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GCSE Biology · GCSEChapter 4 · Mitosis, the cell cycle and stem cells
GCSE · Chapter 4

Mitosis, the cell cycle and stem cells

By the end of this chapter you will be able to

  • Put the stages of the cell cycle in order and describe what happens in each
  • Describe mitosis and say why the daughter cells are genetically identical
  • Compare embryonic and adult stem cells, and describe the issues with therapeutic cloning
  • Explain how plant meristems and cancer relate to cell division
  • [H] Explain the checks made during the cell cycle, and interpret a graph of DNA content

You should already know

  • Cells and their nuclei (Chapter 2)
  • Genes, DNA and chromosomes at KS3 Biology

Where this leads

  • Sexual reproduction and meiosis (Chapter 15)
  • Growth and specialised cells in plants (Chapter 8)
  • Vaccination and immunity (Chapter 14)

GCSE Biology. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe the cell cycle: growth, DNA replication, mitosis and division, Describe mitosis and explain why it produces identical cells for growth and repair, Explain chromosomes and the nucleus, and the number of chromosomes in body cells, Compare embryonic and adult stem cells, and explain therapeutic cloning, Explain stem cells in plants and cancer as uncontrolled division, [H] Describe the stages of the cell cycle in more detail.

Exercise P  Do you remember?

1
FWhich part of a cell holds the chromosomes? Write the word.
2
FWhich organelle releases energy from glucose during aerobic respiration? Write the word.
3
FWhat is 2 multiplied by 2 multiplied by 2? Write the number.
4
FName the process by which a cell makes new cells for growth and repair. Write the word.
5
FIn one sentence, what is DNA? Write the word.
4.1

The cell cycle

GCSE
Describe the stages of the cell cycle: growth, DNA replication, mitosis and division
Video lesson: The cell cycleComing soon
Context. Ali is a single-celled organism. Every time it divides, it makes two cells that are the same as the parent. Over time, a bigger body, such as a leaf or a finger, is built from many cells. “How does a cell know when to divide?” asks Ali.

The cell cycle is the sequence of events from one cell division to the next. It has two main parts: interphase, when the cell grows and copies its DNA, and mitosis, when the nucleus divides. Mitosis is followed by cytokinesis, when the cytoplasm divides and two cells form.

During interphase the cell grows, makes new organelles, and replicates (copies) its DNA so that each chromosome has two identical halves, called chromatids.

Interphase: growth (G1)DNA replication (S)Interphase: growth (G2)Mitosis and divisionone cell cycle
The cell cycle: growth, DNA replication, more growth, then mitosis and division

Key ideas

  • The cell cycle: interphase (growth and DNA replication), then mitosis, then cytokinesis.
  • Interphase is not a stage of mitosis; it is the time before the nucleus divides.
  • DNA is copied during interphase, so each chromosome has two identical chromatids.
  • Two daughter cells form at the end of the cycle.
Worked example 4.1.1
Put these in the correct order: mitosis, growth, DNA replication. Write the order, using the words.

Method. Use the order of the cell cycle.

  1. Start. The cell grows first, during interphase.
    growth first
  2. Copy. Then the DNA is copied, ready for division.
    DNA replication second
  3. Divide. Last, the nucleus divides by mitosis.
    mitosis last
  4. Write the answer.
    growth, DNA replication, mitosis

Check. Growth comes before the copies are made. ✓

Worked example 4.1.2
Name the two main parts of the cell cycle. Write the words.

Method. Recall the two parts.

  1. Recall. The cycle has interphase, when the cell grows and copies its DNA, and mitosis, when the nucleus divides.
    interphase and mitosis
  2. Name them. So the two parts are interphase and mitosis.
    interphase and mitosis
  3. Write the answer.
    interphase and mitosis

Check. Cytokinesis follows mitosis. ✓

Worked example 4.1.3
In the cell cycle, what is the name of the two identical halves of a replicated chromosome? Write the word.

Method. Recall the name for each half.

