The Cell Cycle

Essential Question

Why must a cell grow and copy its DNA before it divides?

Your body makes new cells all the time, to grow, to repair damage, and to replace cells that wear out. But a cell cannot simply split in two. In this lesson, you’ll see why cells divide, follow a cell through the phases of the cell cycle, and use data to calculate how long each phase lasts and how fast a population of dividing cells grows.

Why Cells Divide

• Growth: more cells make a bigger organism
• Repair and replacement of damaged or worn-out cells
• A cell that grows too big has too little surface for its volume
• Dividing keeps cells small enough to work

Organisms grow by adding cells, repair injuries by replacing damaged cells, and replace cells that wear out. Cell division also keeps cells small. As the cubes show, a bigger object has less surface area for each unit of volume (the surface area to volume ratio from Unit 6). Nutrients and wastes cross the cell membrane, so a cell that grows too large cannot exchange materials fast enough to supply all of its volume. Two smaller cells have more surface area in total than one large cell with the same volume.

The Cell Cycle Has Phases

• Interphase: G₁, S, and G₂
• M phase: mitosis, then cytokinesis
• Most of the cycle is interphase
• Some cells leave the cycle and stop dividing (G₀)

The cell cycle is the series of events from one cell division to the next. It has two main parts. During interphase, the cell grows and prepares to divide. Interphase has three phases: G₁ (the first gap, when the cell grows), S (synthesis, when the DNA is copied), and G₂ (the second gap, when the cell finishes growing and prepares to divide). During M phase, the cell divides. Mitosis divides the nucleus, and cytokinesis divides the cytoplasm. In many kinds of cells, a cell spends about 90 percent or more of the cycle in interphase and only a short time in M phase. The times in the ring are sample values for a human cell dividing quickly in a dish. Cycle lengths vary widely. Some cells divide in less than a day, while others, such as most mature nerve cells, leave the cycle and do not divide again (a state called G₀).

Copying the DNA in S Phase

• Each chromosome is one long DNA molecule
• S phase: each DNA molecule is copied
• A copied chromosome has 2 sister chromatids
• Sister chromatids are identical and joined at the centromere

A chromosome is a single, very long DNA molecule wrapped around proteins. As you saw in the nucleic acids lesson, DNA is a double helix made of nucleotides. Before a cell divides, it must make a complete copy of every chromosome, and this happens in S phase. After copying, each chromosome consists of two identical copies of the DNA, called sister chromatids, joined at a region called the centromere. The cell still counts it as one chromosome. When the cell divides, the sister chromatids are separated, so each new cell gets one complete copy of the DNA. (Unit 9 covers how DNA is copied.)

Following One Cell Through the Cycle

• G₁: the cell grows and makes proteins
• S: the DNA is copied
• G₂: the cell checks the copies and prepares
• M: the cell divides into two cells

The drawings follow one cell that has 4 chromosomes. In G₁, each chromosome is a single DNA molecule, and the cell grows and makes proteins and organelles. In S phase, the DNA is copied, so each chromosome gets two sister chromatids. In G₂, the cell keeps growing and prepares to divide. In M phase, the nucleus divides and then the cytoplasm divides, making two new cells. Each new cell starts its own cell cycle in G₁ with 4 chromosomes. (A human body cell has 46.)

Populations Grow by Doubling

• One trip through the cycle: 1 cell becomes 2
• Each cycle doubles the number of cells
• After n cycles: 2ⁿ times as many cells
• Real populations grow more slowly

Each time a cell completes the cell cycle, one cell becomes two. If every cell divides once per cycle, the number of cells doubles each cycle. The graph shows sample data for 1,000 cells with a 24-hour cell cycle. After n cycles, there are 1,000 × 2ⁿ cells. The curve gets steeper as it goes, which is called exponential growth. Real populations grow more slowly, because some cells die, some leave the cycle, and cells can run out of space and nutrients.

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Worked Examples

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

Example 1: Percent of Time in Each Phase

In a human cell growing in a dish, the phases last 11 h (G₁), 8 h (S), 4 h (G₂), and 1 h (M). Find the length of the whole cell cycle, the percent of the cycle spent in each phase, and the percent spent in interphase.

Phase	Time (h)
G1	11
S	8
G2	4
M	1

Show the solution

1. Total: 11 h + 8 h + 4 h + 1 h = 24 h
2. Percent in each phase: G₁: 11 ÷ 24 × 100 = 45.8%; S: 8 ÷ 24 × 100 = 33.3%; G₂: 4 ÷ 24 × 100 = 16.7%; M: 1 ÷ 24 × 100 = 4.2%
3. Interphase: 11 h + 8 h + 4 h = 23 h, and 23 ÷ 24 × 100 = 95.8% (or 100% − 4.2%)
4. Answer: The cycle lasts 24 h. G₁ is 45.8%, S is 33.3%, G₂ is 16.7%, M is 4.2%, and interphase is 95.8%.

Example 2: A Population of Dividing Cells

A dish starts with 1,000 cells. Every cell divides once every 24 hours, and no cells die. How many cells are in the dish after 6 days? How many days until there are more than 100,000 cells?

Day	Cells
0	1,000
1	2,000
2	4,000
3	8,000
4	16,000
5	32,000
6	64,000

Show the solution

1. Each day: the number of cells doubles, so after n days there are 1,000 × 2ⁿ cells.
2. Day 6: 1,000 × 2⁶ = 1,000 × 64 = 64,000 cells
3. More than 100,000: day 6 gives 64,000, which is not enough. Day 7 gives 1,000 × 2⁷ = 128,000 cells.
4. Answer: 64,000 cells after 6 days. It takes 7 days to pass 100,000 cells.

Example 3: When Does the DNA Double?

The graph shows the amount of DNA in one cell during a 24-hour cell cycle. Write a claim, evidence, and reasoning: during which phase does the cell copy its DNA, and why must it do this before it divides?

Show the solution

1. Claim: the cell copies its DNA in S phase.
2. Evidence: the DNA per cell stays at 1 unit during G₁ (0 to 11 h), rises to 2 units between 11 h and 19 h (S phase), and stays at 2 units until the cell divides at 24 h. After division, each daughter cell has 1 unit.
3. Reasoning: the cell makes two complete sets of DNA before it divides, so there is one set for each new cell. A cell that divided without copying its DNA first could pass on only half of it.
4. Answer: S phase. The DNA doubled from 1 to 2 units, so each daughter cell can receive a complete copy.

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Practice

Name the phases of the cell cycle, explain what happens to a chromosome in S phase, calculate the percent of time spent in each phase, and predict how fast a population of dividing cells grows. Try it on your own first, then check your work with the answer key.

• The Cell Cycle Practice Worksheet (PDF)
• The Cell Cycle Practice Answer Key (PDF)

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Lesson Slides

• 7.1 The Cell Cycle Slides (PDF)

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Big Ideas

• Cells divide to grow, repair tissue, and stay small enough to work
• The cell cycle has interphase (G₁, S, G₂) and M phase
• S phase copies the DNA, so each chromosome has 2 sister chromatids
• Most of the cycle is spent in interphase
• Each trip through the cycle doubles the number of cells

Back to the essential question: a cell has to grow so that each new cell has enough cytoplasm and organelles, and it has to copy its DNA so that each new cell gets a complete set. Those jobs happen in G₁, S, and G₂, before M phase divides the cell. Next, you’ll follow the chromosomes through mitosis, the stage that splits the copied DNA between the two new cells.

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