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121 Effect of Substitution Mutation on Phenotype
Key Idea: The change of a single DNA nucleotide can change the amino acid sequence of the resulting protein, causing large scale phenotypic effects.
` Point mutations are mutations where only one nucleotide in a DNA strand is affected. Point mutations may result from deletion of a nucleotide, insertion of an additional nucleotide, or substitution of one nucleotide for another. ` Sometimes the point mutation has no phenotypic effect (e.g. it does not result in a change in the amino acid). At other times, the mutation can disrupt the biological function of the encoded protein (e.g. the mutation changes an amino acid in a key position). ` Sickle cell disease (below) is an inherited blood disorder affecting red blood cells (RBCs). It occurs as a result of a point mutation, and produces RBCs with a deformed sickle cell appearance and a decreased ability to carry oxygen.
Many aspects of metabolism are also affected.
The sickle cell mutation has adenine substituted at this point instead of thymine.
Normal red blood cells
Red blood cells containing normal haemoglobin have a flattened disc shape.
The mutation causes the amino acid valine to be produced instead of glutamic acid.
A faulty b-chain subunit is produced because the amino acid substitution causes the b-chain to fold and behave differently.
Template DNA
HBB gene
p
The haemoglobin beta gene (HBB gene), is located on chromosome 11. It encodes for the b-chain subunit of the haemoglobin molecule.
q
Chromosome 11.
The mutant protein, haemoglobin S is produced. Haemoglobin S changes the shape of RBCs. When the cells are exposed to low oxygen levels the RBCs become deformed into a sickle shape (right).
Sickle cells
1. Identify the type of mutation that causes sickle cell disease:
2. Write the mRNA sequence for part of the mutant DNA strand shown above:
3. (a) Name the amino acid produced by sickle cell gene mutation:
(b) Explain how this amino acid substitution results in the production of a faulty b-chain subunit:
