(Posted:22-Jul-2015)

A mutation is a change in the base sequence of DNA that usually results in insertion of a different amino acid into a protein and the appearance of an altered phenotype.Mutatins results from three types of molecular changes: (1)The first type is the base substitution.This occurs when one base is inserted in place of another.It takes place at the time of DNA replication either because the DNA polymerase makes an error or because a mutagen alters the hydrogen bonding of the base being used as template in such a manner that the wrong base is inserted.When the base substitution results in a codon that simply causes a different amino acid to be inserted,the mutation is called a missense mutation;when the base substitution generates a termination codon that stops protein synthesis prematurely,the mutation is called a nonsense mutation. Nonsense mutations always destroy protein function. (2)The second type of mutation is the frameshift mutation.This occurs when one or more base pairs are added or deleted,which shifts the reading frame on the ribosome and results in incorporation of the wrong amino acids,"downstream" from the mutation and in the production of an inactive protein. (3) The third type of mutation occurs when transposons or inserted sequences are integrated into the DNA.These newly inserted pieces of DNA can cause profound changes in the genes into which they insert and in adjacent genes. Mutations can be caused by chemicals,radiations or viruses. Chemicals act in several different ways: (1)Some,such as nitrous acid and alkylating agent alters the existing base so that it forms a hydrogen bond preferentially with the wrong base,for example,adenine would no longer pair with thymine but with cytosine. (2)Some chemicals such as benzopyrene,which is found in tobacco smoke,bind to existing DNA bases and cause frameshift mutations. X-ray and ultraviolet light can cause mutations also. (1)X-ray have high energy and can damage DNA in three ways: (a)By breaking the covalent bonds that hold the ribose phosphate chain together,(b)By producing free radicals that can attack the the base,and (c)By altering the electrons in the base and thus changing the hydrogen bonding. (2)Ultraviolet radiation,which has lower energy than X-ray , cause the cross linking of the adjacent pyrimidin bases to form dimers.This creoss linking for example of adjacent thymines to form a thymine dimer,results in inability of the DNA to replicate properly. Certain viruses,such as bacterial viruses, cause a high frequency of mutations when their DNA is inserted into the bacterial chromosome.