Biology (0610) | Mitosis

Welcome to Chapter 17.2: Understanding Mitosis!

Hello future biologists! We are now diving into the fascinating world of Inheritance. Before we can talk about how traits are passed down, we must first understand how our bodies grow and replace old cells.
Mitosis is the essential process that makes you, *you*, and helps you heal quickly when you get a scrape.

This chapter is for students aiming for grades C and above, covering the core principles of how cells divide to produce exact copies. Don't worry if cell division seems complicated—we'll break it down step-by-step!

Section 1: What is Mitosis? The Goal of Copying

Mitosis is a type of cell division. Its primary job is simple: to take one cell and turn it into two new cells that are exactly identical to the original.

The Definition of Mitosis (Supplement 1)

Mitosis is the process of nuclear division that results in two daughter cells that are genetically identical to the parent cell.

• The term nuclear division is used because it is the nucleus (which contains the genetic material) that divides first.
• Genetically identical means the new cells have the exact same set of chromosomes (and therefore the same genes) as the cell they came from.

Analogy: Think of mitosis as cloning a document. You need an exact duplicate, not a modified version.

Quick Review: Key Takeaway 1

Mitosis creates two daughter cells that are genetically identical, ensuring every cell in a developing organism or a damaged tissue is carrying the same, correct instructions.

Section 2: The Chromosome Checklist – Preparation and Separation

For mitosis to work perfectly, the cell has to make sure that when it divides, both new cells receive a full and correct set of genetic instructions.

Step 1: Exact Replication Before Mitosis (Supplement 3)

Before the actual division starts, the cell needs to make a complete backup copy of its DNA. This process is called exact replication of chromosomes.

• Every single chromosome copies itself.
• Each original chromosome now consists of two identical strands, held together.
• If the original cell has 46 chromosomes (like a human body cell), it temporarily contains 92 copies, ready to be split evenly.

Step 2: Chromosome Separation and Maintenance (Supplement 4)

During the process of mitosis (the nuclear division itself), the copied chromosomes are separated.

The two identical copies of each chromosome (which formed during replication) pull apart and move to opposite ends of the cell.

Crucially, this separation ensures that the original chromosome number is maintained in each daughter cell.

Example: If the parent cell has 46 chromosomes (diploid), each of the two daughter cells must also end up with 46 chromosomes (diploid). Mitosis maintains this number.

Quick Review: Key Takeaway 2

Before mitosis, chromosomes replicate exactly. During mitosis, the copies separate to ensure the chromosome number is maintained (e.g., 46 $\to$ 46) in the new cells.

Section 3: The Essential Roles of Mitosis in Life (Supplement 2)

Mitosis isn't just a complicated biological theory; it is fundamental to how all multicellular life functions. There are four main roles you must remember:

1. Growth

When you grow, your cells aren't just getting bigger; you are making more of them. Mitosis produces the countless new cells needed to increase the size and complexity of an organism.

2. Repair of Damaged Tissues

If you cut your skin or break a bone, mitosis immediately kicks into action. Cells around the injury divide rapidly to produce new, identical cells to fill the gap and fix the damage.

Think of mitosis as a repair crew, rushing to replace the broken pavement with new, identical slabs.

3. Replacement of Cells

Many cells in your body have a limited lifespan. Your red blood cells only last about 120 days, and the cells lining your gut are constantly sloughed off. Mitosis continuously replaces these worn-out cells, keeping tissues healthy and functional.

4. Asexual Reproduction

In simpler organisms (like yeast or bacteria) or in plants, mitosis is the method of reproduction.
Asexual reproduction is the production of new organisms from a single parent, resulting in offspring that are genetically identical to that parent—a perfect clone!

Examples of asexual reproduction include budding in yeast or runners on strawberry plants.

Quick Review: Key Takeaway 3

The four roles of mitosis are Growth, Repair, Replacement, and Asexual Reproduction.

Section 4: Stem Cells – The Unspecialised Builders (Supplement 5)

Most cells in your body are specialised (like nerve cells or muscle cells), meaning they have a specific job and can't usually turn into another type of cell. However, there are some special cells that can divide by mitosis and then become specialised.

What are Stem Cells?

Stem cells are unspecialised cells that have the remarkable ability to:

1. Divide by mitosis to produce more stem cells (self-renewal).
2. Produce daughter cells that can become specialised for specific functions.

This is vital for growth and repair, especially in tissues like bone marrow (where blood cells are made).

Did you know? Stem cells in plants are found mainly in the tips of shoots and roots, known as meristems. These allow the plant to grow continuously!

Quick Review: Key Takeaway 4

Stem cells are unspecialised cells that divide by mitosis and then specialise to perform specific functions, supporting tissue repair and replacement.