Lesson 3 of 9 / Mutation, cell division and cancer
From a base change to sickling
How can one amino-acid substitution affect whole tissues?
In this lesson: Link gene mutation to protein structure and disease using sickle-cell anaemia.
About 7 min
The key ideaA beta-globin substitution changes intermolecular interactions, linking altered protein behaviour to red-cell and tissue effects.
Explore the idea
Keep the sequence; change oxygen conditions
The genotype is not changing with oxygen. Oxygen conditions affect the behaviour of the altered haemoglobin; this is an illustrative cell, not a clinical prediction.
Explanation
In the classic sickle-cell variant, a substitution changes a beta-globin coding DNA triplet from GAG to GTG, giving an mRNA codon change from GAG to GUG. The corresponding glutamate is replaced by valine. Glutamate is charged and hydrophilic; valine is non-polar.
This creates a hydrophobic surface feature that promotes association of deoxygenated haemoglobin S molecules into fibres. Polymerisation can distort red cells into sickled shapes and make them less flexible. The key mechanism is altered interaction between proteins, not loss of every haem group.
Rigid cells can obstruct small vessels and reduce tissue oxygen delivery. Repeated sickling also damages cells and shortens their survival, contributing to anaemia. The causal chain runs from DNA sequence to amino-acid property to molecular association to cellular behaviour and physiological effect.
The phenotype depends on genotype and conditions. Heterozygotes produce both normal and variant beta-globin, and oxygen conditions influence polymerisation. This example teaches a mechanism; it should not be used to diagnose an individual from a single symptom or a simplified diagram.
Step by step
- 1
State the sequence consequence
Distinguish coding DNA from mRNA.
- 2
State the property change
Charged glutamate is replaced by non-polar valine.
- 3
Build the causal chain
Protein fibres, cell deformation, vessel obstruction or cell loss.
Worked example
Work through the evidence
Why is saying "the mutation makes red cells sickle" incomplete at H2?
One way to explain it
It skips the protein mechanism: changed beta-globin chemistry promotes deoxygenated haemoglobin association into fibres, which changes red-cell shape and flexibility.
Why this answer works
- H2 explanations connect molecular and cellular levels.
- The protein alteration mediates the physiological effect.
Is this true? "Sickle-cell disease is caused by an extra chromosome 21."
It involves a beta-globin gene variant; trisomy 21 is a different chromosome-number condition.