Cell Cycle Regulation and Cancer

Essential Question

What happens when a cell stops following the rules for dividing?

Cells do not divide all the time. In a healthy body, cells divide when new cells are needed and stop when they are not, and the cell cycle has built-in checkpoints that help control this. In this lesson, you’ll see how those controls work, what happens when they fail, and how cell counts show the difference between normal and unregulated growth.

Cells Divide When the Body Needs Them

• Cells divide when they receive the right signals
• A cut speeds up division near the wound
• Division returns to normal as the wound closes
• Division speeds up, slows down, and stops as needed

Cells in a healthy body divide when they receive the right signals. The graph shows sample data for skin cells near a small cut. After the injury, more cells divide to replace the damaged ones. The percent of dividing cells peaks on day 2 and falls back to the normal level as the wound closes. This shows that cell division is regulated. It speeds up, slows down, and stops depending on what the body needs.

Checkpoints Control the Cell Cycle

• Checkpoints are control points in the cycle
• G₁ checkpoint: size, conditions, undamaged DNA
• G₂ checkpoint: DNA fully copied and undamaged
• M checkpoint: chromosomes attached to the spindle

The cell cycle has built-in checkpoints, points where the cell checks whether it is safe to go on. At the G₁ checkpoint, the cell checks that it is big enough, that conditions are right, and that its DNA is not damaged. At the G₂ checkpoint, it checks that all of the DNA has been copied and that the copies are undamaged. At the M (spindle) checkpoint, which acts at metaphase, it checks that every chromosome is attached to spindle fibers before the sister chromatids separate. If a check fails, the cell can pause the cycle while repairs are made. If the damage cannot be repaired, the cell can start a process of programmed cell death, which keeps the damaged DNA from being passed on.

Normal Cells Stop When They Are Crowded

• Normal cells in a dish stop dividing when they touch
• They form a single layer
• Unregulated cells ignore the stop signal and keep dividing
• They pile up in layers and clumps

Normal cells grown in a dish divide until they cover the bottom in a single layer. When the cells touch their neighbors, signals tell them to stop dividing. Cells that have lost this control behave differently. They ignore the stop signals and keep dividing, so they pile up in layers and clumps. In a body, this kind of unregulated growth can crowd out normal cells and interfere with how a tissue works.

Cancer Is a Loss of Control

• Cancer: cells divide without the normal controls
• A mutation can damage a gene that controls the cycle
• In many cancers, several mutations build up
• A tumor is a mass of unregulated cells

Cancer is a group of diseases in which cells divide without the normal controls. It can begin when a mutation, a change in the DNA, damages a gene that helps control the cell cycle, such as a gene needed for a checkpoint. A damaged cell can then move through the cycle without being stopped, and each daughter cell inherits the mutation. Mutations can come from mistakes made when DNA is copied, or from damage caused by things such as ultraviolet radiation and certain chemicals. In many cancers, several mutations build up in the same line of cells over time, so cancer does not start from a single change. A mass of cells that divide without control is called a tumor. A benign tumor stays in one place. A malignant tumor can invade nearby tissue and spread to other parts of the body, which is called metastasis. This lesson describes cell biology in general. It is not medical advice.

Reading Growth Data

• Normal cells slow down and level off
• Unregulated cells keep doubling
• Compare the groups at the same times
• Use numbers from the data as evidence

The graph compares sample data for two groups of cells grown in a dish, each starting with 1,000 cells. The normal cells grow quickly at first and then level off near 10,000, because crowding tells them to stop dividing. The unregulated cells double every day and do not level off during the 6 days, reaching 64,000 cells. For the first few days, the two groups look alike, so the later days are what show the difference. In a real dish, the unregulated cells would eventually run out of nutrients and space too.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Normal or Unregulated?

The table shows the number of cells in two dishes over 6 days. Write a claim, evidence, and reasoning: which dish contains cells that are ignoring the signals that stop division?

