Molecular basis of mutation: Any change in the DNA
base sequence is known as the mutation. Some times this change may do not have
a detectable phenotypic effect. A variety of different genetic forms can be
observed as a mutational change in its nucleotide sequence. These mutational
genetic forms of a gene are known as the alleles. Wild type allele of that gene
is the form in which the given gene exists originally. Mutant alleles or
mutants are the altered forms of the genes which are obtained due to mutations.
Types of mutations: Mutation is classified in two
types. These are,
Point mutation- It is base pair
substitution. In this point mutation, an alteration takes place that changes
only single base pair.
Frame shift mutation- It is the insertion
or deletion mutation. A single base or more than one base can be deleted or
inserted by this type of mutation. It changes the reading frame.
FRAME SHIFT MUTATION
Intercalating agents like acridine dye (acridine orange or
proflavin) cause this frame shift mutation. A planner structure is possessed by
them. Intercalation of those agents between the bases of the DNA double helix
is observed. Because of this reason, the double helix structure gets distorted.
Distortion of the structure can be obtained either by the insertion of some
bases within the DNA sequence or by some deletion of bases from the DNA
sequences and causes frame shift mutation. During replication, the distance
between two consecutive base pairs is doubled by the intercalation of such dye
In point mutation, transition and transversion are observed.
Transition- (a) Purine to Purine or (b) Pyrimidine to
Transversion- (a) Purine to Pyrimidine or (b)
Pyrimidine to Purine.
Each pair of the base is able to undergo one kind of
transition and two kinds of transversion.
Classification of point mutation: There are three
types of point mutation can be obtained. These are as follows,
Silent mutation or nutral mutation
It is also known as nutral mutation. Silent mutation is a type of mutation
which does not possess any effect on protein synthesis.
From the above figure we can see that, the amino acid remain
the same inspite of the change in the DNA base sequence. This event results in
no such detectable effect on the protein synthesis due to the degeneracy of the
code. This is the biological importance of degeneracy of the code.
In case of missense mutation, a change in the amino acid is observed by the
alteration of DNA base sequence. But this change is not observed at the active
site of the protein. As a result, there is no effect on the protein activity
due to this mutation. The mutation does not change the three dimensional
protein structure. This results in no loss of activity of protein.
In the above figure, we can see that, the structure of the
protein is changed. But this change is not at the active centre of the protein.
Due to this reason, the activity of the protein remains the same.
Another case may takes place in the missense mutation. In
this case, the single amino acid replacement is observed at the active centre
of the protein. This changed protein possesses some biological activity like
the original one.
Some times, this type of mutation fails to function at
higher than the normal temperatures. This is known as the temperature sensitive
mutations. As for example: Sickle cell anemia.