Cell Division and Growth
5. Cancer and Uncontrolled Cell Division
Learning outcomes
- I can describe how cancer results from uncontrolled cell division.
- I can explain how mutations can affect the cell cycle.
- I can distinguish between normal and cancerous cell growth.
- I can identify factors that may increase cancer risk.
- I can explain the importance of early detection and treatment.
Cancer and Uncontrolled Cell Division
The cell cycle is normally a carefully controlled process. Cells divide when new cells are needed for growth, repair, or replacement, and they stop dividing when enough cells have been produced.
Sometimes this control system becomes damaged.
Changes in DNA called mutations can affect genes that control the cell cycle. If enough important controls are disrupted, cells may begin dividing when they should not.
This uncontrolled cell division can lead to cancer.
What Is Cancer?
Cancer is a group of diseases involving abnormal cells that grow and divide uncontrollably.
Normally, cells respond to signals that tell them:
- when to divide
- when to stop dividing
- when to repair damaged DNA
- when to die if they are badly damaged
Cancer cells can lose some of these controls.
As a result, they may:
- divide too frequently
- ignore signals telling them to stop
- survive when damaged cells would normally die
- form masses of abnormal cells
- invade surrounding tissues
- sometimes spread to other parts of the body
Normal Cell Division
Normal cells do not divide continuously.
The cell cycle includes:
Interphase → Mitosis → Cytokinesis
Before dividing, a cell passes through several control points called checkpoints.
These help ensure that division only continues when conditions are suitable.
For example, cells may check:
- Is the cell large enough?
- Has the DNA been copied correctly?
- Is the DNA damaged?
- Are the chromosomes correctly prepared for separation?
If a serious problem is detected, the cell cycle may stop.
Why Cell-Cycle Control Is Important
Imagine that cells divided whenever they wanted.
Tissues would quickly become disorganized.
Normal cell-cycle regulation ensures that cells divide mainly when new cells are required.
For example:
Skin injury → cells divide → damaged cells replaced → division slows
This produces controlled growth.
Cancer cells may fail to respond correctly to these normal controls.
What Is a Mutation?
A mutation is a change in the DNA sequence.
DNA contains genes that provide instructions for making proteins.
Some of these proteins control:
- cell division
- DNA repair
- cell-cycle checkpoints
- programmed cell death
A mutation in one of these genes can change the way the cell behaves.
Not every mutation causes cancer.
Many mutations have little effect, and cells also have mechanisms for repairing DNA damage.
Cancer usually develops through the accumulation of multiple important genetic changes.
How Mutations Can Affect the Cell Cycle
Think of cell-cycle control as having an accelerator and brakes.
Some genes encourage cell division when it is needed.
Other genes slow or stop cell division.
Mutations can cause problems in either system.
For example:
Accelerator stuck on
A mutation causes a growth-promoting signal to remain active.
Result:
too much cell division
Or:
Brakes fail
A mutation prevents a cell-cycle control protein from stopping division.
Result:
damaged cells continue dividing
Over time, additional mutations can make the cell increasingly abnormal.
Proto-Oncogenes and Oncogenes
Some normal genes help stimulate cell growth and division when appropriate.
These are called proto-oncogenes.
If certain mutations make these genes excessively active, they can become oncogenes.
An oncogene may send signals that encourage the cell to divide too often.
A simple analogy is:
Normal proto-oncogene = accelerator working normally
Oncogene = accelerator stuck down
This can contribute to uncontrolled cell division.
Tumour Suppressor Genes
Other genes help slow cell division or prevent damaged cells from continuing through the cell cycle.
These are called tumour suppressor genes.
They act somewhat like brakes.
If mutations damage tumour suppressor genes, the cell may lose important controls.
The result can be:
damaged DNA → checkpoint fails → cell divides anyway
Both overactive growth signals and failed growth-control systems can therefore contribute to cancer.
DNA Repair Genes
Cells also contain genes involved in repairing damaged DNA.
If DNA repair systems are damaged by mutations, mistakes may accumulate more quickly.
This can increase the chance that additional genes controlling cell growth will eventually be affected.
Cancer development therefore often involves several genetic changes occurring over time.
