Cell Division and Growth
2. Mitosis
Learning outcomes
- I can define mitosis.
- I can describe the stages of mitosis.
- I can explain how mitosis produces genetically identical cells.
- I can identify the role of chromosomes during mitosis.
- I can explain the importance of mitosis for growth and tissue repair.
Mitosis
Mitosis is the process in which the nucleus of a cell divides to produce two genetically identical nuclei.
It is an essential part of the cell cycle and allows organisms to:
- grow
- replace worn-out cells
- repair damaged tissues
- reproduce asexually in some organisms
Mitosis ensures that each new cell receives the same genetic information as the original parent cell.
What Is Mitosis?
Mitosis is the division of the nucleus.
Before mitosis begins, the cell has already copied its DNA during interphase.
Each chromosome therefore consists of two identical copies called sister chromatids.
During mitosis, these copies are separated so that each new nucleus receives one complete set of chromosomes.
The overall sequence is:
Prophase → Metaphase → Anaphase → Telophase
A common memory aid is:
P → M → A → T
Chromosomes and Mitosis
DNA is organized into structures called chromosomes.
Before the cell divides, each chromosome is copied.
The two identical copies are called sister chromatids.
They are joined together at a region called the centromere.
During mitosis:
- chromosomes condense
- chromosomes line up
- sister chromatids separate
- identical sets of chromosomes move to opposite sides of the cell
- new nuclei form
This careful movement of chromosomes is what allows the two new cells to receive the same genetic information.
Before Mitosis Begins
Mitosis does not begin with DNA replication.
DNA replication happens before mitosis, during interphase.
The cell must copy its DNA first so that there are two complete sets available.
For example:
Before replication:
1 chromosome
After replication:
1 replicated chromosome containing 2 sister chromatids
During mitosis:
the sister chromatids separate
After mitosis:
each new nucleus receives 1 copy
This ensures genetic continuity.
Prophase
During prophase:
- chromosomes condense
- chromosomes become more visible
- each chromosome contains two sister chromatids
- the nuclear envelope begins to break down
- spindle fibres begin to form
The chromosomes condense because long strands of DNA would be difficult to move safely through the cell.
Condensing the DNA helps prevent it from becoming tangled or damaged.
A key clue for identifying prophase is:
visible condensed chromosomes that are not yet lined up
Metaphase
During metaphase:
- chromosomes move to the middle of the cell
- they line up along the cell's equator
- spindle fibres attach to the chromosomes
The chromosomes must line up correctly so that each sister chromatid can be pulled toward the correct side of the cell.
A key clue for identifying metaphase is:
chromosomes lined up across the middle
Anaphase
During anaphase:
- sister chromatids separate
- spindle fibres shorten
- the chromatids move toward opposite ends of the cell
Once the sister chromatids separate, each chromatid is considered an individual chromosome.
A key clue for identifying anaphase is:
chromosomes moving apart toward opposite poles
This is one of the most important stages because it ensures that the two new nuclei receive identical chromosome sets.
Telophase
During telophase:
- chromosomes reach opposite ends of the cell
- chromosomes begin to uncoil
- new nuclear envelopes form
- two nuclei are produced
At this point, nuclear division is nearly complete.
A key clue for identifying telophase is:
two groups of chromosomes with new nuclei forming around them
Mitosis in One Sequence
The stages can be summarized as follows:
| Stage | Main Event |
|---|---|
| Prophase | Chromosomes condense and spindle forms |
| Metaphase | Chromosomes line up in the middle |
| Anaphase | Sister chromatids separate |
| Telophase | Two nuclei form |
A simple way to remember them is:
Prophase = prepare
Metaphase = middle
Anaphase = apart
Telophase = two nuclei
What Happens After Mitosis?
Mitosis divides the nucleus, but the whole cell still needs to divide.
After mitosis, cytokinesis usually occurs.
During cytokinesis:
- the cytoplasm divides
- the cell membrane separates
- two daughter cells are produced
Therefore:
Mitosis = nuclear division
Cytokinesis = division of the cytoplasm
Animal Cell Cytokinesis
In animal cells, the cell membrane pinches inward.
This forms a cleavage furrow.
The furrow deepens until the cell separates into two daughter cells.
Plant Cell Cytokinesis
Plant cells have rigid cell walls, so they cannot pinch inward easily.
Instead, a cell plate forms between the two new nuclei.
