brief - gene mutations
TRANSCRIPT
A silent mutation A silent mutation occurs when substitution of a base
still codes for the same amino acid as the original base
This is possible due to the degenerate nature of the genetic code
This mutation would have no effect on the production of the final polypeptide (protein)
Point Mutations
Non-sense mutationPoint Mutations
Nonsense mutation A non-sense mutation occurs when a substitution of a
base occurs leading to a premature ‘stop codon’ being coded for
This would lead to the premature end to protein synthesis
Successful synthesis of the final protein would be very unlikely and it would be almost certainly not be able to function normally
Stop codons are – UAA, UAG and UGA
Point Mutations
Nonsense mutations This type of mutation is responsible for Thalassemia.
Thalassemia is an autosomal recessive blood disease.
It results in a reduced rate or no synthesis of one of globin chains that makes up haemoglobin
The result is anaemia
Point Mutations
A mis-sense mutation A mis-sense mutation occurs when a change in base
leads to a different amino acid being coded for
The polypeptide will have a single amino acid that is different
In the diagram GAA -> GAG
Leads to the addition of Asp
instead of Glu
Point Mutations
Mis-sense mutation This type of mutation is
responsible for sickle cell disease
It occurs due to a mutation in the haemoglobin gene
The hydrophilic amino acid Glutamic acid is replaced by the hydrophobic amino acid Valine
This distorts the shape of the red blood cell and decreases its elasticity
Point Mutations
Deletion of Bases This occurs when a nucleotide is lost
from the DNA sequence
Deleting a base can lead to the codons no longer being read properly
This is due to the fact bases are read in triplets, therefore the subsequent bases would all be shifted forward by one base
Usually the amino acid sequence of the ‘new’ code will be entirely different
Frame-shift Mutations
Causes of mutations A mutation can occur by mistake
during DNA replication
These can spontaneously occur
Mutations can occur when DNA polymerase makes a mistake, this happens about once every 100,000,000 bases
Mutations can also be caused by the environment (i.e. ionizing radiation)
Sickle Cell Anaemia – From Mutation to Symptoms