Day	Dish A (cells)	Dish B (cells)
0	1,000	1,000
1	2,000	1,900
2	4,000	3,400
3	8,000	5,600
4	16,000	8,000
5	32,000	9,500
6	64,000	10,000

Show the solution

1. Claim: the cells in Dish A are ignoring the signals that stop division.
2. Evidence: Dish A doubled every day, from 1,000 cells to 64,000 cells on day 6. Dish B slowed down and leveled off near 10,000 cells (9,500 on day 5 and 10,000 on day 6).
3. Reasoning: normal cells respond to crowding and checkpoint signals by slowing down and stopping, which gives a leveling-off curve like Dish B. Cells that keep doubling when the dish is crowded are not responding to those signals.
4. Answer: Dish A. It kept doubling to 64,000 cells, while Dish B leveled off near 10,000.

Example 2: Percent Growth Each Day

Use the growth data. Find the percent increase in the number of cells from day 4 to day 5 and from day 5 to day 6, for the normal cells and for the unregulated cells.

Day	Normal cells	Unregulated cells
4	8,000	16,000
5	9,500	32,000
6	10,000	64,000

Show the solution

1. Formula: percent increase = (new − old) ÷ old × 100
2. Normal, day 4 to 5: (9,500 − 8,000) ÷ 8,000 × 100 = 18.75%, about 18.8%
3. Normal, day 5 to 6: (10,000 − 9,500) ÷ 9,500 × 100 = 5.3%
4. Unregulated: (32,000 − 16,000) ÷ 16,000 × 100 = 100%, and (64,000 − 32,000) ÷ 32,000 × 100 = 100%
5. Pattern: the growth of the normal cells keeps slowing down, while the unregulated cells grow by a steady 100% per day.
6. Answer: Normal cells: 18.8% and then 5.3%. Unregulated cells: 100% each day.

Example 3: A Failed Checkpoint

A cell finishes S phase, but some of its DNA is damaged and the damage has not been repaired. Use the checkpoint model to predict what a working G₂ checkpoint does. Then predict what happens if the protein that runs this checkpoint is broken by a mutation.

Show the solution

1. Where: the G₂ checkpoint (number 2) checks that the DNA is copied and undamaged.
2. Working checkpoint: the cell detects the damage and pauses the cycle so repairs can be made. If the damage cannot be repaired, the cell can start programmed cell death.
3. Broken checkpoint: the cell does not stop. It enters M phase and divides, and both daughter cells inherit the damaged DNA.
4. Why it matters: cells that keep dividing with damaged DNA can pick up more mutations, which is how unregulated growth can begin.
5. Answer: A working checkpoint pauses the cycle for repair or starts cell death. A broken one lets the cell divide and pass on the damage.

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Practice

Name what each checkpoint checks, compare normal and unregulated growth data, explain why normal cells stop dividing, and predict the effect of a broken checkpoint. Try it on your own first, then check your work with the answer key.

• Cell Cycle Regulation and Cancer Practice Worksheet (PDF)
• Cell Cycle Regulation and Cancer Practice Answer Key (PDF)

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Lesson Slides

• 7.3 Cell Cycle Regulation and Cancer Slides (PDF)

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Big Ideas

• Cells divide when the body needs them, and stop when it does not
• Checkpoints check the cell before the cycle goes on
• Normal cells stop dividing when they are crowded
• Cancer can begin when mutations in genes that help control the cell cycle let cells divide without the normal checks
• Unregulated cells keep doubling; normal cells level off

Back to the essential question: cells divide when the body needs new cells, and checkpoints stop the cycle if the cell is too small, the DNA is damaged, or the chromosomes are not ready. When a mutation breaks these controls, a cell can divide without stopping and pass the damage to every daughter cell, which is how cancer can begin. Next, you’ll look at viruses, which cannot divide on their own and instead use a host cell to make copies of themselves.

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