The cell plate develops into:
- new cell membranes
- a new cell wall
This separates the original plant cell into two daughter cells.
Why Are the Daughter Cells Genetically Identical?
Mitosis produces genetically identical cells because the DNA is copied before division and then separated equally.
The process follows this sequence:
- DNA is replicated.
- Each chromosome forms two identical sister chromatids.
- Chromosomes line up during metaphase.
- Sister chromatids separate during anaphase.
- One identical set moves to each side of the cell.
- Two nuclei form.
- Cytokinesis produces two cells.
As a result, each daughter cell receives the same chromosome information.
Parent Cell and Daughter Cells
Suppose a parent cell has four chromosomes.
Before mitosis, each chromosome is replicated.
The cell therefore has four replicated chromosomes.
During mitosis, the sister chromatids separate.
Each new nucleus receives:
4 chromosomes
After cytokinesis:
Daughter Cell 1 → 4 chromosomes
Daughter Cell 2 → 4 chromosomes
The chromosome number remains the same.
This is one reason mitosis is different from meiosis, which produces cells with half the usual chromosome number.
Chromosome Number in Humans
Most human body cells contain 46 chromosomes.
When a body cell undergoes mitosis:
Parent cell → 46 chromosomes
Daughter cell 1 → 46 chromosomes
Daughter cell 2 → 46 chromosomes
The daughter cells therefore maintain the normal chromosome number.
This is essential for growth and tissue maintenance.
Mitosis and Growth
Multicellular organisms grow mainly by increasing their number of cells.
A fertilized egg begins as a single cell.
That cell divides by mitosis.
1 cell → 2 cells
Then:
2 → 4
4 → 8
8 → 16
Repeated mitosis eventually produces the enormous number of cells in a mature organism.
Growth therefore depends heavily on repeated rounds of mitosis.
Mitosis and Tissue Repair
Cells can be damaged by:
- cuts
- burns
- friction
- chemicals
- normal wear and tear
When cells are damaged or lost, nearby cells may divide by mitosis to replace them.
For example, when the skin is cut:
- cells are damaged
- nearby cells enter the cell cycle
- DNA is copied
- mitosis occurs
- new cells are produced
- damaged tissue is gradually replaced
This makes mitosis essential for wound healing and tissue repair.
Replacing Worn-Out Cells
Some cells in the body are constantly being lost.
These must be replaced.
Examples include cells associated with:
- skin
- the digestive tract
- blood-cell production
Mitosis allows new cells to replace old ones.
Without mitosis, tissues would gradually lose cells and would not function properly.
Asexual Reproduction
Some organisms use mitosis for asexual reproduction.
In asexual reproduction:
- only one parent is required
- cells divide by mitosis
- offspring are genetically very similar to the parent
Examples can include:
- some single-celled organisms
- some plants
- some fungi
- certain simple animals
Mitosis therefore has a role not only in growth and repair, but also in reproduction.
Mitosis Under the Microscope
Mitosis can be observed in tissues where cells are actively dividing.
One common example is an onion root tip.
Root tips contain regions of rapidly dividing cells.
Under a microscope, students may identify cells based on chromosome position.
Look for:
- chromosomes visible but not lined up → prophase
- chromosomes across the centre → metaphase
- chromosomes moving apart → anaphase
- two nuclei forming → telophase
Why Are Most Cells Not Seen in Mitosis?
When looking at an onion root tip, many cells appear to be in interphase rather than mitosis.
This is because cells usually spend much more time in interphase.
Mitosis is only one relatively short part of the complete cell cycle.
Therefore, in a random sample of cells, it is normal to observe more cells in interphase.
Mitosis and the Cell Cycle
Mitosis is part of a larger process called the cell cycle.
The sequence is:
Interphase → Mitosis → Cytokinesis
During interphase:
- the cell grows
- DNA is replicated
- the cell prepares for division
During mitosis:
- the nucleus divides
During cytokinesis:
- the cytoplasm divides
It is important not to confuse mitosis with the entire cell cycle.
What Happens If Mitosis Goes Wrong?
Mitosis must be carefully controlled.
If chromosomes do not separate correctly, daughter cells may receive incorrect amounts of genetic material.
Cells also contain mechanisms called checkpoints that help control division.
If cell-cycle control is lost, cells may begin dividing uncontrollably.
This uncontrolled division can contribute to the formation of tumours.
