Life Science · Unit 1: Cells
How Cells Divide
How does your body make new cells to fill in a scrape, and how does each new cell get a full set of instructions? You step a model cell through interphase and the four stages of mitosis, then count real onion root-tip cells in each stage. Afterwards you can describe the cell cycle and use counts to argue which part takes the longest.
Where this lesson sits
- Virginia SOL LS.2 d
- Practices LS.1 c, LS.1 d, LS.1 e
Essential question: How does my body make new cells to heal a cut without mixing up the instructions?
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Life Science (grade 7) LS.2 d You are here
This lesson
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Biology
Preview it at high school level
In Biology you will learn that interphase has three parts. In G1 the cell grows. In S it copies its DNA. In G2 it gets ready to divide. You will learn names for the parts you saw here. The two joined copies of a chromosome are sister chromatids, held together at a spot called the centromere. Spindle fibers are the thin fibers that pull them apart. The splitting of the cell into two is called cytokinesis. You will also learn why cells divide instead of growing very large: a big cell has too little outside surface for all the stuff inside it, so materials cannot move in and out fast enough.
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Biology II Biology II: Advanced Survey of Biology Topics, BIOII.2 e; Genetics, GEN.4 a and c
Preview where it goes next
In Virginia's Biology II courses, cell division comes back in two ways. In Advanced Survey of Biology Topics, you study how cells send signals to each other that control when they grow, divide and become specialized. In Genetics, you compare mitosis with meiosis and ask how meiosis makes offspring different from each other. You also study what happens when chromosomes are shared out unevenly, so that a cell ends up with one too many or one too few.
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College
Watch first Copy First, Then Share · 2:29
Scrape your knee, and the spot is often covered with new skin in about one to two weeks. Where did those new skin cells come from? Every cell comes from a cell that was already there. One cell splits and becomes two. But most cells carry a full set of instructions. How does each new cell get a complete set, with nothing missing?
What do you think? Decide before you read.
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Scientists disagree: Before a cell divides, it copies every chromosome. Then one copy of each goes to each new cell. Scientists see this in stained cells under a microscope: the chromosomes look doubled before they separate, and each new cell ends up with the same number as the parent.
What scientists think
Scientists disagree: Before a cell divides, it copies every chromosome. Then one copy of each goes to each new cell. Scientists see this in stained cells under a microscope: the chromosomes look doubled before they separate, and each new cell ends up with the same number as the parent.
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Scientists disagree: A cell does grow a little before it divides. Then it splits into two smaller cells, so most cells stay small. Growing mostly adds more cells, though some cells do get bigger. An adult has many more cells than a baby, not giant cells.
What scientists think
Scientists disagree: A cell does grow a little before it divides. Then it splits into two smaller cells, so most cells stay small. Growing mostly adds more cells, though some cells do get bigger. An adult has many more cells than a baby, not giant cells.
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Scientists disagree: When scientists photograph a root tip, where cells divide often, most cells are not dividing at that moment. They are in a long stage of growing and copying. Counting cells in each stage shows which stages take the most time.
What scientists think
Scientists disagree: When scientists photograph a root tip, where cells divide often, most cells are not dividing at that moment. They are in a long stage of growing and copying. Counting cells in each stage shows which stages take the most time.
Explore: heal a scrape, one cell at a time
In the simulation, the left panel shows a strip of skin cells with a gap in it. You set how long one cell cycle takes and press start. A counter shows how many cells there are. The right panel follows one cell with 4 chromosomes through each stage. A counter shows how many chromosomes the cell has and whether each one is copied. A switch lets you skip the copying step.
1. Fill the gap
- Cells now
- 4
- Gap spaces filled
- 0 of 12
- Time so far
- 0 time units
A strip of skin cells with a gap. Press Start.
2. Follow one cell
In this sketch, the strip starts with 4 cells, every cell divides at the same time, the gap has 12 spaces, and each cell divides twice. A model cell has 4 chromosomes instead of the 46 in most human body cells. Cells are flat shapes, and time units are not to scale. Interphase is drawn as the longer part of the cycle, because the lesson says it is usually the longest; the exact split is not to scale.
Rule to keep: a cell copies its chromosomes first, then each daughter cell gets one copy of each, so it has the same number as the parent cell. A copied chromosome is still one chromosome. In a real scrape some cells also crawl into the gap.
Try this.
- Predict how long the gap will take to fill if you make the cell cycle shorter. Then test it and watch the counter.
- Before you step the single cell, predict how many chromosomes each new cell will have at the end. Step through and count.
- Turn on “skip the copying.” Predict how many chromosomes each new cell will get now. Then run it and compare.
Explain: the cell cycle
- cell cycle
- interphase
- mitosis
- chromosome
- daughter cell
- prophase
- metaphase
- anaphase
- telophase
All living things grow and reproduce. When you grow, the number of cells in your body goes up. When you heal, new cells replace the ones you lost. In both cases, cells that already exist divide.
Inside the nucleus are chromosomes, the threads that carry a cell’s instructions (DNA). You meet the nucleus and DNA in other lessons. A typical human body cell that has a nucleus has 46 chromosomes.
The cell cycle is the series of steps a cell goes through from when it forms until it divides. It has two main parts.
- Interphase is the part of the cycle when the cell grows and copies its chromosomes. It is usually the longest part. The cell is busy, not resting.
- Mitosis is the part of the cycle when the cell divides into two new cells with the same chromosomes.
Each new cell is called a daughter cell. The cell that divided is the parent cell.
Copying matters. A copied chromosome is still one chromosome. It is made of two identical copies joined together. So a cell with 4 chromosomes still has 4 after copying, but each one now has two copies.
