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B3.1 · Explain the cell cycle and mitosis in growth and repair

Learn to explain the cell cycle and mitosis in growth and repair through clear examples and targeted practice.

Ontario Grade 10 Science

Biology: Tissues, Organs, and Systems

How cells make new cells for growth and repair

A small scrape heals as new cells replace damaged ones. A growing plant also adds cells as it gets larger. These changes depend on cells making new cells. The cell cycle is the repeating series of events in which a cell grows, copies its genetic material, and divides. Mitosis is one part of that cycle. In this lesson, you will follow a cell through the cycle and see why mitosis matters for growth and repair.

What you will learn

  • Describe the main parts of the cell cycle.
  • Explain how mitosis distributes copied genetic material.
  • Connect cell division to growth and repair.
  • Use observations and a model to explain what cells are doing.

1. A Grade 9 bridge: cells and genetic material

Cells are the basic units of living things. Many organisms are made of many cells. As a reminder, a tissue is a group of similar cells that work together. When an organism grows or repairs a tissue, it needs more cells or replacement cells.
A cell contains DNA, the genetic material that carries instructions for cell activities. DNA is organized into structures called chromosomes. Before a cell divides, it copies its DNA. This gives the cell two matching sets of genetic instructions to share between the new cells.
  • Growth and repair can require new cells.
  • DNA carries genetic instructions and is organized into chromosomes.
  • DNA must be copied before a cell divides.

2. The cell cycle: prepare, divide, and begin again

The cell cycle is the sequence of events from when a cell forms until it divides to make new cells. Most of the cycle is interphase. During interphase, the cell grows, carries out its usual activities, and copies its DNA. Interphase is preparation for division; it is not mitosis.
Mitosis is the division of the cell’s nucleus, where the copied chromosomes are separated into two groups. It is commonly described in four stages: prophase, metaphase, anaphase, and telophase. In prophase, chromosomes become visible as compact structures. In metaphase, they line up across the middle of the cell. In anaphase, matching copies separate and move to opposite ends. In telophase, a new nucleus forms around each group.
After mitosis, cytokinesis divides the rest of the cell, including its cytoplasm. Cytoplasm is the material inside the cell outside the nucleus. This produces two daughter cells, meaning the two cells formed from one dividing cell. In the usual cell cycle model, the daughter cells have the same genetic information as each other and as the original cell before its DNA was copied.
The order matters. Copying DNA first gives each new cell a set of instructions. Separating the copies during mitosis helps ensure that both new cells receive genetic information.
  • Interphase includes cell growth and DNA copying.
  • Mitosis separates copied chromosomes into two groups in the nucleus.
  • Cytokinesis divides the rest of the cell.
  • The usual result is two cells with matching genetic information.

3. What the cycle explains: growth and repair

During growth, cell division increases the number of cells. For example, a young plant becomes larger as cells divide and the resulting cells grow. Cell division is part of growth, although division alone does not explain every change in an organism’s size.
During repair, cell division can replace cells that are damaged or lost. When a scrape heals, new cells help restore the affected tissue. The same basic cycle supports both growth and repair: a cell prepares, copies its DNA, separates the copies by mitosis, and divides by cytokinesis.
A model helps show the sequence, but it simplifies what happens. Real cells are tiny and their stages are not always easy to distinguish in a single observation. A drawing or prepared microscope image can show examples of cells at different stages. Comparing several cells can help you identify patterns in the sequence.
  • More cells can contribute to an organism’s growth.
  • New cells can replace damaged or lost cells during repair.
  • A model shows an ordered sequence, but an individual image may not show every stage clearly.

4. Evidence, models, and safe observation

Evidence for the sequence can come from comparing images of cells in different stages of division. A cell with chromosomes lined up across its middle fits the model of metaphase. A cell with chromosome copies moving toward opposite ends fits anaphase. These observations support an explanation of how genetic material is separated.
A microscope image is a snapshot, not a recording of one cell moving through every stage. The cell-cycle model connects observations into an ordered explanation. It is important not to claim that one image alone proves the whole sequence.
Use only teacher-provided images or prepared slides, and follow classroom microscope and slide-handling instructions. Do not collect or prepare biological samples at home. This lesson’s examples of observations are descriptions of possible evidence, not invented measurements.
  • Look for chromosome position and movement when identifying stages.
  • Compare multiple observations to understand the sequence.
  • Prepared slides and teacher-provided images are appropriate classroom materials.

Worked example

Following a cell through the cycle

A cell grows, copies its DNA, shows chromosomes lined up across the middle, and later forms two new cells. Identify the preparation step, the mitosis stage shown, and the final cell-division process.
  1. Identify preparation
    Growth and DNA copying happen during interphase. This prepares the cell for division.
  2. Match the chromosome position
    Chromosomes lined up across the middle identify metaphase. Their position is the evidence used to name this stage.
  3. Name the final division
    Cytokinesis divides the rest of the cell after mitosis, producing two daughter cells.
Answer: The preparation step is interphase, the pictured mitosis stage is metaphase, and cytokinesis divides the cell into two daughter cells.
Check: The answer keeps interphase, mitosis, and cytokinesis distinct: preparation, nuclear division, and division of the rest of the cell.

