Cell Division » Mitosis Process

What you'll learn this session

Study time: 30 minutes

The purpose and importance of cell division
The cell cycle and its different phases
The detailed stages of mitosis (prophase, metaphase, anaphase, telophase)
The significance of mitosis in growth, repair and asexual reproduction
How to identify the different stages of mitosis under a microscope

Introduction to Cell Division

Cell division is one of the most amazing processes in biology! It's how a tiny embryo grows into a full human, how your skin heals after a cut and how some plants and animals can reproduce without a mate. For single-celled organisms, it's their way of reproducing. For us humans and other multicellular organisms, it's how we grow and repair damaged tissues.

Key Definitions:

Cell division: The process where a parent cell divides to form two or more daughter cells.
Mitosis: A type of cell division that results in two identical daughter cells, each with the same number of chromosomes as the parent cell.
Chromosomes: Thread-like structures made of DNA that carry genetic information.
Chromatin: The loosely packed form of DNA when the cell isn't dividing.
Chromatids: The two identical copies of a chromosome that are joined together at the centromere.
Centromere: The region where sister chromatids are joined together.

🔬 Why Cells Divide

Cells divide for three main reasons:

Growth: To increase the size of an organism
Repair: To replace damaged or dead cells
Reproduction: To create new organisms (asexual reproduction)

📊 Cell Size Limitations

Cells can't grow indefinitely because:

The surface area to volume ratio would decrease
Nutrients and waste wouldn't move in and out fast enough
The cell's DNA couldn't control a very large cell effectively

The Cell Cycle

The cell cycle is the series of events that take place in a cell leading to its division and duplication. It consists of interphase (where the cell grows and replicates its DNA) and the mitotic phase (where the cell actually divides).

📅 Interphase

The longest phase of the cell cycle where the cell:

Grows in size (G1)
Replicates its DNA (S phase)
Prepares for division (G2)

📈 Mitotic Phase

Consists of:

Mitosis (nuclear division)
Cytokinesis (cytoplasm division)

Results in two identical daughter cells

⏱ Timing

The cell cycle varies in length:

Skin cells: ~24 hours
Liver cells: ~1 year
Nerve cells: rarely divide

The Stages of Mitosis

Mitosis is the process of nuclear division, which is followed by cytokinesis (division of the cytoplasm). The entire process ensures that each daughter cell receives an identical set of chromosomes. Let's explore each stage in detail:

Prophase

Prophase is the first and longest stage of mitosis. During this busy phase:

Chromatin condenses into visible chromosomes
Each chromosome appears as two sister chromatids joined at the centromere
The nuclear envelope begins to break down
The nucleolus disappears
Spindle fibres form between the centrioles as they move to opposite poles of the cell

How to identify: Chromosomes become visible but are scattered throughout the nucleus; nuclear membrane starts to disappear.

Metaphase

During metaphase:

Chromosomes line up along the equator (middle) of the cell
Each chromosome attaches to spindle fibres at its centromere
The chromosomes are now at their most condensed and easiest to see

How to identify: Chromosomes are aligned in a straight line across the middle of the cell.

Anaphase

Anaphase is the shortest but most dramatic stage:

The centromeres split, separating the sister chromatids
Each chromatid (now called a chromosome) is pulled towards opposite poles of the cell
The cell begins to elongate as the chromosomes move apart

How to identify: Chromosomes appear to be moving to opposite ends of the cell, creating a V-shape.

Telophase

The final stage of mitosis:

Chromosomes reach the poles of the cell and begin to decondense
Nuclear envelopes reform around each set of chromosomes
Nucleoli reappear
Spindle fibres disappear
Cytokinesis usually begins during this stage

How to identify: Two distinct nuclei forming at opposite ends of the cell; chromosomes becoming less distinct.

Cytokinesis

Although not technically part of mitosis (which is just nuclear division), cytokinesis is the division of the cytoplasm that usually follows:

In animal cells: A cleavage furrow forms and pinches the cell in two
In plant cells: A cell plate forms in the middle and develops into a new cell wall

After cytokinesis, two genetically identical daughter cells have been formed.

🌱 Mitosis in Plants vs Animals

Similarities:

Same basic stages
Same end result (two identical daughter cells)

Differences:

Plant cells have no centrioles
Plant cells form a cell plate during cytokinesis
Animal cells form a cleavage furrow during cytokinesis

🔬 Observing Mitosis

Mitosis can be observed under a microscope using:

Onion root tips (plants)
Whitefish blastula (animals)
Special stains that highlight chromosomes

These tissues have many cells actively dividing, making them ideal for studying mitosis.

Case Study Focus: Cancer and Mitosis

Cancer is essentially a disease of uncontrolled cell division. Normal cells have checkpoints in the cell cycle that prevent them from dividing when they shouldn't. In cancer cells, these checkpoints are damaged, leading to uncontrolled mitosis.

Many cancer treatments work by targeting rapidly dividing cells:

Chemotherapy: Damages cells during DNA replication or mitosis
Radiation therapy: Damages DNA, preventing successful mitosis
Targeted therapies: Block specific molecules involved in cancer cell growth

Understanding mitosis has been crucial for developing these life-saving treatments.

The Importance of Mitosis

Mitosis is essential for life as we know it. Here's why it matters:

💫 Growth

Mitosis allows a single fertilised egg to develop into a complex organism with trillions of cells. It's how you grew from a tiny embryo to your current size!

🩹 Repair

When you cut your skin or break a bone, mitosis helps replace damaged cells. Your body is constantly replacing old cells with new ones through mitosis.

🌱 Asexual Reproduction

Some organisms like starfish, plants and single-celled organisms use mitosis to create genetically identical offspring.

Summary

Mitosis is a remarkable process that ensures genetic continuity as cells divide. Through the carefully orchestrated stages of prophase, metaphase, anaphase and telophase, followed by cytokinesis, a parent cell divides to form two genetically identical daughter cells. This process is essential for growth, repair and asexual reproduction.

Understanding mitosis helps us comprehend how organisms develop, how wounds heal and even how diseases like cancer occur. It's one of the fundamental processes that make life possible!

🧠 Test Your Knowledge!