Alteration of the sequence of bases in DNA can alter the structure of proteins - Biology 7402 - AQA A Level

Welcome to Your Guide on DNA Mutations!

In this chapter, we are diving into the heart of genetic variation. You will learn how tiny changes in the DNA base sequence can lead to big changes in the proteins that make our bodies work. Don’t worry if this seems a bit abstract at first—we’ll use plenty of analogies to make these microscopic changes easy to visualize!

Why is this important? DNA is like a master instruction manual. If you change even one letter in a recipe, you might end up with a totally different dish. In biology, these "typos" are called mutations, and they are the foundation of everything from genetic diseases to the evolution of new species.

Prerequisite Refresh: The "Three-Letter Word" Rule

Before we start, remember these two key rules of the genetic code:

1. The Triplet Code: DNA is read in groups of three bases (e.g., AGT, GCA). Each group is called a triplet and codes for one specific amino acid.
2. Degenerate Code: There are 64 possible triplets but only 20 amino acids. This means some amino acids are coded for by more than one triplet. Think of it like synonyms: "Big" and "Large" mean the same thing.

1. What are Gene Mutations?

A gene mutation is a change in the base sequence of a chromosome. These usually happen 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.

Mutagenic Agents

While mutations happen naturally, certain external factors can make them happen much more often. These are called mutagenic agents. Examples include:
• High-energy radiation: Such as X-rays or UV light.
• Toxic chemicals: Like those found in cigarette smoke.

Key Takeaway: Mutations are spontaneous "typos" in the DNA manual, and mutagenic agents make these typos more frequent.

2. The "Mutation Menu": Types of Mutations

AQA requires you to know six specific ways DNA can be altered. Let’s use the sentence "THE CAT SAT" (all three-letter words, just like DNA triplets) to see what happens.

1. Substitution

One base is swapped for a different one.
Original: THE CAT SAT
Mutation: THE RAT SAT
Effect: Only one triplet is changed. Because the code is degenerate, the new triplet might still code for the same amino acid (a silent mutation).

2. Deletion

One base is completely removed.
Original: THE CAT SAT
Mutation: THE ATS AT...
Effect: This causes a frame shift. Every triplet after the mutation is changed because the "reading frame" has shifted to the left.

3. Addition (Insertion)

An extra base is squeezed in.
Original: THE CAT SAT
Mutation: THE CBA TSA T...
Effect: Just like deletion, this causes a frame shift, but the reading frame shifts to the right. All downstream triplets are altered.

4. Inversion

A group of bases detaches, flips 180 degrees, and reattaches.
Original: THE CAT SAT
Mutation: THE TAC SAT
Effect: Only the amino acids in that specific inverted section are likely to change.

5. Duplication

One or more bases are repeated.
Original: THE CAT SAT
Mutation: THE CAT CAT SAT
Effect: This also causes a frame shift downstream from the point of duplication.

6. Translocation

A group of bases from one chromosome breaks off and is inserted into a different chromosome.
Effect: This is a major change that often leads to significant alterations in the types of proteins produced and can even lead to cancer.

Quick Review: Which mutations cause a frame shift? Addition, Deletion, and Duplication. These are usually the most damaging!

3. How Mutations Affect Proteins

The goal of a gene is to create a polypeptide (a chain of amino acids). The shape of this protein depends entirely on the order of those amino acids.

Scenario A: Change in a Single Triplet (Substitution)

If a substitution occurs, one of three things happens:
1. No change: The new triplet codes for the same amino acid (due to the degenerate nature of the code). The protein remains perfect.
2. Mis-sense: A different amino acid is placed in the chain. The protein might still work, or it might change shape slightly.
3. Non-sense: The mutation creates a "Stop" codon. The cell stops building the protein too early, making it non-functional.

Scenario B: The Frame Shift (Addition/Deletion)

A frame shift is like a "domino effect." Since the cell reads DNA in strict sets of three, adding or removing one base changes every single triplet after that point.
• The primary structure (amino acid sequence) is completely different.
• The tertiary structure (3D shape) changes because the new amino acids form hydrogen and ionic bonds in different places.
• The Result: The protein (e.g., an enzyme) will have a different shaped active site and will no longer work.

Did you know? Sickle cell anaemia is caused by a single substitution mutation. Just one base change is enough to change the shape of haemoglobin!

4. Summary Table for Revision

Common Mistakes to Avoid

• Confusing Gene and Chromosome Mutations: These are gene mutations (changes in the base sequence). Chromosome mutations involve having extra or missing whole chromosomes (like Down's Syndrome).
• Forgetting "Degenerate": Always mention the degenerate nature of the genetic code when explaining why a substitution might have no effect.
• Active Site: If a question asks about enzymes, specifically state that the active site changes shape so it is no longer complementary to the substrate.

Quick Check: Can you explain it?

1. Why does an addition mutation usually have a bigger impact than a substitution?
2. What do we call a mutation that doesn't change the amino acid sequence?
3. How do mutagenic agents affect the rate of mutation?

Key Takeaway: The structure of a protein is determined by the sequence of bases in DNA. Change the bases, and you change the protein's shape and function!

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