Base Pair Substitutions -Transitions,Transversion,Inversion

Gene Mutation
Random Mutations Vs Directed Mutations
Rate of Mutations
Effects of Mutations on the Phenotype
How Does a Mutation Act?
Base Pair Substitutions - Transitions, Transversion, Inversion
Framshift Mutations - Deletion, Insertion
Effect of Change of One Amino Acid in Phenotype
Missense Mutations
Codon Changes and Amino Acid Replacments in Haemoglobins
Nonsense Mutations
Mutation in Termination Codons
Silent Codons
Genetic Supression
Intragenic Suppression
Extragenic OR Intergenic Suppression

Base Pair Substitutions -Transitions,Transversion,Inversion

	Base Pair Substitutions -Transitions,Transversion,Inversion - Gene mutations are of two main types, base pair substitutions or switches and frameshift mutations. Base pair substitutions are the most common mutations. They result in the incorporation of wrong bases during replication or repair of DNA. Examples of base pair switches are from A- T to G-C, C-G or T-A. In base pair switches one base of a triplet codon is substituted by another, resulting in a changed codon.
(1) Original (wild type) message of reading frame. CAT GAT CAT GAT CAT GAT CAT................... (2) Substitution or replacement. A replaced by G CAT GAT CGT GAT CAT GAT CAT..................
If the mutated codon specifies another amino acid it will result in amino acid substitution in the polypeptide chain during translation. Substitutions usually result in a simple change of one amino acid in the polypeptide chain synthesized. Such changes are called missense mutations. If a single base substitution modifies a codon to a termination codon, the chain may terminate at the point pf mutation (e.g. CAGàUAG). Such mutations are called nonsense mutations. They result in incomplete polypeptide chains which may not be biologically active. Base pair substitutions are of two main types, transitions and transversions.
Transitions If a purine base is replaced by another purine base (A by G or G by A) or a pyrimidine by another pyrimidine (T by C or C by T) the substitution is called a transition. Transitions are by far the most common types of mutations. Transversions If a purine base is substituted by a pyrimidine, or vice versa, the substitution is called a transversion. It will be seen that each base pair can undergo one kind of transition and two kinds of transversions. In general, transition mutations code for chemically similar amino acids while transversions show a greater possibility of inserting amino acids with different charges. Although transitions and transversions can cause nonsense mutations, the chances of missense mutations are greater.
	Mutations leading to base pair substitutions presumably take place in two steps. Let us consider a mutation in a DNA double strand in which the purine base A is substituted by another purine base G (transition) When the DNA replicates it will give rise to two chains, one normal like the parent chain and the other mutant. Since the mutated base G pairs with C, the mutant DNA will have G-C at the point of mutation. Thus both chains have altered bases at the point of mutation.

Inversion

If a segment of DNA is removed and reinserted in a reverse direction it results in an inversion. As in substitution, the message is out of phase only in the triplets involved in the inversion.

CAT GAT TAC GAT CAT GAT CAT........

Massage out of frame

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Suppressor Genes Frameshift Suppressions Molecular Basis of Mutations Spontaneous Mutations Tautomerism Induced Mutations Chemical Mutagens Base Analogues Agents Modifying Purines and Pyrimidines OR Agents Labilize the Bases Agents Producing Distortions in DNA Radiations Types of Mutations

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