Mutation and cancer

Welcome to Mutation and Cancer!

In this chapter, we are going to explore what happens when the "instruction manual" of life—our DNA—gets a typo. We will learn how these tiny changes, called mutations, can sometimes lead to the development of cancer. While the word "mutation" might sound like something out of a science-fiction movie, it is a natural biological process that we can understand step-by-step. Don't worry if it seems a bit technical at first; we will use plenty of analogies to make it clear!

3.2.11.1 Gene Mutations

A gene mutation is a change in the base sequence of DNA. Think of your DNA as a long sentence made of three-letter words (codons). If you change just one letter, the whole meaning of the sentence might change.

When do mutations happen?

Mutations usually arise spontaneously during DNA replication (the S-phase of the cell cycle). Even though our cells are very good at copying DNA, they occasionally make mistakes.

What are Mutagens?

While mutations happen naturally, certain things can increase how often they occur. These are called mutagens.
Examples include:
1. Physical agents: Such as UV light or X-rays.
2. Chemical agents: Such as those found in cigarette smoke.

Types of Gene Mutations

The syllabus requires you to know two main types:

1. Base Substitution: One nucleotide base is swapped for another.
Analogy: Changing the sentence "THE CAT SAT" to "THE RAT SAT." Only one word changed.

2. Base Deletion: One nucleotide base is completely removed. This is often more serious because it causes a frameshift.
Analogy: If you delete the 'C' in "THE CAT SAT," and the letters must stay in groups of three, it becomes "THE ATS AT..." Everything after the mistake is now scrambled!

The "Backup Plan": Degenerative Nature of the Genetic Code

Quick Review: Remember that the genetic code is degenerate. This means that more than one triplet (set of three bases) can code for the same amino acid.

Because of this, not all mutations are bad! If a substitution happens, the new triplet might still code for the exact same amino acid. In this case, the protein produced remains perfect. This is called a silent mutation.

Quick Review: Mutation Basics

• Mutations = changes in DNA base sequence.
• Occur spontaneously during replication.
• Substitution: One base swapped (may or may not change the protein).
• Deletion: One base removed (usually changes everything after it).

3.2.11.2 Mutations and Cancers

Cancer is essentially uncontrolled cell division. To understand how this happens, we need to look at the "gas pedal" and the "brakes" of the cell.

The Two Types of Tumours

When cells divide out of control, they form a mass called a tumour.
• Benign Tumours: These are "gentle" tumours. They grow slowly, are surrounded by a membrane, and do not spread to other parts of the body.
• Malignant Tumours: These are cancerous. They grow rapidly, interfere with surrounding tissues, and can spread through the blood or lymphatic system to form secondary tumours (metastasis).

The Genetic Control of Cell Division

Your cells have two main types of genes that manage the cell cycle. Think of a car:

1. Proto-oncogenes: These stimulate cell division. They are like the gas pedal of the car, telling the cell to "Go" and divide when needed.
• When mutated: They become oncogenes. This is like the gas pedal getting stuck down. The cell receives a constant signal to divide far too quickly.

2. Tumour Suppressor Genes: These slow down cell division or stop it if there is DNA damage. They are like the brakes of the car.
• When mutated: The gene is inactivated. This is like the brakes failing. The cell can no longer stop itself from dividing, even if it is damaged.

Cancer Treatment and the Cell Cycle

Many drugs used to treat cancer (chemotherapy) work by disrupting the cell cycle. If we can stop the cell from replicating its DNA (S-phase) or stop it from physically pulling apart (Mitosis), we can stop the tumour from growing.
Common Mistake to Avoid: Remember that these drugs also affect normal cells that divide quickly (like hair cells), which is why cancer treatments often have side effects.

Summary and Key Takeaways

Key Point 1: Mutations are changes in the DNA sequence that happen spontaneously but can be increased by mutagens like radiation.

Key Point 2: Deletion mutations are usually more damaging than substitutions because they change how the rest of the DNA "sentence" is read (frameshift).

Key Point 3: Cancer is caused by mutations in genes that control the cell cycle. Oncogenes are like a stuck gas pedal (too much growth), and mutated tumour suppressor genes are like broken brakes (no stopping growth).

Key Point 4: Malignant tumours are the dangerous ones because they can spread and invade other tissues.

Don't worry if the names "proto-oncogene" and "tumour suppressor" feel long—just remember the Car Analogy (Gas vs. Brakes) and you'll do great!