Genetic Variation and Mutations Revision Notes | GCSE Edexcel Higher Biology

1. What Are Genetic Variants and Why Do They Matter?

Your DNA isn't identical to anyone else's - not even your siblings. These differences are called genetic variants, and they're happening in your cells right now. Some change how you look, others affect how your body works, and many do absolutely nothing at all.

Think of DNA as an instruction manual for building proteins. If someone changes a word in the manual, three things can happen: the instructions still work fine, they work slightly differently, or they don't work at all. This is exactly what happens with genetic variants.

Trap: Many students think all genetic variants cause diseases. Actually, most variants are harmless - they're just natural differences between people, like having different eye colours or blood types.

2. How Coding DNA Variants Change Protein Activity

Coding DNA contains the instructions for making proteins. When variants occur in these regions, they can change the protein that gets made. Picture a recipe where someone changes 'add salt' to 'add sugar' - you'll get a very different result.

The DNA code works in groups of three bases (called codons). Each codon specifies which amino acid gets added to the growing protein chain. Change one base, and you might change the amino acid - which changes the protein's shape and function.

Worked example: How a Single Base Change Affects Protein Function

A gene has the sequence GAG (coding for glutamic acid). A mutation changes this to GTG (coding for valine). Explain the consequences.

1. GAG → GTG (A changed to T) — Single base substitution mutation
2. Glutamic acid → Valine — Different amino acids have different properties
3. Glutamic acid is charged and hydrophilic Valine is uncharged and hydrophobic — This changes how the protein folds
4. Protein shape changes → Function may be altered — This specific change causes sickle cell anaemia

Answer: The single base change alters the amino acid, changing protein shape and potentially destroying its function

This is where it gets interesting - not all base changes matter equally. Some create 'silent' mutations where the new codon still codes for the same amino acid. Others cause dramatic changes that completely break the protein.

3. Non-Coding DNA and Gene Expression Control

Here's something that surprises many students: most of your DNA doesn't actually code for proteins. These non-coding regions act like switches, controlling when and how much protein gets made from each gene.

Think of genes like light bulbs and non-coding DNA like the dimmer switches. The bulb (gene) is always there, but the switch controls whether it's on, off, or somewhere in between. Variants in these switch regions can dramatically change gene expression without touching the protein-coding sequence at all.

These regulatory sequences can be right next to the gene or thousands of bases away. Some act as accelerators (enhancers), others as brakes (silencers). When variants occur in these regions, they can turn genes on when they should be off, or reduce expression when more protein is needed.

4. Why Most Mutations Don't Matter

Mutations are happening in your cells constantly - about 3 billion times per day. If they all caused problems, we'd be in serious trouble. Fortunately, most mutations are harmless for three key reasons.

First, many mutations occur in non-essential DNA regions. Second, the genetic code has built-in redundancy - multiple codons can code for the same amino acid. Third, even when the protein changes slightly, it often still works well enough to do its job.

The key insight is that proteins are remarkably tolerant of small changes. Like a car that still runs with a slightly different brand of oil, most proteins can handle amino acid substitutions without losing function.

5. When Mutations Do Cause Problems

While most mutations are harmless, some cause serious problems by dramatically altering protein shape. This is particularly devastating for enzymes, which need precise shapes to bind their substrates, and structural proteins, which must maintain their strength.

Picture an enzyme as a lock and its substrate as a key. If a mutation changes the lock's shape, the key no longer fits. The enzyme becomes useless, even though it's still a protein. Similarly, if a mutation weakens a structural protein like collagen, tissues become fragile and prone to damage.

Worked example: Analysing the Impact of Protein Shape Changes

A mutation in the gene for hexokinase (a glycolysis enzyme) changes the active site shape. Predict the consequences for cellular respiration.

1. Hexokinase catalyses: glucose + ATP → glucose-6-phosphate + ADP — First step of glycolysis pathway
2. Changed active site → glucose cannot bind properly — Enzyme-substrate specificity is lost
3. Glucose cannot enter glycolysis → No pyruvate production — Blocks the entire respiratory pathway
4. Reduced ATP production → Cell energy crisis — Could be lethal if severe enough

Answer: The mutation would severely impair cellular respiration by blocking glucose metabolism, potentially causing cell death

Trap: Students often think mutations always affect the whole organism. Actually, the impact depends on which protein is affected and how essential it is. A mutation in a digestive enzyme might cause stomach problems, while one in a structural protein could affect bones or skin.

6. Exam Technique - Genetic Variation Questions

Genetic variation questions test your ability to link DNA changes to their consequences. Examiners love to give you a scenario and ask you to predict what happens next. The key is working through the chain: DNA → protein → function → phenotype.

Common exam mistakes include confusing mutations with genetic variants (all mutations are variants, but not all variants are harmful), and assuming all DNA changes affect proteins (remember non-coding DNA). Always read carefully to see if the question is asking about coding or non-coding regions.