Cell Division and Mitosis

Introduction

Every living organism depends on cells. New cells are needed when you grow, when damaged tissue is repaired, and when old cells are replaced. In many eukaryotic organisms, this happens through a carefully controlled series of events called the cell cycle. One important part of that cycle is mitosis, the process that separates copied chromosomes into two nuclei.

This aiMOOC is designed for Grades 7–8. You will learn how a cell prepares to divide, how chromosomes move during mitosis, how the cytoplasm separates during cytokinesis, and why accurate cell division matters.

The diagram above gives you a visual overview of the main stages. As you study it, remember one key idea: DNA is copied before mitosis begins. Mitosis sorts the copied chromosomes so that each new cell receives a complete set.

Learning Goals

By the end of this aiMOOC, you should be able to explain the difference between the cell cycle, mitosis, and cytokinesis; describe the main stages of mitosis in the correct order; connect chromosome movement to the formation of two daughter cells; compare cytokinesis in plant and animal cells; interpret simple microscope images of dividing cells; and explain how cell division supports growth, repair, replacement, and some forms of asexual reproduction.

The Cell Cycle

The cell cycle is the ordered sequence of growth, DNA copying, preparation, and division that a cell can pass through. For a typical dividing eukaryotic cell, it can be organized into two broad parts: interphase and the mitotic phase.

Interphase: Growth and Preparation

Interphase is not a stage of mitosis. It happens before mitosis and usually takes up much more of the cell cycle. Interphase includes three main parts:

G1 phase: The cell grows, carries out normal functions, and makes proteins and organelles.
S phase: The cell copies its DNA. Each chromosome is duplicated so that it consists of two identical sister chromatids joined at a centromere.
G2 phase: The cell continues growing, checks its preparation, and makes materials needed for division.

A chromosome may look like a single long DNA-containing structure before it is copied. After DNA replication, each duplicated chromosome has two sister chromatids. The chromosome has not become "two chromosomes" yet simply because it was copied; the sister chromatids remain connected until they separate during anaphase.

The Mitotic Phase

After interphase, the cell enters the mitotic phase. In a simplified Grade 7–8 model, this includes mitosis, which separates the copied chromosomes into two nuclei, and cytokinesis, which divides the cell's cytoplasm.

The four commonly taught stages of mitosis are prophase, metaphase, anaphase, and telophase. Scientists often describe an additional stage called prometaphase between prophase and metaphase. In this course, prometaphase is treated as part of the transition from prophase to metaphase unless it is named separately.

Why Cells Divide

Cells divide for several important reasons.

Growth: A multicellular organism becomes larger mainly by increasing its number of cells.
Repair: New cells replace cells damaged by cuts, injuries, or normal wear.
Replacement: Some cells are continually lost and must be replaced, such as many cells in the skin and digestive system.
Asexual reproduction: Some single-celled eukaryotes and some multicellular organisms can produce new individuals using mitotic cell division.

Mitosis helps keep the chromosome number the same from the parent cell to the daughter cells. Barring copying errors or mutations, the daughter cells receive the same genetic information.

The Stages of Mitosis

A useful memory aid is PMAT: Prophase, Metaphase, Anaphase, Telophase. The letters help you remember the order, but understanding what the chromosomes are doing is more important than memorizing names.

Prophase: Prepare and Pack

During prophase, the copied DNA coils tightly into visible chromosomes. Each duplicated chromosome contains two sister chromatids. The mitotic spindle begins to form from microtubules. In animal cells, structures associated with spindle organization move toward opposite sides of the cell.

As prophase progresses, the nucleolus disappears and the nuclear envelope begins to break down. When prometaphase is described separately, the complete breakdown of the nuclear envelope and attachment of spindle microtubules to chromosomes are usually placed in that stage.

A helpful idea is: Prophase prepares the chromosomes for movement.

Metaphase: Meet in the Middle

During metaphase, the duplicated chromosomes line up near the middle of the cell. Spindle microtubules connect the chromosomes to opposite sides of the spindle.

This alignment matters because the cell is preparing to separate the sister chromatids evenly. If each chromatid is connected correctly, one copy of each chromosome can move toward each future daughter cell.

A helpful idea is: Metaphase means chromosomes meet near the middle.

Anaphase: Apart They Go

During anaphase, the sister chromatids separate at their centromere regions. Once separated, each former chromatid is considered a daughter chromosome. Spindle action moves the daughter chromosomes toward opposite poles of the cell.

This fluorescent microscope image shows a cell in early anaphase. The chromosomes are being pulled apart as the spindle separates the genetic material.

A helpful idea is: Anaphase means the chromosome copies move apart.

Telophase: Two Nuclei Form

During telophase, the separated chromosomes arrive near opposite poles. New nuclear envelopes form around the two chromosome groups, the chromosomes begin to loosen, and the spindle breaks down.

