Chapter 4 of 8

All 8 chapters
  1. 1Cell structure in detail, readunderstood
  2. 2Microscopes and size, readunderstood
  3. 3Diffusion, osmosis and active transport, readunderstood
  4. 4Cell division, readunderstood
  5. 5Enzymes, readunderstood
  6. 6Photosynthesis, readunderstood
  7. 7Testing photosynthesis, readunderstood
  8. 8Respiration, readunderstood

S4.1.4 Chapter 4

Cell division

When I cut my finger, it heals. Where do the new cells come from?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Describe how mitosis makes two identical cells for growth and repair
  • Explain what stem cells are and how they could treat disease

A baby grows into an adult. A cut on your finger heals. A worn skin cell is replaced. All of these need new cells, and new cells come from old cells dividing.

The nucleus holds the genetic material. In a human body cell the DNA is in 46 chromosomes. A chromosome is a long thread of DNA, coiled up so that it is short enough to fit in the nucleus.

A cell that divides goes round the cell cycle. First the cell grows. It makes more of its parts, such as mitochondria and ribosomes, and it copies its DNA. Then the nucleus divides. Last the rest of the cell divides.

A ring of three stages with arrows going round. The stages are cell grows and copies its DNA, mitosis: the nucleus divides, and the cell divides in two. The last arrow goes back to the first stage.cell growsand copiesits DNAmitosis:the nucleusdividesthe celldivides intwo
The cycle goes round and round. Each new cell can start the cycle again.

The part where the nucleus divides is mitosis. Before it starts, the cell has copied every chromosome. Each chromosome is now two identical copies joined together, like an X.

In mitosis the copied chromosomes line up across the middle of the cell. The two copies of each one are pulled to opposite ends. Then a new nucleus forms round each set. The cytoplasm and membrane divide, and there are two cells.

The stages of mitosis down the page, for a cell with 4 chromosomes, 2 long and 2 short. They are original cell with 4 chromosomes, chromosomes copied, lined up in the middle, pulled apart, and two new cells, each with 4 chromosomes. A thin outline marks the nucleus in the original cell, in the copied cell and in each new cell.original cell with4 chromosomeschromosomes copiedlined up in themiddlepulled aparttwo new cells,each with 4chromosomes
A human cell has 46 chromosomes. This model has 4, 2 long and 2 short, so the steps are clear to count. Each new cell gets one copy of every chromosome. The thin outline is the nucleus.

The two new cells have the same chromosomes as the first cell, and the same genes. They are genetically identical. The cell copies, and then splits the copies, so nothing is lost and nothing is new.

Worked example

A human skin cell has 46 chromosomes. It divides by mitosis. How many chromosomes are in each new cell?

  1. The cell copies every chromosome first. That makes 46 × 2 = 92 copies, joined in 46 X shapes.
  2. The 92 copies are shared between two new cells.
  3. 92 ÷ 2 = 46.

Answer: 46 chromosomes

Mitosis is how an organism grows and repairs itself. It is also how some living things make new individuals that are copies of themselves. One cell makes two, then four, then eight. The number doubles with every division.

A line graph. The x axis is number of divisions, from 0 to 5. The y axis is number of cells, from 0 to 32 in steps of 8. The points are at 0 divisions and 1 cell, 1 and 2, 2 and 4, 3 and 8, 4 and 16, and 5 and 32.01234508162432number of divisionsnumber of cells
The line gets steeper because each division doubles a bigger number.

Worked example

One cell divides by mitosis once every hour. How many cells are there after 5 hours?

  1. Each division doubles the number of cells.
  2. 1 cell becomes 2, then 4, then 8, then 16, then 32.
  3. Five doublings: 2 × 2 × 2 × 2 × 2 = 32.

Answer: 32 cells

Now to the cells that make all the others. A stem cell is an unspecialised cell. It can divide again and again, and it can change into other kinds of cell. A cell that has changed into one kind, such as a nerve cell, is specialised.

Stem cells are found in several places. An embryo has stem cells that can become almost any kind of cell in the body. Bone marrow, the soft middle of some bones, has stem cells that make new blood cells all through your life. A plant has them in the tips of its shoots and roots, where it grows.

Three boxes joined by arrows, from left to right: stem cell, divides many times, and cells change into special kinds.stemcelldividesmanytimescellschangeintospecialkinds
A stem cell is not yet a nerve cell, a blood cell or a muscle cell. An embryo's stem cells can become almost any of them. Stem cells in bone marrow make only blood cells.
A table with the columns Source and Can become. Embryo: almost any cell. Bone marrow: blood cells. Shoot tip: any plant cell.SourceCan becomeembryoalmost any cellbone marrowblood cellsshoot tipany plant cell
Embryonic stem cells come from an embryo in its first few days.