Normal Cells vs Cancer Cells
Normal and cancerous cells can behave very differently.
| Normal Cells | Cancer Cells |
|---|---|
| Division is controlled | Division may become uncontrolled |
| Respond to stop signals | May ignore stop signals |
| DNA damage may stop the cycle | May continue dividing despite damage |
| Usually remain organized in tissues | May disrupt normal tissue organization |
| Damaged cells may die | Abnormal cells may avoid cell death |
| Usually remain in their tissue | Some can invade other tissues |
What Is a Tumour?
When abnormal cells divide repeatedly, they can produce a mass of cells called a tumour.
Not all tumours are cancerous.
Tumours can broadly be described as:
- benign
- malignant
Benign Tumours
A benign tumour is an abnormal growth that does not invade nearby tissues or spread to distant parts of the body.
Benign tumours can still cause health problems.
For example, they may:
- press against organs
- block blood vessels
- interfere with normal tissue function
However, they do not behave like malignant cancers.
Malignant Tumours
A malignant tumour is cancerous.
Malignant cells can:
- grow uncontrollably
- invade nearby tissues
- damage surrounding structures
- sometimes enter blood or lymph vessels
- spread to other parts of the body
The ability to invade and spread is one of the most serious features of cancer.
Metastasis
The spread of cancer from its original location to another part of the body is called metastasis.
A simplified sequence is:
- A malignant tumour develops.
- Some cancer cells break away.
- Cells enter blood or lymph vessels.
- They travel to another part of the body.
- Some leave the circulation.
- They begin growing in another tissue.
- A secondary tumour may form.
Cancer that has spread can be more difficult to treat than cancer that remains localized.
What Causes Mutations?
Mutations can occur for many reasons.
Some happen naturally when DNA is copied.
Others can be caused by environmental factors.
A substance or form of radiation that can increase the chance of cancer is called a carcinogen.
Examples of factors that can damage DNA or increase cancer risk include:
- tobacco smoke
- ultraviolet radiation
- ionizing radiation
- certain chemicals
- some infections
However, exposure to a risk factor does not mean a person will definitely develop cancer.
Cancer risk is influenced by a combination of genetics, environment, age, exposures, and other factors.
Tobacco and Cancer Risk
Tobacco smoke contains many harmful chemicals, including carcinogens.
These chemicals can damage DNA.
Repeated exposure increases the chance that important mutations will accumulate.
Smoking is strongly associated with several cancers, particularly lung cancer, as well as cancers in other parts of the body.
This demonstrates an important relationship:
greater exposure to carcinogens → greater opportunity for DNA damage → increased cancer risk
It does not mean every exposed person develops cancer.
Ultraviolet Radiation
Ultraviolet, or UV, radiation from sunlight can damage DNA in skin cells.
Cells have DNA repair mechanisms, but excessive UV exposure can increase the chance that mutations remain.
Over time, these mutations can contribute to skin cancer.
Risk can be reduced by limiting excessive UV exposure and using appropriate sun protection.
Ionizing Radiation
High-energy ionizing radiation can damage DNA.
Sources can include:
- X-rays
- gamma radiation
- radioactive materials
Medical imaging using ionizing radiation is carefully controlled so that useful diagnostic information can be obtained while unnecessary exposure is minimized.
Radiation can therefore have both important medical uses and potential biological risks.
Infections and Cancer
Some infections can increase the risk of particular cancers.
Certain viruses can interfere with genes controlling cell growth or create long-term changes in infected tissues.
One example is human papillomavirus (HPV), some types of which are associated with cervical cancer and several other cancers.
Another example is hepatitis B or C infection, which can increase the risk of liver cancer.
This does not mean that cancer itself is generally contagious.
Inherited Cancer Risk
Cancer itself is usually not directly inherited from a parent.
However, people can inherit particular gene variants that increase their risk of developing certain cancers.
For example, inherited changes in some DNA-repair or tumour-suppressor genes can make it easier for additional mutations to accumulate.
Therefore:
inherited mutation ≠ guaranteed cancer
Instead:
inherited mutation → potentially increased risk
Age and Cancer Risk
Cancer becomes more common with increasing age.
One reason is that cells accumulate mutations over time.
The longer an organism lives, the more opportunities there are for:
- DNA copying errors
- environmental DNA damage
- failures in DNA repair
- combinations of mutations to accumulate
This helps explain why many cancers are more common in older adults.
Risk Factor Does Not Mean Cause in Every Individual
It is important to understand the meaning of risk.
Suppose a behavior doubles the risk of developing a particular disease.
