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B. Biology: Tissues, Organs, and Systems of Living Things · Tissues, Organs, and Systems of Living Things

Cancer: When Cell Division Is Out of Control

B2.5 B3.1 B2.1 A1.8 A1.12 A1.13 A2.1About 70 minutes
Learning goals
  • I can explain how the cell cycle is normally controlled and what goes wrong in cancer.
  • I can compare images or data from cancerous and non-cancerous cells and describe the differences in cell division.
  • I can use doubling calculations and graphs to predict how fast an uncontrolled population of cells grows.
  • I can predict how uncontrolled cell division affects an organism, and describe ways to lower risk and treat cancer.

Words to know

cancer
A group of diseases in which cells divide without control and can invade other tissues.
mutation
A change in the DNA of a cell.
carcinogen
Something that can cause cancer by damaging DNA, such as chemicals in tobacco smoke or UV radiation.
tumour
A mass of cells formed by abnormal, uncontrolled division.
benign tumour
A tumour that stays in one place and does not invade other tissues.
malignant tumour
A cancerous tumour whose cells can invade nearby tissues and spread.
metastasis
The spread of cancer cells to other parts of the body through the blood or lymph.
apoptosis
Programmed cell death: a damaged or unneeded cell destroys itself in a controlled way.

A system with brakes

In the last two lessons you saw how one cell becomes two. In a healthy body, this happens only when it should: when you are growing, when a wound needs repair, or when old cells need replacing. Every day your body makes billions of new cells, and almost all of them divide at the right time and stop at the right time. This lesson is about what happens when the controls fail. Cancer is a serious topic, and some of you may know someone who has had it. The science here can help you understand what doctors are doing and why prevention and early detection matter.

How division is normally controlled

Think of the cell cycle like a car with an accelerator and brakes.

  • Growth signals (accelerator): chemical messages from nearby cells tell a cell when to divide.
  • Checkpoints (brakes): proteins check the cell at key points. Is the DNA damaged? Was it copied correctly? Are chromosomes attached to the spindle? If not, the cycle stops until the problem is fixed.
  • Contact inhibition: normal cells in a dish stop dividing when they touch neighbours on all sides, forming one neat layer.
  • Apoptosis: if the damage can't be fixed, the cell destroys itself in a controlled way. This protects the body.
  • Limited divisions: most normal body cells can divide only a limited number of times.

What goes wrong in cancer

The controls are themselves made from instructions in DNA. If mutations damage the genes for the accelerator (stuck "on") or the brakes (not working), a cell can divide when it shouldn't. Usually several mutations must build up in one cell before it becomes cancerous, which is one reason many cancers are more common in older adults.

Cancer cells:

  • keep dividing without growth signals and ignore checkpoints,
  • do not stop when crowded (no contact inhibition), so they pile up,
  • avoid apoptosis, so damaged cells survive,
  • can keep dividing almost without limit,
  • often have large, irregular nuclei and abnormal numbers of chromosomes,
  • may invade nearby tissue and spread (metastasis).
Normal cellsCancer cellsone neat layer; division stopswhen cells touch their neighbourscells pile up in layers, irregular shapes,large nuclei; division does not stop
Cells growing in a lab dish (simplified side view). Normal cells form a single layer and stop dividing when crowded. Cancer cells ignore this signal and pile up, which is one of the features pathologists look for in images.

Comparing the rate of division

The expectation asks you to compare the rate of cell division in cancerous and non-cancerous cells using images or videos. Here is what scientists look for:

  • Time-lapse videos of cells in a dish show that cancer cells keep dividing, generation after generation, long after normal cells in the same dish have stopped.
  • Micrographs (microscope images) of tissue samples: a pathologist counts how many cells are in mitosis. Tumours often have a higher mitotic index than the normal tissue around them, and the dividing cells look disorganized.