  1. Recall. After DNA replication each chromosome has two identical copies, called chromatids.
    two identical copies
  2. Name it. The halves are called chromatids.
    chromatids
  3. Write the answer.
    chromatids

Check. The chromatids are joined at the centromere. ✓

Watch out

Watch out: interphase is not mitosis. Interphase is the long period of growth and DNA copying before the nucleus divides.

Exam tip

Exam tip. When you order the cell cycle, always put growth and DNA replication before mitosis.

Try it (with your teacher). Draw the cell cycle as a circle with four labelled stages, in order. Mark where DNA is copied, and where the nucleus divides. Check your order with your teacher.

Exercise 4A  The cell cycle

1
FName the part of the cell cycle when the cell grows and copies its DNA. Write the word.
2
FName the part of the cell cycle when the nucleus divides. Write the word.
3
FName the process in which the cytoplasm divides, making two cells. Write the word.
4
FPut these in order: mitosis, growth, DNA replication. Write the order.
5
FWhat is the name of the molecule that is copied during interphase? Write the word.
6
FHow many chromatids does a chromosome have after DNA replication? Write the number.
7
FIn the cell cycle, which stage is the longest? Write the word.
8
FDoes the cell grow before or after the DNA is copied? Write the word.
9
FWhich stage follows DNA replication in the cell cycle? Write the word.
10
PSWhy must DNA be copied before mitosis? Write one sentence. Accept any sentence that says each daughter cell needs a full copy of the DNA.
11
PSA cell has 4 chromatids in total after DNA replication. How many chromosomes are there? Write the number.
12
PSDescribe how a cell grows during interphase. Write one sentence. Accept any sentence that says the cell makes new organelles and increases its cytoplasm.
13
RChallenge: explain why the cell cycle must be completed fully before a cell can divide again. Write one sentence. Accept any sentence that says the chromosomes must be copied and the cell must grow, so each daughter cell has everything it needs.
4.2

Mitosis and identical cells

GCSE
Describe the stages of mitosis, Explain why mitosis makes genetically identical cells for growth and repair
Video lesson: Mitosis and identical cellsComing soon
Context. Priya cuts her knee. Within a few days, new skin cells have replaced the damaged ones. The new cells look the same as the old ones and carry the same genes. The body makes these cells by mitosis.

Mitosis makes two daughter nuclei, each with the same number and type of chromosomes as the parent cell. This produces genetically identical cells. A human body cell has 46 chromosomes, in 23 pairs. After mitosis, each daughter cell also has 46 chromosomes.

Mitosis is used for growth (making more cells as an organism gets bigger), for repair (replacing damaged or dead cells), and for asexual reproduction in some organisms.

a replicated chromosome: two identical chromatidsone chromatidreplicate
A replicated chromosome has two identical chromatids
metaphase: chromosomes line upanaphase: chromatids move to opposite ends
Metaphase and anaphase of mitosis
StageWhat happens
Prophasechromosomes condense and become visible; the nuclear membrane breaks down
Metaphasechromosomes line up across the centre of the cell
Anaphasethe two chromatids of each chromosome are pulled to opposite ends
Telophasetwo new nuclear membranes form around each set of chromosomes
Cytokinesisthe cytoplasm divides, making two cells
The stages of mitosis and cytokinesis

Key ideas

  • Mitosis makes two genetically identical nuclei, each with 46 chromosomes in humans.
  • Mitosis is used for growth, repair and some asexual reproduction.
  • The stages are prophase, metaphase, anaphase, telophase, then cytokinesis.
  • Chromosomes line up in the middle in metaphase.
Worked example 4.2.1
How many chromosomes does each daughter cell have after mitosis in a human body cell? Write the number.

Method. Mitosis keeps the chromosome number the same.

  1. Parent. A human body cell has 46 chromosomes.
    46
  2. Daughters. Mitosis gives each daughter cell the same number as the parent.
    same number
  3. Answer. So each daughter cell has 46 chromosomes.
    46
  4. Write the answer.
    46

Check. 23 pairs make 46 chromosomes. ✓

Worked example 4.2.2
Name the stage in which the chromosomes line up across the centre of the cell. Write the word.

Method. Recall the stage where the chromosomes are in the middle.