This does not mean every person with that behavior will develop the disease.
Similarly, a person with few known risk factors can still develop cancer.
Risk factors change probability, not certainty.
Reducing Cancer Risk
Not all cancers are preventable, but some risks can be reduced.
Examples include:
- avoiding tobacco
- limiting excessive UV exposure
- using appropriate sun protection
- receiving recommended vaccines against cancer-associated infections
- following workplace safety procedures around hazardous chemicals or radiation
- participating in recommended screening programs when appropriate
These actions cannot guarantee that cancer will never occur, but they can reduce certain risks.
Why Early Detection Is Important
Cancer is often easier to treat when it is detected before it has grown extensively or spread to distant tissues.
Early detection may allow doctors to:
- remove a smaller tumour
- treat a localized area
- begin treatment before metastasis occurs
- improve the chance of successful treatment for many cancer types
This is why screening programs exist for some cancers.
Screening aims to detect cancer, or sometimes precancerous changes, before serious symptoms develop.
Cancer Screening
Different screening methods are used for different cancers and populations.
Examples can include:
- mammography
- cervical screening
- colorectal screening
- certain imaging tests
- laboratory tests
Screening recommendations depend on factors such as:
- age
- sex
- family history
- personal medical history
- cancer type
Screening does not prevent every cancer, but it can help detect some cancers earlier.
Diagnosing Cancer
If cancer is suspected, doctors may use several methods to investigate.
These can include:
- medical imaging
- blood tests
- physical examination
- biopsy
A biopsy involves removing a sample of tissue so that it can be examined.
Pathologists can examine cells and tissue organization under a microscope to look for abnormal features.
Cancer Treatment
Cancer is not one single disease, so treatment varies considerably.
Possible treatments include:
- surgery
- radiation therapy
- chemotherapy
- targeted therapy
- immunotherapy
- hormone therapy for certain cancers
- combinations of treatments
The most appropriate treatment depends on factors such as:
- cancer type
- location
- stage
- genetic characteristics of the cancer
- overall health of the patient
Surgery
If a tumour is localized, doctors may be able to remove it surgically.
Surgery can be especially effective when the cancer has not spread to distant tissues.
This is another reason early detection can be important.
Radiation Therapy
Radiation therapy uses high-energy radiation to damage the DNA of cancer cells.
Cancer cells that receive enough damage may stop dividing or die.
Treatment is carefully planned to target cancer while reducing damage to surrounding healthy tissues.
Chemotherapy
Chemotherapy uses drugs that kill cancer cells or prevent them from dividing.
Because many chemotherapy drugs affect rapidly dividing cells, they can also affect some healthy cells that divide frequently.
This can contribute to side effects.
Researchers continue developing treatments that target cancer cells more specifically.
Targeted Therapy
Cancer cells often contain particular molecular changes.
Targeted therapies are designed to interfere with specific molecules or pathways that cancer cells depend upon.
This reflects an important development in cancer biology:
Different cancers can have different mutations, even when they occur in the same organ.
Understanding these mutations can sometimes help doctors select treatments.
Immunotherapy
The immune system normally identifies and destroys many abnormal cells.
Some cancer cells develop ways to avoid immune attack.
Immunotherapy aims to help the immune system recognize or attack cancer cells more effectively.
This has become an important treatment approach for some cancers.
Why Cancer Can Be Difficult to Treat
Cancer cells originate from the body's own cells.
This creates a major challenge.
A treatment must damage or remove cancer cells while limiting harm to normal cells.
Cancer cells can also continue mutating.
This means that some cancer cells may eventually become resistant to a treatment.
Different cells within the same tumour may even contain different combinations of mutations.
Worked Example: Cell-Cycle Mutation
A mutation damages a gene that normally stops cells from dividing when DNA is damaged.
What could happen?
The checkpoint may fail.
A damaged cell could continue through the cell cycle.
It could then pass mutations to its daughter cells.
Over time, additional mutations could accumulate and contribute to cancer.
Worked Example: Normal vs Cancerous Growth
A person's skin is cut.
Nearby cells divide.
When the wound has healed, cell division slows.
This is:
normal controlled cell growth
Now imagine cells continue dividing even when no new cells are needed.
This is:
uncontrolled cell division
and may contribute to tumour formation.
Worked Example: Cancer Risk
Two people have different levels of UV exposure.