Some tumours can become cancerous.
This shows why accurate regulation of mitosis is important.
Worked Example: Identifying Metaphase
A cell has several chromosomes arranged in a line across its centre.
Which stage is shown?
The key clue is:
chromosomes lined up in the middle
Therefore, the stage is:
metaphase
Worked Example: Identifying Anaphase
A student observes two groups of chromosomes moving toward opposite ends of a cell.
Which stage is occurring?
This indicates:
anaphase
because the sister chromatids have separated.
Worked Example: Genetic Identity
A body cell contains 12 chromosomes.
It undergoes normal mitosis.
How many chromosomes should each daughter cell receive?
Each daughter cell receives:
12 chromosomes
The chromosome number remains the same because mitosis produces genetically identical cells.
Worked Example: Growth
Why does mitosis allow an organism to grow?
Mitosis increases the number of cells.
Repeated mitotic divisions produce more cells, increasing the size of tissues and the organism.
A complete answer could be:
Mitosis allows growth by producing genetically identical daughter cells, increasing the total number of cells in the organism.
Worked Example: Tissue Repair
A person cuts their skin.
Why is mitosis important?
Mitosis produces new skin cells.
These cells replace damaged or lost cells.
Therefore:
mitosis allows damaged tissue to be repaired.
Identifying the Stages Quickly
When identifying mitosis stages from images, focus on chromosome position.
Prophase
Chromosomes visible but scattered.
Metaphase
Chromosomes in the middle.
Anaphase
Chromosomes moving apart.
Telophase
Two groups at opposite ends; new nuclei forming.
This is usually more reliable than trying to memorize the exact shape of the entire cell.
Common Misconceptions
Mitosis is the same as the whole cell cycle.
Incorrect. Mitosis is only the division of the nucleus.
DNA is copied during mitosis.
DNA is copied before mitosis during interphase.
Mitosis produces four cells.
Normal mitosis followed by cytokinesis produces two daughter cells.
The daughter cells have half the chromosomes of the parent cell.
Incorrect. Mitosis maintains chromosome number.
Chromosomes are always X-shaped.
No. The X shape represents a condensed replicated chromosome with two sister chromatids.
Mitosis is only important during childhood.
Mitosis continues throughout life for cell replacement and tissue repair.
Animal and plant cells divide in exactly the same way after mitosis.
Their nuclei divide similarly, but cytokinesis differs because plant cells have rigid cell walls.
Did You Know?
Your body depends on enormous numbers of mitotic divisions throughout life.
Some tissues replace cells regularly, while others divide much less frequently.
This means the rate of mitosis is carefully controlled according to the needs of each tissue.
If that control is lost, cells may divide when they should not, which can contribute to tumour formation.
Key Terms
Mitosis – Division of the nucleus to produce two genetically identical nuclei.
Chromosome – A structure containing DNA.
Sister chromatids – Identical copies of a replicated chromosome.
Centromere – The region joining sister chromatids.
Prophase – The stage when chromosomes condense and the spindle begins to form.
Metaphase – The stage when chromosomes line up in the middle of the cell.
Anaphase – The stage when sister chromatids separate and move toward opposite ends.
Telophase – The stage when two new nuclei form.
Spindle fibres – Structures that help move chromosomes during mitosis.
Cytokinesis – Division of the cytoplasm after nuclear division.
Daughter cells – The two cells produced after cell division.
Cleavage furrow – The inward pinching of an animal cell during cytokinesis.
Cell plate – The structure that separates daughter plant cells during cytokinesis.
Key Takeaways
- Mitosis is the division of the nucleus.
- It usually produces two genetically identical nuclei.
- DNA must be replicated before mitosis begins.
- Replicated chromosomes contain two identical sister chromatids.
- The main stages of mitosis are prophase, metaphase, anaphase and telophase.
- In prophase, chromosomes condense.
- In metaphase, chromosomes line up in the middle.
- In anaphase, sister chromatids separate.
- In telophase, two nuclei form.
- Cytokinesis usually follows mitosis and divides the cell into two daughter cells.
- Mitosis maintains the chromosome number of the parent cell.
- Mitosis is essential for growth.
- Mitosis replaces old and damaged cells.
- Mitosis is important for tissue repair.
- Some organisms also use mitosis for asexual reproduction.
- Accurate chromosome separation is essential so that each daughter cell receives a complete set of genetic information.