  1. Recall. In metaphase the chromosomes line up across the middle of the cell.
    line up
  2. Name it. So the stage is metaphase.
    metaphase
  3. Write the answer.
    metaphase

Check. Anaphase follows, when the chromatids separate. ✓

Worked example 4.2.3
A single body cell divides by mitosis twice, with no cell death. How many cells are there at the end? Write the number.

Method. Each division doubles the number of cells.

  1. First division. One cell becomes 2 cells.
    1 x 2 = 2
  2. Second division. Each of the 2 cells divides, giving 2 x 2 = 4 cells.
    2 x 2 = 4
  3. Answer. So there are 4 cells at the end.
    4
  4. Write the answer.
    4

Check. 2 to the power 2 is 4. ✓

Watch out

Watch out: the daughter cells from mitosis are identical to the parent, unlike the cells made by meiosis in Chapter 15. Do not say that mitosis halves the number of chromosomes.

Exam tip

Exam tip. Say “genetically identical” and “same number of chromosomes” for mitosis, not just “the same”.

Try it (with your teacher). With your teacher, look at a labelled photograph or a prepared slide of a root tip. Find cells in different stages, and draw two of them with labels. Do not handle live roots without the teacher.

Exercise 4B  Mitosis and identical cells

1
FName the type of cell division that makes identical cells for growth and repair. Write the word.
2
FHow many chromosomes does a human body cell have? Write the number.
3
FHow many chromosomes does each daughter cell have after mitosis in a human body cell? Write the number.
4
FName the stage of mitosis in which the chromatids are pulled to opposite ends of the cell. Write the word.
5
FName the stage in which the nuclear membranes re-form around the two sets of chromosomes. Write the word.
6
FName the stage in which the chromosomes condense and become visible. Write the word.
7
FName one use of mitosis in the body. Accept any one: growth, repair, or replacing dead cells.
8
PSA single cell divides by mitosis three times, with no cell death. How many cells are there at the end? Write the number.
9
PSA cell has 46 chromosomes. How many chromosomes will be in each of the four cells formed from two rounds of mitosis? Write the number.
10
PSA human cell is at metaphase. Write one feature you would see in the cell. Accept any sentence that says the chromosomes are lined up across the middle of the cell.
11
PSExplain why a graze heals by mitosis rather than by making sperm or egg cells. Write one sentence. Accept any sentence that says mitosis makes identical body cells to replace the damaged ones.
12
RChallenge: explain why two daughter cells made by mitosis carry the same genes. Write two sentences. Accept any answer that says the DNA is copied before division, and each daughter cell receives one copy of each chromosome.
4.3

Stem cells in animals and therapeutic cloning

GCSE
Compare embryonic and adult stem cells, Describe therapeutic cloning and its issues
Video lesson: Stem cells in animals and therapeutic cloningComing soon
Context. Ruth has a condition that damages some of her body cells. Her doctor explains that stem cells might one day help to replace them. “Where do stem cells come from?” Ruth asks. “And why do some come from embryos?”

Stem cells are undifferentiated cells that can divide and develop into different types of specialised cells. Embryonic stem cells come from an early embryo, and they can become almost any type of cell. Adult stem cells are found in some tissues, such as bone marrow, and they can only make a limited range of cell types, mostly those of their own tissue.

Therapeutic cloning is a way of making embryonic stem cells that are genetically identical to a patient. The nucleus of a body cell from the patient is placed into an egg cell whose nucleus has been removed. The egg then develops into an early embryo, and stem cells are taken from it. Because the cells match the patient, the body is less likely to reject them.

fertilised eggembryonic stem cellsany cell type (nerve, muscle)many cell typesadult stem cells (bone marrow)only some cell types (e.g. blood)
Stem cells: embryonic stem cells can become many cell types; adult stem cells in bone marrow make blood cells
FeatureEmbryonic stem cellsAdult stem cells
Found inan early embryosome adult tissues, such as bone marrow
Range of cells madealmost any cell type (pluripotent)a limited range (multipotent)
Ethical issuesthe embryo is destroyed to collect themfew ethical concerns
Useresearch and possible treatmentstreatments already used, such as bone marrow transplants
Embryonic and adult stem cells compared

Key ideas

  • Stem cells can divide and specialise into different cell types.
  • Embryonic stem cells can make almost any cell type.
  • Adult stem cells in bone marrow make blood cells.
  • Therapeutic cloning makes a patient-matched embryo, but there are ethical concerns and it is not yet widely used.
Worked example 4.3.1
Name the type of stem cell that comes from an early embryo. Write the words.