Person A regularly receives intense UV exposure without protection.
Person B has much lower exposure.
Which person has a greater risk of developing UV-related DNA damage?
Person A
because greater UV exposure provides more opportunities for DNA damage.
However, this does not mean Person A will definitely develop cancer.
Worked Example: Early Detection
Cancer is discovered while it remains confined to one small area.
Why can this be an advantage?
The cancer may be easier to remove or treat before cells spread to distant tissues.
Therefore:
early detection can improve treatment options and outcomes for many cancers.
Cancer and the Cell Cycle
The relationship between the cell cycle and cancer can be summarized as:
Normal cell
↓
DNA mutation
↓
Cell-cycle control affected
↓
Damaged cell continues dividing
↓
Additional mutations may accumulate
↓
Uncontrolled cell division
↓
Tumour may develop
↓
Some cancer cells may invade surrounding tissues
↓
Some may metastasize
Cancer usually develops through multiple steps rather than from one single event.
Common Misconceptions
Cancer is simply very fast mitosis.
Cancer involves abnormal regulation of cell growth, survival, and division. Uncontrolled cell division is central, but cancer biology is more complex than simply "fast mitosis."
Every mutation causes cancer.
Incorrect. Most individual mutations do not cause cancer. Cancer usually involves multiple important genetic changes.
Every tumour is cancerous.
Incorrect. Benign tumours do not invade or metastasize.
Cancer is contagious.
Cancer itself is generally not contagious, although some infections can increase the risk of particular cancers.
Having a cancer risk factor means you will get cancer.
Risk factors change the probability of developing cancer. They do not determine the outcome with certainty.
Cancer is always inherited.
Most cancers are not directly inherited, although inherited genetic variants can increase susceptibility to certain cancers.
Cancer can always be prevented.
Some cancer risks can be reduced, but not all cancers are preventable.
Did You Know?
Your cells experience DNA damage surprisingly often.
Most of the time, cells successfully repair the damage or prevent seriously damaged cells from continuing to divide.
Cancer usually develops only after a cell lineage accumulates a combination of changes that allows it to escape several of these normal control systems.
This is why cancer is best understood not as a single mutation, but as a breakdown of multiple layers of cellular control.
Key Terms
Cancer – A group of diseases involving abnormal cells that grow and divide uncontrollably.
Mutation – A change in the DNA sequence.
Cell-cycle checkpoint – A control point that helps determine whether a cell should continue through the cell cycle.
Tumour – An abnormal mass of cells produced by excessive cell growth or division.
Benign tumour – A non-cancerous tumour that does not invade nearby tissues or spread to distant sites.
Malignant tumour – A cancerous tumour capable of invading surrounding tissues.
Metastasis – The spread of cancer cells from the original site to another part of the body.
Carcinogen – A substance or exposure that can increase cancer risk.
Oncogene – A gene that can promote excessive cell growth when abnormally activated.
Tumour suppressor gene – A gene that normally helps control cell division or prevent damaged cells from continuing to divide.
Biopsy – Removal of a tissue sample for examination.
Chemotherapy – Treatment using drugs that kill cancer cells or interfere with their growth.
Radiation therapy – Treatment using high-energy radiation to damage cancer cells.
Immunotherapy – Treatment that helps the immune system attack cancer.
Key Takeaways
- Cancer results from the loss of normal controls over cell growth and division.
- Mutations are changes in DNA.
- Mutations affecting cell-cycle control genes can contribute to cancer.
- Normal cells respond to signals controlling when they divide.
- Cancer cells may ignore signals that normally stop division.
- Cancer usually develops after multiple genetic changes accumulate.
- Benign tumours do not invade surrounding tissues or metastasize.
- Malignant tumours can invade nearby tissues and may spread.
- The spread of cancer to other parts of the body is called metastasis.
- Tobacco smoke, UV radiation, ionizing radiation, certain chemicals, some infections, inherited genetic variants, and age can influence cancer risk.
- A risk factor increases probability; it does not guarantee that cancer will develop.
- Some cancer risks can be reduced by limiting harmful exposures and using appropriate preventive measures.
- Early detection is important because many cancers are easier to treat before they have spread.
- Cancer treatments can include surgery, radiation therapy, chemotherapy, targeted therapy, and immunotherapy.
- Understanding the cell cycle helps explain both normal growth and how uncontrolled cell division can lead to cancer.