Worked example

Explaining a growing tissue

A tissue sample from a growing organism contains many cells. Some show chromosome copies moving toward opposite ends of the cell. Explain what this observation means and how it can relate to growth.
  1. Interpret the observation
    Copies moving toward opposite ends match anaphase. This is part of mitosis, when copied chromosomes are separated.
  2. Connect the stage to new cells
    After mitosis, cytokinesis divides the cell. The two resulting cells can add to the tissue’s cell number.
  3. Explain the growth link
    A larger number of cells can contribute to tissue growth. The observation shows a cell dividing; it does not by itself describe every process involved in growth.
Answer: The observed cells are in anaphase. Their division can produce more cells, which can contribute to growth of the tissue.
Check: The chromosome movement identifies anaphase; the connection to growth is the increase in cell number after division.

Worked example

Using evidence to explain repair

After a small scrape, new cells help replace damaged cells. A set of classroom images shows one cell with chromosomes separated into two groups and another with two nuclei. Explain how these observations fit the cell cycle and repair.
  1. Identify the first image
    Two separated chromosome groups fit anaphase or the transition toward telophase. The observation shows the copied genetic material being divided between the two sides.
  2. Interpret the second image
    Two nuclei indicate that nuclear division has reached telophase. Cytokinesis can then divide the rest of the cell.
  3. Connect division to repair
    The cycle can produce new cells that replace damaged or lost cells in the scraped tissue. The images support the stages of division; the repair context explains why making replacement cells is useful.
Answer: The images fit late mitosis: chromosome groups separate, and two nuclei form. Cytokinesis can then make two cells. New cells can help replace damaged cells during repair.
Check: The explanation uses the observed features as evidence and does not confuse the images with a complete record of one cell’s entire cycle.

Common mistakes and how to avoid them

Calling interphase a stage of mitosis.
Correction: Interphase is part of the cell cycle, but it occurs before mitosis and includes growth and DNA copying.
Saying mitosis divides the whole cell.
Correction: Mitosis divides the nucleus. Cytokinesis divides the rest of the cell.
Saying DNA is copied during anaphase.
Correction: DNA is copied during interphase. Anaphase separates the copied chromosomes.
Claiming that one microscope image shows the entire cell cycle.
Correction: An image shows a cell at one moment. Use observations from different stages to build an explanation of the sequence.

Lesson summary

  • The cell cycle includes interphase, mitosis, and cytokinesis.
  • In interphase, the cell grows and copies its DNA.
  • Mitosis separates copied chromosomes through prophase, metaphase, anaphase, and telophase.
  • Cytokinesis divides the rest of the cell and produces two daughter cells.
  • Cell division supports growth by adding cells and repair by replacing damaged or lost cells.

Check your understanding

Question 1

When does a cell copy its DNA in the basic cell-cycle model?
  1. During interphase
  2. During metaphase
  3. During cytokinesis
  4. After the two daughter cells have formed
Show answer and explanation
During interphase
DNA copying happens during interphase, before mitosis separates the copied chromosomes.

Question 2

A cell has chromosomes lined up across its middle. Which mitosis stage does this show?
  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase
Show answer and explanation
Metaphase
Chromosomes lined up across the middle are the defining observation for metaphase in this model.

Question 3

Which statement best connects mitosis to repair?
  1. Mitosis can help produce new cells to replace damaged or lost cells.
  2. Mitosis copies DNA after the cell has divided.
  3. Mitosis breaks down the tissue around a scrape.
  4. Mitosis turns one cell directly into a complete tissue.
Show answer and explanation
Mitosis can help produce new cells to replace damaged or lost cells.
Cell division can supply new cells that help replace damaged or lost cells. It does not create a whole tissue in one step.

Question 4

What does cytokinesis do?
  1. Copies the cell’s DNA
  2. Lines chromosomes up across the middle
  3. Divides the rest of the cell after mitosis
  4. Moves chromosome copies to opposite ends
Show answer and explanation
Divides the rest of the cell after mitosis
Cytokinesis divides the cell’s cytoplasm and the rest of the cell after nuclear division.

Key terms

Cell cycle
The repeating sequence in which a cell grows, copies its genetic material, and divides.
DNA
Genetic material that carries instructions for cell activities.
Chromosome
A structure made of organized DNA that carries genetic information.
Interphase
The part of the cell cycle when a cell grows, carries out its activities, and copies its DNA.
Mitosis
Division of a cell’s nucleus that separates copied chromosomes into two groups.
Cytokinesis
Division of the rest of the cell, including its cytoplasm.
Daughter cells
The cells formed when one cell divides.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation B3.1. It is a study resource, not an official curriculum publication.

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