Method. Recall the source of each type.

  1. Recall. Stem cells from an early embryo are called embryonic stem cells.
    from an early embryo
  2. Name it. So the answer is embryonic stem cells.
    embryonic stem cells
  3. Write the answer.
    embryonic stem cells

Check. Adult stem cells come from adult tissues. ✓

Worked example 4.3.2
Name a tissue where adult stem cells are found, and say one cell type they make. Write the words.

Method. Recall the adult stem cell source.

  1. Recall. Adult stem cells are found in bone marrow.
    bone marrow
  2. Name the cell. Bone marrow stem cells make blood cells.
    blood cells
  3. Write the answer.
    bone marrow; blood cells

Check. Bone marrow transplants use these cells. ✓

Worked example 4.3.3
In therapeutic cloning, which cell provides the nucleus? Write the words.

Method. Recall where the genetic material comes from.

  1. Recall. The nucleus comes from a body cell of the patient.
    body cell of the patient
  2. Name it. So the nucleus is from a body cell of the patient.
    body cell of the patient
  3. Write the answer.
    a body cell of the patient

Check. This makes the embryo match the patient. ✓

Watch out

Watch out: a stem cell is not the same as a specialised cell. Stem cells can divide and make specialised cells; a specialised cell can usually only do one job.

Exam tip

Exam tip. Compare embryonic and adult stem cells using the range of cell types they can make. “Better” is not enough on its own.

Try it (with your teacher). Write a short paragraph, with your teacher, that compares embryonic and adult stem cells. Use the words range of cell types, source and ethical issues.

Exercise 4C  Stem cells in animals and therapeutic cloning

1
FWhat is a stem cell? Write the words.
2
FName the stem cells that come from an early embryo. Write the words.
3
FName the stem cells found in bone marrow. Write the words.
4
FWhich type of stem cell can make almost any type of cell: embryonic or adult? Write the word.
5
FWhich cells do adult stem cells in bone marrow mainly make? Write the words.
6
FName the process in which the nucleus of a patient’s body cell is put into an egg cell with its nucleus removed. Write the words.
7
PSIn therapeutic cloning, why is the body less likely to reject the new cells? Write one sentence. Accept any sentence that says the cells are genetically identical to the patient, so they are not seen as foreign.
8
PSGive one ethical issue with embryonic stem cells. Accept any one: the embryo is destroyed to collect the cells, or the embryo is created for research.
9
PSGive one reason adult stem cells are used in treatments already. Write one sentence. Accept any sentence that says bone marrow transplants use adult stem cells to make blood cells.
10
PSA scientist says embryonic stem cells can be used to make any type of cell. Is this a fair summary? Write yes or no.
11
RChallenge: explain why therapeutic cloning is a possible treatment, but is not yet widely used. Write two sentences. Accept any answer that gives one reason it can help a patient, and one of the issues, such as ethics, cost or that it is still being researched.
4.4

Stem cells in plants and cancer

GCSE
Describe meristems and their use in plants, Explain cancer as uncontrolled cell division
Video lesson: Stem cells in plants and cancerComing soon
Context. Ali buys a potted plant. A week later it has new leaves growing from the tips of its stems. “Where do the new cells come from?” he asks. In a different part of the body, a lump of cells that keeps growing is called a tumour. Both involve cell division, but in very different ways.

Plants have meristems: regions where cells keep dividing by mitosis throughout the life of the plant. They are found at the tips of roots and shoots. Meristem cells can become different types of plant cell, so plants can keep growing. Meristem tissue can also be used to grow many identical plants quickly.