One use is already established. A bone marrow transplant gives a patient healthy stem cells. These make new blood cells. Doctors use transplants to treat some blood cancers. Gardeners also use plant stem cells to grow many identical plants from one cutting.

Other uses are being researched. Stem cells could one day replace damaged nerve cells after a spinal injury, or cells in a heart that has been harmed. These uses are not yet part of ordinary treatment.

There are concerns, and people weigh them differently. Some people object to using embryos, because they see an embryo as the start of a human life. Others point to the diseases the cells could help treat. Stem cells that keep dividing could also form a tumour, so they must be tested with great care.

Try it

Act out mitosis with paper strips

You need: a sheet of paper, a pencil, tape, a loop of string or a circle drawn on a big sheet.

Be safe: Tear the paper. If you cut it instead, use blunt scissors and cut away from yourself.

  1. Tear the paper into 8 strips: 4 long and 4 short. Mark each strip with a pencil.
  2. Put 2 long and 2 short strips inside the loop. This is the cell with 4 chromosomes.
  3. Copy each chromosome: tape a matching strip across each one to make an X. You now have 4 X shapes.
  4. Line the 4 X shapes up across the middle of the loop.
  5. Pull each X apart. Put one strip of each X on the left and the other on the right.
  6. Draw a line down the middle. Count and compare the two sides.

What you will see: Each side has 2 long and 2 short strips, the same as the cell had at the start. The two new cells are identical.

If you cannot try it, this is what happens.
A table with the columns Step and Strips. Start: 4. Copied: 8. Pulled apart: 4 and 4.StepStripsstart4copied8pulled apart4 and 4

A wrong idea many people have

A stem cell is a cell from the stem of a plant.

Why it feels right: The word stem makes people think of the stem of a plant.

The test that breaks it: Bone marrow has stem cells, and a bone has no stem like a plant. The cells are called stem cells because other cells grow from them, as branches grow from a stem.

What is true: A stem cell is an unspecialised cell that can divide and become other kinds of cell. It can be in an animal or a plant.

Key words

chromosomes
long threads of DNA in the nucleus, which carry the genes.
cell cycle
the series of steps a cell goes through to grow and divide.
mitosis
the division of the nucleus that makes two identical nuclei.
stem cell
an unspecialised cell that can divide and change into other kinds of cell.
specialised
changed to do one job, such as a nerve cell does.

Back to your guess

The answer is C: Cells next to the cut divide and make new cells like themselves.

Cells next to the cut divide by mitosis. Each division makes two new cells that are identical to the first, and they fill the gap.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. What does mitosis make?

    Tap your answer.

    1. Not this one. Halving the chromosomes happens when sex cells are made. In mitosis each new cell gets the full set. Try another.

    2. Not this one. Mitosis makes two cells, and they are identical to each other and to the first cell. Try another.

    3. Yes, that is right. The cell copies its chromosomes and then shares them equally. Each new cell has the same genetic material.

    See the answer

    Two cells that are identical to the first cell

    The cell copies its chromosomes and then shares them equally. Each new cell has the same genetic material.

    • If you chose A: Halving the chromosomes happens when sex cells are made. In mitosis each new cell gets the full set.
    • If you chose B: Mitosis makes two cells, and they are identical to each other and to the first cell.

  2. Which is a use of mitosis in the body?

    Tap your answer.

    1. Yes, that is right. Mitosis makes new cells for growth and repair. Respiration and osmosis are different processes.

    2. Not this one. That is aerobic respiration, which happens in the mitochondria. Go to S4.1.1. Try another.

    3. Not this one. That is osmosis, which moves water across the membrane. It does not make new cells. Go to S4.1.3. Try another.

    See the answer

    Replacing skin cells that are lost from a cut

    Mitosis makes new cells for growth and repair. Respiration and osmosis are different processes.

    • If you chose B: That is aerobic respiration, which happens in the mitochondria. Go to S4.1.1.
    • If you chose C: That is osmosis, which moves water across the membrane. It does not make new cells. Go to S4.1.3.

  3. A cell is about to divide by mitosis. What happens first?

    Tap your answer.

    1. Not this one. That comes later. The chromosomes must be copied before they can be pulled apart. Try another.

    2. Yes, that is right. The cell copies its DNA while it grows, so each chromosome is two copies joined. Then they line up, are pulled apart, and the cell splits.