Think of a recipe book. If you tear it in half, each person gets half the recipes. If you copy it first, each person gets the whole book. The comparison breaks in one place: a cell does not decide to copy. Its molecules do the copying, step by step.
Explain: the four stages of mitosis
Mitosis has four stages. They always come in the same order.
| Stage | What the chromosomes do |
|---|---|
| Prophase | The chromosomes coil up and get thick enough to see. |
| Metaphase | The chromosomes line up in a row across the middle of the cell. |
| Anaphase | The two copies of each chromosome are pulled to opposite ends. |
| Telophase | Two new nuclei form, and the cell pinches into two. |
Prophase is the stage when the copied chromosomes coil up and become visible. Metaphase is the stage when they line up across the middle. Anaphase is the stage when the copies are pulled apart. Thin fibers in the cell do the pulling. Telophase is the stage when a new nucleus forms at each end and the cell splits. A plant cell has a stiff wall, so it cannot pinch. A new wall forms across its middle instead.
- prophase
- metaphase
- anaphase
- telophase
- daughter cell
- blank
- metaphase
- blank
- blank
- daughter cell
Word bank: anaphase, prophase, telophase
- blank
- blank
- blank
- blank
- blank
Check your labels
- prophase
- metaphase
- anaphase
- telophase
- daughter cell
Count the chromosomes in a model cell. It starts with 4. In interphase, each of the 4 is copied. In anaphase, one copy of each goes to each end. Each daughter cell ends up with 4 chromosomes, the same as the parent cell. Skip the copying, and each daughter cell would get only 2.
Mistakes can happen, but in normal mitosis each daughter cell gets a complete set.
A one-celled living thing, such as an amoeba, reproduces by dividing. Its two daughter cells are two new amoebas. A different kind of division, meiosis, makes sex cells with half the parent’s genetic material. You study it in Reproduction and Meiosis.
Elaborate: count onion root-tip cells
Near the tip of an onion root, cells divide often. A stained root tip is a snapshot. Each cell is caught at one moment of its cycle. If cells are caught at random moments, a longer stage holds more cells.
| Stage | Cells counted |
|---|---|
| Interphase | 20 |
| Prophase | 10 |
| Metaphase | 3 |
| Anaphase | 2 |
| Telophase | 1 |
| Total | 36 |
Source: The Biology Project, University of Arizona, onion (Allium cepa) root-tip cell cycle activity, completed-photo page dated 1997 and revised August 2004. It sorts 36 photographed cells. The activity says to assume one cycle takes 24 hours. That is a round number for the activity, not a measured time.
- Which stage has the most cells? What percent of the 36 cells is that?
- Find the percent for each stage. Do your five percents add up to about 100?
- If one cycle takes 24 hours, about how many hours does interphase take?
- The stage with the most cells is blank.
- This stage takes the most time because blank.
- One limit of these counts is blank.
Claim, evidence, reasoning
Make a claim about which part of the cell cycle takes the longest, and what that tells you about how a cell gets ready to divide.
Start with: The counts show that blank takes the longest part of the cell cycle.
Start with: Out of 36 cells, blank were in interphase. That is about blank percent.
Start with: A longer stage holds more cells at one moment, because blank. During this stage the cell blank.
Compare with a model answer
Interphase takes the longest part of the cell cycle. Out of 36 onion root-tip cells, 20 were in interphase. That is about 56 percent, more than all four stages of mitosis together. A longer stage holds more cells at any moment. During interphase the cell grows and copies every chromosome, so it is ready to give each daughter cell a full set. One limit: these are only 36 photos, and the page does not say how they were picked. The 24-hour cycle is also an assumption.
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Design problem
Does temperature change how fast onion roots grow longer? Plan an investigation.
One way to do it
- Question: Do onion roots grow longer each day in a warm place than in a cool place?
- Independent variable: temperature, such as a cool spot near 15 °C and a warm spot near 25 °C, checked with a thermometer.
- Dependent variable: root length in millimetres, measured with a ruler each day for 5 days.
- Constants: the same kind and size of onion, the same cup and amount of water, the same light.
- Control: onions kept at normal room temperature, to compare with.
- Repeated trials: at least 5 onions at each temperature. Find the mean length for each group.
- Safety: wash your hands after touching onions. Do not taste anything. Your teacher handles any blades.
Roots grow when cells near the tip divide and the new cells stretch. Faster growth in the warm group would suggest that temperature speeds this up. Seeing the dividing cells themselves would need stained slides, which your teacher prepares.
Evaluate: check your thinking
A scrape fills in because skin cells near the wound divide and make new cells, and some cells also crawl into the gap. Before each cell divides, it copies every chromosome in interphase. Then mitosis gives one copy of each chromosome to each daughter cell, so the instructions are not mixed up or lost.
Science practices you used: making predictions and testing them with a model, organizing and reading counts in a table, finding percents to compare stages, and planning a fair test with repeated trials.
How we measure it
Light microscope and stained root tips
A thin onion root tip is stained so the chromosomes show up, and each cell is sorted into a stage by what its chromosomes look like. The photo is a snapshot of many cells at one moment, not a movie of one cell.
Counting cells in each stage
The number of cells in each stage is divided by the total to give a percent. If cells are caught at random moments, a stage that lasts longer holds more cells.
Finish line
Test yourself
8 questions
Practice and MCAT-style, with explained answers.
Next · Lesson 5
Diffusion and Osmosis
Why does salted cucumber get wet, while limp lettuce in plain cold water turns crisp?
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