Molecular basis of mutation: Any change in the DNAbase sequence is known as the mutation. Some times this change may do not havea detectable phenotypic effect. A variety of different genetic forms can beobserved as a mutational change in its nucleotide sequence. These mutationalgenetic forms of a gene are known as the alleles. Wild type allele of that geneis the form in which the given gene exists originally.
Mutant alleles ormutants are the altered forms of the genes which are obtained due to mutations. Types of mutations: Mutation is classified in twotypes. These are,1. Point mutation- It is base pairsubstitution. In this point mutation, an alteration takes place that changesonly single base pair.
2. Frame shift mutation- It is the insertionor deletion mutation. A single base or more than one base can be deleted orinserted by this type of mutation. It changes the reading frame. FRAME SHIFT MUTATIONIntercalating agents like acridine dye (acridine orange orproflavin) cause this frame shift mutation. A planner structure is possessed bythem. Intercalation of those agents between the bases of the DNA double helixis observed. Because of this reason, the double helix structure gets distorted.
Distortion of the structure can be obtained either by the insertion of somebases within the DNA sequence or by some deletion of bases from the DNAsequences and causes frame shift mutation. During replication, the distancebetween two consecutive base pairs is doubled by the intercalation of such dyebetween them. POINT MUTATIONIn point mutation, transition and transversion are observed.Transition- (a) Purine to Purine or (b) Pyrimidine toPyrimidine.Transversion- (a) Purine to Pyrimidine or (b)Pyrimidine to Purine. Or Each pair of the base is able to undergo one kind oftransition and two kinds of transversion.
Classification of point mutation: There are threetypes of point mutation can be obtained. These are as follows,1. Silent mutation or nutral mutation2. Missense mutation3. Nonsense mutation SILENT MUTATION:It is also known as nutral mutation. Silent mutation is a type of mutationwhich does not possess any effect on protein synthesis.From the above figure we can see that, the amino acid remainthe same inspite of the change in the DNA base sequence. This event results inno such detectable effect on the protein synthesis due to the degeneracy of thecode.
This is the biological importance of degeneracy of the code. MISSENSE MUTATION:In case of missense mutation, a change in the amino acid is observed by thealteration of DNA base sequence. But this change is not observed at the activesite of the protein. As a result, there is no effect on the protein activitydue to this mutation.
The mutation does not change the three dimensionalprotein structure. This results in no loss of activity of protein.In the above figure, we can see that, the structure of theprotein 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. Inthis case, the single amino acid replacement is observed at the active centreof the protein. This changed protein possesses some biological activity likethe original one.
Some times, this type of mutation fails to function athigher than the normal temperatures. This is known as the temperature sensitivemutations. As for example: Sickle cell anemia.