By the end of telophase, one cell contains two newly formed nuclei with matching chromosome sets.

A helpful idea is: Telophase finishes nuclear division.

Cytokinesis: Dividing the Cytoplasm

Cytokinesis is the physical division of the cytoplasm. It often begins before mitosis is completely finished and produces two separate daughter cells.

Animal Cells

In a typical animal cell, a ring of proteins beneath the cell membrane tightens. This creates a cleavage furrow that pinches the parent cell into two daughter cells.

Plant Cells

A plant cell cannot simply pinch inward because it has a rigid cell wall. Instead, membrane-bound structures gather at the center and form a cell plate. The cell plate grows outward and becomes part of the new boundary between the two daughter cells.

The difference between animal and plant cytokinesis is a good example of how cell structure affects cell processes.

Looking at Mitosis Under a Microscope

Scientists can observe mitosis in tissues where cells divide rapidly. Onion root tips are commonly used in school microscopy because growth near the root tip includes many actively dividing cells.

In a microscope image, you do not usually see every cell dividing at once. Many cells will be in interphase because interphase usually lasts much longer than mitosis. A smaller number may show condensed chromosomes characteristic of prophase, chromosomes aligned at metaphase, separated chromosomes during anaphase, or two forming nuclei during telophase.

When identifying a stage, focus on evidence in the chromosome pattern, not only on the overall shape of the cell.

How to Read a Mitosis Image

Ask yourself these questions: Are the chromosomes loose or condensed? Are they gathered in the middle? Are two chromosome groups moving apart? Are two nuclei beginning to form? Is a cleavage furrow or cell plate visible? These clues let you justify your stage identification with evidence.

Accuracy, Checkpoints, and Cell-Cycle Control

Cells do not divide successfully just by moving through a list of stages. The cell cycle includes control points called checkpoints. At checkpoints, the cell can pause if important conditions are not met.

For example, a cell should not begin dividing its chromosomes if DNA copying has not been completed properly. During mitosis, the cell also needs to make sure chromosomes are correctly connected to the spindle before the sister chromatids separate.

Cell-cycle control is important because errors can cause daughter cells to receive incorrect genetic information. Loss of normal control over cell division is one feature of cancer, although cancer is a complex group of diseases involving many biological changes.

Mitosis and Meiosis Are Different

Mitosis and meiosis are both forms of nuclear division in eukaryotic cells, but they have different purposes.

Mitosis usually produces two daughter cells with the same chromosome number as the parent cell and is important in growth, repair, replacement, and some forms of asexual reproduction.

Meiosis is used to produce cells for sexual reproduction. It includes two divisions and reduces the chromosome number by half. For Grades 7–8, the most important distinction is that mitosis maintains chromosome number while meiosis reduces it.

Common Misconceptions

Misconception: Interphase is one of the stages of mitosis. Correction: Interphase is part of the cell cycle but occurs before mitosis.

Misconception: DNA is copied during prophase. Correction: DNA replication occurs earlier, during S phase of interphase.

Misconception: Mitosis and cytokinesis mean exactly the same thing. Correction: Mitosis separates copied chromosomes into two nuclei; cytokinesis divides the cytoplasm.

Misconception: Every cell in a tissue is dividing all the time. Correction: Cells can spend long periods in interphase, and some specialized cells divide rarely or not at all.

Misconception: Plant and animal cells complete cytokinesis in the same way. Correction: Animal cells usually form a cleavage furrow, while plant cells form a cell plate.

Interactive Tasks

Quiz: Test Your Knowledge

During which part of the cell cycle is DNA copied before mitosis? (S phase) (!Prophase) (!Anaphase) (!Cytokinesis)

What is the main result of mitosis? (Two nuclei with matching chromosome sets) (!Four cells with half the chromosome number) (!One cell without chromosomes) (!Two cells with no nuclei)

What happens to chromosomes during metaphase? (They line up near the middle of the cell) (!They are copied for the first time) (!They disappear from the cell) (!They form a cell plate)

What happens during anaphase? (Sister chromatids separate and move apart) (!DNA is copied) (!The cell enters G1 phase) (!The cell wall dissolves)

What forms around chromosome groups during telophase? (New nuclear envelopes) (!New centromeres) (!New cell walls in all cells) (!New DNA copies)

Which process divides the cytoplasm? (Cytokinesis) (!Replication) (!Metaphase) (!Interphase)

How does a typical animal cell complete cytokinesis? (It forms a cleavage furrow) (!It forms a cell plate) (!It copies its DNA) (!It removes all chromosomes)

How does a typical plant cell complete cytokinesis? (It forms a cell plate) (!It forms only a cleavage furrow) (!It separates sister chromatids) (!It skips chromosome division)

Why are many onion root tip cells useful for studying mitosis? (The root tip contains many actively dividing cells) (!Every root cell is always in anaphase) (!Onion cells have no chromosomes) (!Root tips divide without DNA)

