17.2 Mitosis Revision Notes

1. Overview

Mitosis is a fundamental biological process that allows organisms to grow, heal, and reproduce asexually. It ensures that every new cell created contains an exact copy of the genetic information required for the organism to function, maintaining continuity across generations of cells.

Key Definitions

Mitosis: Nuclear division giving rise to genetically identical cells.
Chromosome: A thread-like structure of DNA, carrying genetic information in the form of genes.
Gene: A length of DNA that codes for a specific protein.
Genetically Identical: Cells that have the exact same DNA sequence and chromosome number.
Daughter Cells: The two new cells produced following the division of a parent cell.
Stem Cells: Unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions.
Differentiation: The process by which a stem cell becomes a specialised cell (e.g., a muscle cell or a neuron).

Core Content

There are no specific Core-only objectives for this topic; all requirements are part of the Supplement/Extended curriculum.

Extended Content (Extended Curriculum Only)

The Nature of Mitosis

Mitosis is defined as nuclear division. Its primary purpose is to ensure that when a cell divides, each resulting daughter cell receives a full, identical set of chromosomes.

Genetic Consistency: Because the DNA is copied exactly, the daughter cells are genetically identical to each other and to the original parent cell.
Chromosome Number: Mitosis maintains the chromosome number. For example, a human body cell with 46 chromosomes will divide to produce two daughter cells, each also containing 46 chromosomes.

The Process: Step-by-Step

Replication: Before mitosis begins, the exact replication of chromosomes occurs. Each chromosome is copied to create two identical "sister" strands (chromatids) joined together.
- 📊A single chromosome (straight line) showing an arrow leading to a replicated chromosome (X-shape), with a label indicating the two identical chromatids.
Alignment and Separation: During mitosis, these copies of chromosomes separate.
Division: The nucleus divides into two, and the cell membrane moves to separate the cytoplasm, forming two distinct cells.
- 📊A mother cell with 4 chromosomes in the center, followed by a stage showing the copies being pulled to opposite sides of the cell, and finally two separate daughter cells each containing 4 chromosomes.

The Role of Mitosis

Mitosis is essential for four main biological functions:

Growth: Increasing the number of cells in an organism (e.g., a seedling growing into a tree).
Repair of damaged tissues: Creating new cells to close a wound or fix an injury.
Replacement of cells: Substituting cells that die naturally or wear out (e.g., skin cells or red blood cells).
Asexual Reproduction: Producing offspring that are clones of the parent (common in bacteria, fungi, and some plants).

Stem Cells

Stem cells are unique because they are unspecialised.

They divide by mitosis to produce more daughter cells.
These daughter cells have the potential to undergo differentiation, where they develop specific structures to perform specific functions (such as becoming a red blood cell, a white blood cell, or a nerve cell).

Key Equations

There are no mathematical equations for this topic. However, remember the ratio: 1 Parent Cell $\rightarrow$ 2 Genetically Identical Daughter Cells.

Common Mistakes to Avoid

❌ Wrong: Thinking that growth is just cells getting larger.
- ✓ Right: Growth involves an increase in the number of cells through mitosis.
❌ Wrong: Stating that chromosomes replicate during mitosis.
- ✓ Right: Chromosomes replicate before mitosis begins (during a phase called interphase).
❌ Wrong: Confusing mitosis with meiosis (the division used for making gametes).
- ✓ Right: Mitosis results in identical cells; meiosis results in non-identical cells with half the chromosome number.
❌ Wrong: Using the term "cell division" when "nuclear division" is specifically asked for in the definition of mitosis.

Exam Tips

Keywords: Whenever you describe mitosis, always use the phrase "genetically identical." It is almost always a marking point in exam schemes.
Command Words: If a question asks you to "State the role of mitosis," ensure you list at least three: growth, repair, and replacement.
Stem Cells: When describing stem cells, you must mention two things: their ability to divide (staying unspecialised) and their ability to differentiate (becoming specialised).
Contextual Questions: You may be asked about mitosis in the context of cancer. Cancer is the result of uncontrolled mitosis (uncontrolled cell division).
Diagrams: You are not required to name the stages (Prophase, Metaphase, etc.), but you should be able to recognise a diagram showing chromosomes being pulled apart to opposite poles.