    3. Not this one. That is the last step. The nucleus divides before the rest of the cell. Try another.

    See the answer

    The cell copies its chromosomes.

    The cell copies its DNA while it grows, so each chromosome is two copies joined. Then they line up, are pulled apart, and the cell splits.

    • If you chose A: That comes later. The chromosomes must be copied before they can be pulled apart.
    • If you chose C: That is the last step. The nucleus divides before the rest of the cell.

  4. What is a stem cell?

    Tap your answer.

    1. Not this one. The name is not about stems. Stem cells are in animals and in plants. Try another.

    2. Not this one. That is a specialised cell. A stem cell has not yet changed to one job. Try another.

    3. Not this one. A stem cell can also change into other kinds of cell. Try another.

    4. Yes, that is right. Stem cells are unspecialised. They can divide and change into other cells, such as blood cells.

    See the answer

    An unspecialised cell that can divide and become other kinds of cell

    Stem cells are unspecialised. They can divide and change into other cells, such as blood cells.

    • If you chose A: The name is not about stems. Stem cells are in animals and in plants.
    • If you chose B: That is a specialised cell. A stem cell has not yet changed to one job.
    • If you chose C: A stem cell can also change into other kinds of cell.

  5. Which use of stem cells is already established in medicine?

    Tap your answer.

    1. Not this one. Scientists are working on this. It could one day be possible, but it is not an ordinary treatment now. Try another.

    2. Yes, that is right. Bone marrow has stem cells that make blood cells. Doctors already use transplants to treat some blood cancers.

    3. Not this one. This is being researched. It could help in future, but it is not an ordinary treatment now. Try another.

    See the answer

    A bone marrow transplant to treat some blood cancers

    Bone marrow has stem cells that make blood cells. Doctors already use transplants to treat some blood cancers.

    • If you chose A: Scientists are working on this. It could one day be possible, but it is not an ordinary treatment now.
    • If you chose C: This is being researched. It could help in future, but it is not an ordinary treatment now.

  6. Where are the stem cells of a plant?

    Tap your answer.

    1. Not this one. A seed has them, but a plant keeps them as it grows, in the tips of the shoots and roots. Try another.

    2. Not this one. Cells in a leaf are specialised. The cells that keep dividing are at the shoot and root tips. Try another.

    3. Yes, that is right. A plant grows at its tips, and the stem cells there divide and then specialise.

    See the answer

    In the tips of the shoots and roots

    A plant grows at its tips, and the stem cells there divide and then specialise.

    • If you chose A: A seed has them, but a plant keeps them as it grows, in the tips of the shoots and roots.
    • If you chose B: Cells in a leaf are specialised. The cells that keep dividing are at the shoot and root tips.

  7. From an earlier chapter

    Which process moves mineral ions into a root hair cell against the concentration gradient?

    Tap your answer.

    1. Yes, that is right. The ions go from a lower to a higher concentration. That needs energy from respiration, so it is active transport.

    2. Not this one. Osmosis moves water, not ions. Go to S4.1.3. Try another.

    3. Not this one. Diffusion moves particles down the gradient. Here they go up it. Go to S4.1.3. Try another.

    See the answer

    Active transport

    The ions go from a lower to a higher concentration. That needs energy from respiration, so it is active transport.

    • If you chose B: Osmosis moves water, not ions. Go to S4.1.3.
    • If you chose C: Diffusion moves particles down the gradient. Here they go up it. Go to S4.1.3.

  8. A cell with 78 chromosomes divides by mitosis. How many chromosomes are in each new cell?

    See the answer

    78 chromosomes

    The cell copies its chromosomes to make 156. They are shared between two cells, so each has 78.

  9. One cell divides by mitosis every hour. Use the graph to say how many cells there are after 6 hours.

    A line graph. The x axis is number of divisions, from 0 to 3. The y axis is number of cells, from 0 to 8 in steps of 2. The points are at 0 divisions and 1 cell, 1 and 2, 2 and 4, and 3 and 8.012302468number of divisionsnumber of cells
    See the answer

    64 cells

    The number doubles with each division: 8 after 3, then 16, 32 and 64. After 6 divisions it is 2 × 2 × 2 × 2 × 2 × 2 = 64.

Keep it

Copy this into your exercise book.

Mitosis: copy the chromosomes, line up, pull apart, split. Two identical cells, for growth and repair. Each division doubles the cells. Stem cells are unspecialised and can become other kinds. Bone marrow transplants are used now.

Draw: A cell with 4 chromosomes dividing into two cells, with the word copied beside the middle step.

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