Which statement correctly compares mitosis and meiosis? (Mitosis maintains chromosome number while meiosis reduces it) (!Mitosis always produces four cells) (!Meiosis is the main process for skin repair) (!Mitosis reduces chromosome number by half)

Memory Game

Interphase	Cell grows and copies DNA before mitosis
Prophase	Chromosomes condense and the spindle begins to form
Metaphase	Duplicated chromosomes line up near the cell center
Anaphase	Sister chromatids separate and move toward opposite poles
Telophase	New nuclear envelopes form around chromosome sets
Cytokinesis	Cytoplasm separates to form two daughter cells
Spindle	Microtubule structure that helps move chromosomes

Drag and Drop

Match the correct terms.	Topic
Chromosomes condense	Prophase
Chromosomes line up near the middle	Metaphase
Sister chromatids separate	Anaphase
New nuclear envelopes form	Telophase
Cytoplasm divides	Cytokinesis

...

Crossword Puzzle

Mitosis	What process separates copied chromosomes into two nuclei?
Chromosome	What DNA-containing structure becomes highly condensed during cell division?
Prophase	In which stage do chromosomes first become clearly condensed?
Metaphase	In which stage do chromosomes line up near the cell center?
Anaphase	In which stage do sister chromatids move apart?
Cytokinesis	What process divides the cytoplasm?

LearningApps

Cloze Text

Open-Ended Tasks

Easy

Mitosis storyboard: Draw six panels showing interphase, the four main stages of mitosis, and cytokinesis; add one clear sentence under each panel.
Cell cycle vocabulary cards: Create a set of illustrated cards for chromosome, sister chromatid, centromere, spindle, cytokinesis, and daughter cell.
Microscope image hunt: Use a teacher-provided image or prepared slide to find three cells in different stages and label the evidence that supports each identification.
PMAT explanation video: Record a one-minute video that explains Prophase, Metaphase, Anaphase, and Telophase in your own words.

Standard

Plant and animal cytokinesis comparison: Make a labeled diagram comparing a cleavage furrow with a cell plate and explain why the two cell types divide differently.
Mitosis model: Build a movable model using safe classroom materials to show how duplicated chromosomes line up and then separate.
Onion root investigation: Examine a prepared onion root tip slide or high-quality image, classify at least twenty cells by stage, and make a bar chart of your observations.
Cell division interview: Interview a science teacher, laboratory worker, nurse, or other relevant professional about where cell division matters in their work and summarize three insights.

Advanced

Mitotic index investigation: Use microscope-image data to estimate the percentage of observed cells that are in mitosis and explain what the result suggests about time spent in different cell-cycle stages.
Cell-cycle checkpoint case study: Create a short case study in which a checkpoint detects a problem, then explain why pausing the cycle could protect the daughter cells.
Mitosis and meiosis comparison project: Produce an infographic or short presentation comparing purpose, number of divisions, chromosome number, and products of mitosis and meiosis.
Tissue growth research project: Research how controlled cell division supports growth or wound repair, use at least two reliable sources, and present your findings as a poster, article, podcast, or video.

Learning Assessment

Stage identification assessment: Analyze an unfamiliar diagram or microscope image, identify the mitotic stage, and justify your answer using at least two visible chromosome clues.
Sequence reasoning assessment: Explain what would go wrong if sister chromatids separated before chromosomes were properly aligned and connected to the spindle.
Growth and repair assessment: Use a real-life example such as a healing scrape to explain how mitosis and cytokinesis increase cell number while maintaining genetic information.
Plant and animal division assessment: Compare cytokinesis in plant and animal cells and connect the difference to the presence of a rigid cell wall.
Cell-cycle error assessment: Predict one possible consequence if DNA were not copied completely before mitosis and explain the reasoning behind your prediction.
Transfer assessment: Given a new tissue sample with many cells in mitosis, propose a reasonable explanation for what the tissue may be doing and identify what additional evidence you would want.

Evidence of Learning

Strong evidence of learning includes accurate use of the terms cell cycle, interphase, chromosome, sister chromatid, centromere, spindle, mitosis, and cytokinesis; a correctly ordered explanation of the main stages of mitosis; diagrams or models that show chromosome behavior clearly; microscope-image classifications supported by visible evidence; a correct comparison of plant and animal cytokinesis; an explanation of how cell division supports growth, repair, replacement, and asexual reproduction; and successful transfer of these ideas to unfamiliar diagrams, tissue examples, or cell-cycle problems.

Products such as a storyboard, labeled model, microscope analysis, graph, infographic, interview summary, or short video can demonstrate both scientific understanding and communication skills.

OERs on the Topic

For additional reliable background reading, you can explore the Cell cycle, Chromosome, DNA replication, Cytokinesis, and Meiosis topics.

Linked Learning Areas

Cell Division and Mitosis

aiMOOC Projects