Cancer is uncontrolled cell division. Cells keep dividing when they should stop, which can form a tumour. The cause is a change (mutation) in the DNA that affects the controls of the cell cycle.

Key ideas

  • Meristems are areas of cell division at the tips of roots and shoots.
  • Meristem cells can become different plant cell types.
  • Cancer is uncontrolled cell division, forming a tumour.
  • Cancer is caused by a change in DNA that affects the controls of the cell cycle.
Worked example 4.4.1
Name the regions of plants where cells keep dividing by mitosis. Write the word.

Method. Recall the plant structure.

  1. Recall. Plants have regions of continuous cell division at the root and shoot tips.
    root and shoot tips
  2. Name them. These regions are called meristems.
    meristems
  3. Write the answer.
    meristems

Check. Meristems keep the plant growing. ✓

Worked example 4.4.2
Describe what cancer is, in one sentence. Accept any sentence that says cancer is uncontrolled cell division that can form a tumour.

Method. Recall the definition.

  1. Recall. Cancer is cells dividing when they should stop.
    dividing when they should stop
  2. Conclude. This can form a tumour, which is a mass of cells.
    mass of cells
  3. Write the answer.
    Accept any sentence that says cancer is uncontrolled cell division that can form a tumour.

Check. Uncontrolled means the usual controls have failed. ✓

Worked example 4.4.3
Name the change in DNA that can lead to cancer. Write the word.

Method. Recall the change in DNA.

  1. Recall. A change in the DNA is called a mutation.
    change in DNA
  2. Name it. A mutation that affects the cell cycle controls can cause cancer.
    mutation
  3. Write the answer.
    mutation

Check. Mutations can also be harmless. ✓

Watch out

Watch out: not every growth is cancer. Some growths are benign (not cancerous). Cancer is cells dividing uncontrollably and spreading to other tissues.

Exam tip

Exam tip. Explain cancer by saying what goes wrong with the controls of the cell cycle. Do not say cancer is a virus or a bacterium.

Try it (with your teacher). Find out, with your teacher, where meristems are in a bean seedling. Draw the root and shoot tips, and mark the meristem areas.

Exercise 4D  Stem cells in plants and cancer

1
FName the regions in plants where cells keep dividing by mitosis. Write the word.
2
FName two places in a plant where meristems are found. Accept any two: root tips, shoot tips.
3
FCan meristem cells become different types of plant cell? Write yes or no.
4
FWhat is the name for uncontrolled cell division that can form a lump of cells? Write the word.
5
FWhat is cancer? Write the words.
6
FWhat kind of change in DNA can lead to cancer? Write the word.
7
FIs a tumour always cancerous? Write yes or no.
8
PSWhy are meristems important to a plant? Write one sentence. Accept any sentence that says they allow the plant to keep growing new cells, roots and shoots throughout its life.
9
PSPlants can be cloned from meristem tissue. Name the advantage, in one sentence. Accept any sentence that says many identical plants can be grown quickly from a small piece of tissue.
10
PSExplain why a mutation in a cell cycle control gene can lead to a tumour. Write one sentence. Accept any sentence that says the cell keeps dividing without the usual control, so cells build up.
11
RChallenge: explain why a plant meristem and a cancer tumour both involve cell division, but only one is normal. Write two sentences. Accept any answer that says meristem division is controlled and provides new growth, while cancer division is uncontrolled and forms a mass.
4.5

The cell cycle in more detail [H]

GCSE
[H] Describe G1, S and G2 phases, [H] Interpret a graph of DNA content through the cell cycle
Video lesson: The cell cycle in more detail [H]Coming soon
Context. Hana’s teacher draws a graph of the amount of DNA in one cell over one cycle. “The DNA goes up, then it drops,” Hana says. “Why does it drop, and why does it stay the same for a while?”

[H] Higher tier only. This section is higher-tier content. It extends the cell cycle in Section 4.1.

[H] Interphase has three phases. In G1 the cell grows and makes proteins and organelles. In the S phase the DNA is replicated. In G2 the cell grows further and prepares for mitosis. Checkpoints during the cycle check that the DNA is intact before the cell moves on.