Two cautions:

  1. Some normal cells divide very fast (cells in bone marrow, the lining of the intestine, hair follicles). The key problem with cancer is loss of control: the cells don't stop.
  2. Growth adds up. Because each new cell can divide again, the number of cells doubles with each round. This is exponential growth, and you learned about exponential models in Grade 9 math.
025050075010000246810doubles every day (1024)doubles every 2 days (32)Time (days)Number of cells
Two populations of cells growing from 1 cell (exponential growth). After 10 days, cells that double every day reach 1024, while cells that double every 2 days reach only 32. A small change in the rate of division makes a huge difference.
Worked example

Let's model a doubling tumour

A single cell becomes cancerous. Its descendants double in number every 3 days and none of them die. (Real tumours are messier, but this is a useful model.) How many cells will there be after 30 days? After how many doublings will there be more than one billion cells?

  1. Number of doublings in 30 days: 30 ÷ 3 = 10.
  2. Number of cells = 1 × 2¹⁰ = 1024. That is still far too small to see or feel.
  3. To pass one billion (1 000 000 000), find n with 2ⁿ > 10⁹. Since 2¹⁰ = 1024 ≈ 10³, 2³⁰ = (2¹⁰)³ ≈ 10⁹. Check: 2³⁰ = 1 073 741 824, which is just over one billion.
  4. So 30 doublings are needed: 30 × 3 days = 90 days in this model.
  5. A clump of about a billion cells is commonly estimated to be roughly 1 cm across. Notice that it took 29 doublings to reach half a billion, and only one more doubling to reach a billion.

After 30 days: 1024 cells. More than a billion cells after 30 doublings (90 days) in this model. This is why a tumour can seem to "appear suddenly" and why early detection matters.

Worked example

Let's compare two tissue samples

A pathologist counts 400 cells in each of two samples. Normal lung tissue: 4 cells in mitosis. Tumour sample: 48 cells in mitosis. Compare the mitotic indexes and predict the effect on the organism.

  1. Normal tissue: 4 ÷ 400 × 100% = 1%.
  2. Tumour: 48 ÷ 400 × 100% = 12%.
  3. 12 ÷ 1 = 12, so the tumour has 12 times the share of dividing cells.
  4. Prediction: the tumour will grow much faster than normal tissue. As it grows it can crowd and damage nearby tissue, block airways or blood vessels, use nutrients that other cells need and, if malignant, spread to other organs.

Mitotic index: normal 1%, tumour 12%. The tumour cells are dividing far more often, so the mass will grow and can harm the lung's ability to exchange gases.

Benign or malignant?

  • A benign tumour grows in one place, often inside a capsule. It is not cancer, although it can still cause problems if it presses on something important (such as in the brain).
  • A malignant tumour is cancer. Its cells can invade nearby tissue and break away into the blood or lymph, starting new tumours elsewhere. This spreading is called metastasis, and it is what makes cancer most dangerous.

Lowering risk

Not every cancer can be prevented, and getting cancer is never a person's fault. But some risks can be lowered:

  • No tobacco: tobacco smoke contains many carcinogens and is linked to lung, mouth, throat and other cancers. Vaping also exposes the lungs to harmful chemicals.
  • Sun safety: UV radiation damages the DNA of skin cells. Use shade, clothing, hats and sunscreen, and avoid tanning beds.
  • Vaccines: the HPV vaccine protects against viruses that can cause cervical and other cancers; the hepatitis B vaccine protects against a virus that can cause liver cancer. Ontario offers both to students in Grade 7.
  • Screening: tests that find some cancers early, before symptoms (you will look at screening programs later in this unit).

Treatment

  • Surgery removes the tumour.
  • Radiation therapy aims high-energy radiation at the tumour to damage the DNA of cancer cells. In 1951, Canadian teams in Saskatoon and London, Ontario, were among the first in the world to treat patients with cobalt-60 radiation machines.
  • Chemotherapy uses drugs that attack dividing cells. Because they also affect normal fast-dividing cells (hair follicles, the lining of the gut, bone marrow), side effects can include hair loss, nausea and a higher risk of infection.
  • Newer treatments include targeted drugs that block a specific faulty protein and immunotherapy, which helps the immune system recognize cancer cells.

