The following two basic types of sub nucleotide changes are noticed in the DNA molecule – Frame shift mutation and substitution mutation.
1. Frame shift mutations:
Addition or deletions of nitrogen bases either from DNA or mRNA are called frame shifts, because, from the site of the alteration, the reading frame of the message alters. In some cases frame shifts are neutralized i.e. deletion may be neutralized by the addition at the same site.
Frame shift mutations are of two categories. These are deletion mutations and insertion mutations. In deletion a single nucleotide in a triplet in deleted. These are found frequently in T bacterophages.
In insertion mutations an extra nucleotide is added to a gene. These insertions can be induced by chemicals such as acirdine, Proflavin etc.
2. Substitution mutations:
This generally occurs during the replication of DNA, when one nitrogenous base is replaced by another changing the triplet codon. The altered codon now codes for a different amino acid resulting in an altered protein. There are two kinds of substitutions – Transitions and Transversions.
The replacement of one purine by another purine or one pyrimidine by another pyrimidine is called trasition. In other words A is replaced by G and T by C or vice versa.
Transitions may be brought about by tautomerization, ionization, base analogs or by deamination. In tautomerization, a normal base, pairs with its tautomer partner [A tautomer is an alternate state of existence in a molecule.
For e.g in cytosine and adenine tautomers, the normal amino group NH2 is converted into the NH (imino) group and the keto group (C = 0) of thymine and Guanine is converted into the OH group in their tautomers.
Normally this does not happen]. Such pairing is calledforbidden base pair. Watson and Crick (1953) opined that the existence of a tautomer to DNA provides a mechanism for mutation.
Ionization is another mechanism by which transition can be brought about. Ionization of a base at the time of DNA replication involves the loss of H from a nitrogen base. Such ionized bases pair with normal bases.
Another method by which transition may be brought about is by the replacement of normal bases by their analogs. These compounds called base analogs are derivatives of the natural bases.
For e.g. cytosine may be replaced y hydroxyl methyl cytosine or by 5 glucosyl hydroxy methyl cytosine there are naturally occurring base analogs of cytosine found in T even phages of coli. Uracil has a naturally occurring analog – 5 hydroxy methly uracil found in some Viruses.
Besides the naturally occurring analogs, artificial analogs like 5 or nouracil, 5 iodouracil, 5 bromocytosine, 5 methyl cytosine may be used s analogs of thymine (first two) and cytosine (last two) which may be used to induce mutations.
Deamination is another method of induction of transition, by replacing the NH2 group of bases with OH group. This may be induced by chemials such as nitrous acid, hydroxylamine, diethyl sulphate etc.
Deamination of cytosine leads to the formation of uracil, and that of adenine forms hypoxanthine. During replication U pairs with A and hypoxanthine pairs with C. As a result A = T is substituted by G = C and G=C by A=T.
Unlike m transition, a purine is replaced by a pyrimidine and twice versa i.e. CG replaced by GC and AT by TA. This change can be induced by alkylating agents like ethylmethane sulphate (EMS) and Methylmethane sulphate (MMS).
The alkylating agents alkylate the nitrogen in the 7th position in a purine and lead to its separation from the DNA strand this gap can be filled by any of the four bases at the time of replication. Trasversions were first postulated by E.Freesein 1959.