cells remain in the preparation stage before cell division occurs.

Multiple sclerosis (MS) is a condition which affects the central nervous system caused by the immune system mistakenly attacking the brain and nerves. MS can lead to symptoms which flare up periodically, such as fatigue, vision problems, muscle spasms, mobility problems, difficulty in thinking, learning or planning, amongst others.

There are several different treatments for MS; two are considered here.

1. Stem cell treatments:  Healthy cells found in the patients bone marrow can be used to rebuild the immune system. Must be used in combination with chemotherapy to remove the harmful immune cells.
2. Disease modifying therapies (DMT): Medicines that can reduce the number of relapses a patient suffers and reduce the symptoms of MS. An example of a DMT is a drug called Lemtrada.

Table 1 gives some extra information about these two treatments for MS.

Table 1

Evaluate the use of stem cells and disease modifying therapies to treat multiple sclerosis.

Heart attacks may be caused if a coronary artery becomes blocked and the flow of oxygenated blood is restricted. This can result in damage to the heart muscle tissue.

Suggest how embryonic cells could be used to treat a patient after a heart attack.

A patient who has suffered a heart attack was offered treatment using stem cells from an embryo, but refused due to the use of embryonic stem cells.

Suggest a reason for this decision.

The patient mentioned in part (c) was offered treatment using their own healthy stem cells removed from their heart tissue.

Evaluate the decision to use these stem cells as an alternative to embryonic stem cells in the treatment of the damaged heart tissue.

Some scientists studied mitosis in yeast cells. Figure 3 shows the concentration of DNA and the number of yeast cells over a 12 hour period.

Figure 3

Describe and explain the shape of the curve for the number of yeast cells between 2 and 8 hours.

Onc cell cycle in Figure 3 from part (a) takes 6 hours.

Calculate the percentage of time taken by mitosis in one cell cycle.

Figure 4 shows cells in two stages of division.

Figure 4

Describe the changes that occur in the cell between image X and image Y.

Cancer cells originate from healthy cells where the cell cycle is not being controlled normally. This can lead to uncontrolled cell division and the formation of tumours. Drugs used to destroy cancer cells also cause damage to healthy cells, although are more effective against the cancer cells.

Figure 5 shows the effect of a chemotherapy cancer treatment on the number of healthy and cancerous cells over a 12-week time period.

Figure 5

Calculate how many more cancer cells than healthy cells were destroyed after the first treatment.

Give your answer in standard form.

With reference to the data in Figure 5, identify a time period in which the cancer cells were dividing by mitosis. 

Another treatment for cancer patients is to receive a stem cell transplant from a donor. The process includes the following steps:

1. The patient is given a high dose of chemotherapy
2. Stem cells are taken from the bone marrow of a closely-related individual (with similar tissue type and genotype)
3. Stem cells are introduced into the patient
4. The patient is monitored for after-effects of the transplant

With reference to the data in Figure 5 and the information provided here, suggest why a high dose of chemotherapy might be necessary.

With reference to the process described in part (c), suggest an issue that might be associated with receiving a stem cell transplant which requires careful monitoring of the patient after the procedure (stage 4).