The DNA content of one cell is double after S phase, because each chromosome now has two chromatids. It halves again when the cell divides in mitosis and cytokinesis.

DNA content of one cell through the cycle0123404812162024time (hours)DNA (relative units)
Line graph of DNA content of one cell through the cell cycle

Key ideas

  • [H] G1: growth; S: DNA replication; G2: preparation for mitosis.
  • [H] Checkpoints check the DNA is intact before the cell divides.
  • [H] DNA content doubles in S phase, and halves again after division.
  • [H] Mitosis itself is much shorter than interphase.
Worked example 4.5.1
[H] In which phase does the DNA content of the cell double? Write the words.

Method. Read the graph where the line rises.

  1. Read. The line rises from 2 to 4 between 4 and 12 hours.
    rises from 2 to 4
  2. Name it. This rise is the S phase, when the DNA is replicated.
    S phase
  3. Write the answer.
    S phase

Check. G1 and G2 do not change the DNA amount. ✓

Worked example 4.5.2
[H] Read the graph. What is the DNA content at 16 hours? Write the number.

Method. Find 16 hours on the time axis.

  1. Find. At 16 hours the line is at the top of its rise.
    top of the rise
  2. Read. The DNA content is 4 units.
    4
  3. Write the answer.
    4

Check. The line is level between 12 and 16 hours. ✓

Worked example 4.5.3
[H] Why does the DNA content fall from 4 to 2 at about 20 hours? Write one sentence.

Method. Think about what happens in division.

  1. Think. When the cell divides, the chromatids separate.
    separate
  2. Conclude. Each daughter cell receives one set, so the DNA halves.
    receives one set
  3. Write the answer.
    The chromatids separate and each daughter cell receives one set, so the DNA halves.

Check. Each daughter has half the DNA of the parent. ✓

Watch out

Watch out: a graph of DNA content and a graph of cell number are different. The DNA doubles before the cell divides; the number of cells doubles only after division.

Exam tip

Exam tip. [H] Name the phase for each part of the graph, and explain any change using the DNA or the chromatids.

Try it (with your teacher). With your teacher, sketch the DNA content graph on squared paper. Mark G1, S, G2 and mitosis on the time axis.

Exercise 4E  [H] The cell cycle in more detail

1
F[H] Name the phase when the cell replicates its DNA. Write the letter or words.
2
F[H] Name the phase when the cell grows and makes new proteins and organelles before DNA replication. Write the letters.
3
F[H] Name the phase when the cell prepares for mitosis after DNA replication. Write the letters.
4
F[H] What do checkpoints in the cell cycle check? Write the words.
5
F[H] From the graph, what is the DNA content at 0 hours? Write the number.
6
F[H] Between which two times does the DNA content stay at 4 units? Write the two times.
7
PS[H] At what time does the DNA content first reach 3 units? Write the number of hours.
8
PS[H] Calculate the change in DNA content from 4 hours to 12 hours. Write the number.
9
PS[H] Explain why the DNA content does not change during G1. Write one sentence. Accept any sentence that says DNA is not copied during G1, so the amount stays the same.
10
R[H] Explain why the cell must check its DNA before it moves on from G2 to mitosis. Write one sentence. Accept any sentence that says damaged DNA would be passed to the daughter cells, so the checkpoint prevents this.
11
R[H] Challenge: explain why the graph shows DNA content, not cell number, and why it falls at about 20 hours. Write two sentences. Accept any answer that says the graph tracks DNA in one cell, and the fall is because the cell has divided into two daughter cells with half the DNA each.

Chapter summary

SkillMethod
Cell cycleInterphase (growth and DNA replication), then mitosis, then cytokinesis.
MitosisMakes two genetically identical nuclei; used for growth, repair and some asexual reproduction.
ChromosomesA human body cell has 46 chromosomes (23 pairs); each daughter cell after mitosis has 46.
Stem cellsEmbryonic: almost any cell type; adult: a limited range, e.g. blood cells from bone marrow.
Therapeutic cloningMakes a patient-matched embryo for stem cells; raises ethical concerns and is not yet widely used.
MeristemsRegions of cell division at root and shoot tips, allowing plant growth throughout life.
CancerUncontrolled cell division caused by a mutation affecting the cell cycle controls; can form a tumour.
[H] G1, S, G2Growth; DNA replication; growth and preparation for mitosis; checkpoints check the DNA.

