Mitosis

HSC Biology | Free Study Notes

In this lesson

• the purpose of mitosis

• how chromosomes behave during mitosis

• the main stages of mitosis

• why mitosis is important in growth and repair

What is mitosis?

Mitosis is a type of cell division that produces two genetically identical daughter cells.

It happens in body cells and is used when an organism needs to:

• grow

• repair damaged tissue

• replace worn-out cells

• reproduce asexually in some organisms

A useful exam point is that mitosis keeps the chromosome number the same in the daughter

cells.

Purpose of mitosis

The main purpose of mitosis is to make new cells that are genetically the same as the original parent cell.

Why this matters

This is important because:

• growing tissues need more cells

• damaged tissues need replacement cells

• organisms must keep the same genetic information in body cells

Mitosis and continuity

In asexual reproduction, mitosis helps ensure continuity because new organisms can be formed with the same genetic code as the parent.

Chromosome behaviour in mitosis

Before mitosis begins, the DNA is replicated.

Both mitosis and meiosis require DNA replication, where the genetic content doubles before division. 

What happens to chromosomes

During mitosis:

• each chromosome has already been copied

• the copied chromosomes are made of sister chromatids

• sister chromatids separate during the process

• each new cell receives one copy of each chromosome

Key idea

Mitosis keeps chromosome number constant, because each daughter cell gets a complete set of chromosomes.

Stages of mitosis

At this level, students should know the basic sequence of stages.

Prophase

In prophase:

• chromosomes become visible

• each chromosome consists of two sister chromatids

• the nuclear membrane begins to break down

Metaphase

In metaphase:

• chromosomes line up across the middle of the cell

Important point

In mitosis, chromosomes line up individually. Homologous pairs do not line up together, which helps distinguish mitosis from meiosis.

Anaphase

In anaphase:

• sister chromatids separate

• they move to opposite poles of the cell

This is one of the most important stage-specific facts to remember for exams. 

Telophase

In telophase:

• chromosomes reach opposite ends of the cell

• new nuclear membranes form around them

• the cell begins to split

Cytokinesis

After mitosis, cytokinesis divides the cytoplasm, producing two separate daughter cells.

Result

The end result is:

• two daughter cells

• genetically identical to each other

• genetically identical to the parent cell, unless a mutation has occurred

Role in growth and repair

Growth

Mitosis is essential for growth because multicellular organisms need to increase their number of cells.

As an organism grows:

• cells divide by mitosis

• new cells are added

• tissues become larger

Repair

Mitosis is also important in repair.

When cells are damaged or die:

• mitosis produces replacement cells

• tissue can heal

• worn-out cells can be replaced

Why identical cells matter

Because mitosis produces genetically identical cells, the new cells usually function in the same way as the cells they replace.

Mitosis compared with meiosis

Students often confuse mitosis and meiosis, so it helps to keep the main contrast clear.

Feature	Mitosis	Meiosis
Number of daughter cells	2	4
Genetic similarity	Identical	Genetically different
Chromosome number	Stays the same	Halved
Main role	Growth, repair, asexual reproduction	Gamete production

Worked example

Exam-style question

Explain why mitosis is important in growth and repair.

Worked answer

Mitosis is important in growth and repair because it produces two genetically identical daughter cells. These new cells can increase the number of cells in a growing organism and replace damaged or worn-out cells in tissues.

Why this works

This answer:

• states the outcome of mitosis

• links mitosis to both growth and repair

• uses the idea of genetically identical daughter cells

Common mistakes

• Confusing mitosis with meiosis.

• Saying mitosis produces gametes.

• Forgetting that DNA is replicated before mitosis.

• Saying homologous chromosome pairs line up in metaphase of mitosis.

• Forgetting that sister chromatids separate during anaphase.

Quick quiz

1. What is the main purpose of mitosis?

2. How many daughter cells are produced by mitosis?

3. Are the daughter cells genetically identical or different?

4. What happens to sister chromatids during anaphase?

5. Why is mitosis important in tissue repair?

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