Careers: oncologists (cancer doctors), pathologists (doctors who diagnose disease from tissue samples), cytotechnologists (who examine cells under the microscope), radiation therapists, oncology nurses and cancer researchers all use the ideas in this lesson every day.

Hands-on activity

The power of doubling

You need: a bag of dried beans or rice, 10 small cups or paper muffin liners, a calculator and paper.

  1. Label the cups "Round 0" to "Round 9". Put 1 bean in Round 0, 2 in Round 1, 4 in Round 2, and keep doubling. How many cups can you fill before you run out or get tired of counting?
  2. Make a table: round number, number of beans. Then graph it with round number on the x-axis.
  3. Use a calculator to continue the table to round 30. Write 2³⁰ in words.
  4. Now model a slower population that doubles only every second round. After 30 rounds, how many would it have? (2¹⁵ = 32 768.) Divide to compare the two.

What to look for: the graph starts flat and then shoots upward (exponential growth). After 30 rounds the fast population has 2³⁰ ≈ 1.07 billion beans, while the slow one has 32 768, about 32 768 times fewer. Talk about what this means for a tumour whose cells divide faster than normal. Dried beans are a choking hazard for small children; keep them away from little ones.

Your turn

1

Name two ways the cell cycle is normally kept under control.

2

What is a mutation, and how can it lead to cancer?

3

Give two differences between a benign and a malignant tumour.

4

Cells double every 2 days, starting from 1 cell. How many cells are there after 16 days?

5

A tissue sample has 6 cells in mitosis out of 300. What is its mitotic index?

6

Why does chemotherapy often cause hair loss?

7

Name two carcinogens or risk factors and one action that lowers each risk.

More practice: warm-up

1

What is a tumour?

2

What is metastasis?

3

What is apoptosis?

4

Is cancer contagious?

5

What is 2¹⁰?

More practice: on your own

1

Two cell populations each start with 1 cell. A doubles every 12 hours; B doubles every 24 hours. How many cells does each have after 5 days?

2

Describe what you would see in a time-lapse video of normal cells versus cancer cells growing in a dish.

3

Explain why cancer is usually more common in older adults.

4

Why is early detection important? Use the idea of doubling.

5

Explain how radiation therapy and chemotherapy both target cancer cells, and why both can harm normal cells.

Challenge

1

A tumour has about 10⁹ cells and doubles every 60 days. Ignoring cell death, about how long until it has 8 × 10⁹ cells? What does your answer suggest about tumours found by screening?

2

Real tumours do not keep doubling at a constant rate forever. Suggest two reasons, using what you know about cells' needs.

3

A drug blocks a protein that tells cells to divide (the "accelerator"). It works well against one type of cancer but not another. Suggest a reason.

Check your understanding

Quiz

1.

Which feature describes cancer cells but not normal cells?

2.

One cell doubles every 4 days. How many cells after 20 days?

3.

What is metastasis?

4.

A sample has 12 dividing cells out of 200. What is the mitotic index?

5.

Why can chemotherapy cause side effects in the gut?

6.

Which is a carcinogen?

For parents and teachers

Big idea: expectation B2.5 asks students to compare the rate of cell division in cancerous and non-cancerous cells using images or videos, and to predict the effect on an organism. The doubling math links to exponential models from Grade 9 math.

Sensitive topic: cancer may have touched your family. Let your teen know they can ask questions or step away and come back. Emphasize that cancer is not anyone's fault and that treatment has improved a lot. This lesson has no joke box on purpose.

How to help: Do the doubling activity together and talk about why early detection matters. If your teen wants to see real images, search for time-lapse videos of cells dividing from university or hospital sources and watch them together.

Common difficulties:

  • Thinking all tumours are cancer (benign tumours aren't).
  • Thinking cancer cells always divide faster than every normal cell. The key problem is loss of control.
  • Adding instead of doubling (1, 2, 3, 4... instead of 1, 2, 4, 8...).