Can you…?

  • I can put the stages of the cell cycle in order and describe each one
  • I can describe mitosis and explain why the daughter cells are identical
  • I can compare embryonic and adult stem cells
  • I can describe therapeutic cloning and one issue with it
  • I can explain meristems and cancer as division that is controlled or uncontrolled
  • [H] I can name G1, S and G2, and read a graph of DNA content

Key terms

cell cycle
the sequence of growth, DNA replication and division between one cell division and the next
interphase
the period of growth and DNA replication before mitosis
mitosis
cell division that makes two genetically identical nuclei
cytokinesis
the division of the cytoplasm to make two cells
chromatid
one of the two identical halves of a replicated chromosome
stem cell
an undifferentiated cell that can divide and develop into specialised cells
therapeutic cloning
making an embryo genetically matched to a patient to provide stem cells
meristem
a region of a plant where cells keep dividing by mitosis
tumour
a mass of cells formed by uncontrolled cell division
mutation
a change in the DNA of a cell
[H] checkpoint
a point in the cell cycle where the cell checks the DNA before moving on

Exercise 4Z  Check-up

1
Put these in order: mitosis, growth, DNA replication. Write the order.
2
How many chromosomes does each daughter cell have after mitosis in a human body cell? Write the number.
3
Name the stage of mitosis in which the chromosomes line up in the middle of the cell. Write the word.
4
Name the stage of mitosis in which the chromatids are pulled to opposite ends. Write the word.
5
Name the stem cells found in bone marrow. Write the words.
6
Which type of stem cell can make almost any cell type? Write the word.
7
Name the uncontrolled division that can form a tumour. Write the word.
8
A cell divides by mitosis three times. How many cells are there at the end? Write the number.
9
Explain in one sentence why therapeutic cloning uses the patient’s own nucleus. Accept any sentence that says the cells will match the patient, so the body is less likely to reject them.
10
Give one reason why plant meristems are important to a plant. Accept any sentence that says they allow the plant to keep growing new roots and shoots.

Total 25 marks. Allow about 25 minutes.

Exercise 4X  Exam-style questions

1
FName the process that makes two genetically identical nuclei. Write the word.[1 mark]
2
FName the stage of the cell cycle in which the DNA is copied. Write the word.[1 mark]
3
FName the stage of mitosis in which the nuclear membranes re-form. Write the word.[1 mark]
4
PSGive one use of mitosis in the body, and one reason why identical cells are needed. Accept any answer that gives growth or repair, and says that the new cells must match the existing ones.[2 marks]
5
PSA human body cell has 46 chromosomes. How many chromosomes are in each daughter cell after mitosis? Write the number, and say why. Accept 46, because mitosis keeps the number the same.[2 marks]
6
RExplain the difference between embryonic stem cells and adult stem cells. Write two sentences. Accept any answer that says embryonic stem cells can make almost any cell type and come from an embryo, while adult stem cells come from adult tissue and make a limited range.[3 marks]
7
FDescribe what happens to a cell during the cell cycle, from growth to division. Write three points. Accept any three of: growth, DNA copying, mitosis, and cytokinesis to make two cells.[3 marks]
8
RExplain how cancer is different from normal cell division in a plant meristem. Accept any answer that says cancer is uncontrolled and forms a mass, while meristem division is controlled and adds new growth.[4 marks]
9
PSA single cell divides by mitosis to make a group of cells. Calculate how many cells there are after 4 divisions, with no cells dying. Write the number. Accept 16.[3 marks]
10
R[H] The DNA content of a cell doubles during the cell cycle. Name the phase in which this happens, and explain why it is needed before division. Accept any answer that names the S phase and says each daughter cell needs a full copy of the DNA.[5 marks]
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