Download Higher Biology: Genome - Gene Mutation

Genetic Mutations
Frame-shift and point mutations
Point mutations – substitution
These only affect a small part of the gene.
Substitution
normal
mutant
g g c c t c c t c
DNA
c c g g a g g a g
mRNA
pro
val
glu
glu
glu
g g c c a c c t c
c c g g u g g a g
protein
pro
Only one amino acid is affected in the primary
structure.
Frame shift mutations – insertion and deletion
These can affect a large part of the gene.
Insertion
normal
mutant
a g a g t c t t c
DNA
u c u c a g a a g
mRNA
ser
pro
glu
glu
lys
a g a g g t c t
u c u c c a g a a
protein
ser
All the amino acids after the insertion will be
affected in the primary structure of the protein.
Frame-shift mutations – insertion and deletion
These can affect a large part of the gene.
Deletion
normal
mutant
a g a g t c t t c
DNA
u c u c a g a a g
ser
ser
glu
mRNA
lys
a g a g c t t c g
u c u c c a g a a
protein
ser
arg
All the amino acids after the deletion will be affected in
the primary structure of the protein.
Summary
Point mutation
Occurs at a single point – substitution.
Generally not too harmful, most of the protein remaining
unaffected.
Only one amino acid affected so the protein will probably be
functional.
(single nucleotide polymorphism)
Summary
Frame-shift mutation
After a deletion or insertion the open reading frame is moved one
base pair forward or backward.
This is generally harmful since all the amino acids in the primary
structure of the protein will have changed from the mutation
onwards.
The protein will probably be non-functioning.
Mutation
Naturally occurring mutations are rare, they occur randomly
and spontaneously.
Mutations can be induced by mutagenic agents such as
gamma rays, X-rays and UV light. Tar in cigarettes, certain
food additives and many chemicals are thought to induce
mutations.
Some mutagens are also carcinogens – cancer-causing
mutations.
Mutation
Most mutant alleles are recessive so are only seen in
the phenotype when two recessive alleles are present.
However, some are dominant (achondroplasia) and
some are sex-linked (haemophilia).
Some mutations give rise to better genes and provide
alternative choices on which natural selection can act.
They are considered to be the raw material of
evolution.