Mechanism of chemical mutagens to induce mutation

Chemical mutagens
Classification of chemical mutagens
Mode of action of chemical mutagens ti induce mutation

1. University Of Agricultural Sciences Bangalore College of Agriculture, Bangalore Mechanism of Chemical Mutagens to I nduce Mutations SUBHASHCHANDRA L. SHABANNAVAR PAMB3177 Jr. M.Sc . Agriculture Department of Genetics and Plant Breeding

2. A mutagen is any substance or agent that can cause a mutation, or change in the sequence or structure of DNA . Mutation is a change in a phenotype, which is sudden , heritable and not produced due to segregation or Recombination. Mutagen

3. Molecular Basis of Mutation Base addition or deletion : Insertion/deletion of one or more bases in a DNA molecule is known as base addition or deletion . If number of bases involved in addition or deletion is a multiple of three bases leads to change in only corresponding amino acids in polypeptide chain . However , If number of bases involved in addition or deletion is not a multiple of three bases, leads to change in the entire reading frame beyond the point of insertion/deletion , such kind of mutations are known as frame shift mutations . Due to frame shift mutation, the entire amino acid composition of the polypeptide chain will be changed .

4. 2 . Base substitution : One nucleotide base of DNA is replaced by another base. There are two types of base substitution namely transition and transversion . The classical example for base substitution is sickle cell anaemia in which adenine in normal blood cell is replaced by thymine in sickle shaped cells . Transition : One purine base of DNA is replaced by another purine base ( A-G ) or one pyrimidine base of DNA is replaced by another pyrimidine base ( C-T ) is known as transition. Transitions occur due to oxidative deamination and T automerization . The possible number transitions are four . Transversion : One purine base of DNA is replaced by one pyrimidine base or vice versa. Transversions occur due to I onizing R adiations and Alkylating agents . Possible number of transversion is eight.

5. Chemical Mutagens Mustard gas was first identified as a chemical mutagen in D. melanogaster ( Auerbach and Robson, 1946) followed by many other chemicals. Chemical mutagens are categorized into four general groups, based on the mechanism by which they interact with DNA . Base analogs Alkylating agents Intercalating agents Agents altering DNA structures

6. Base Analogues Base analogues are very similar to the bases normally found in DNA As a result, they are readily incorporated in DNA during replication . Some of the base analogues are 5-bromouracil (5-BU), 5-chlorouracil (5-CU), 5-iodouracil (5-IU), 2-aminopurine (2-AP) and 2, 6-diaminopurine. Of these 5-BU and 2-AP are the most commonly used base analogues. The mechanism of mutagenic action of 5-BU is more clearly known than that of other bases .

7. 5-BU normally behaves like thymine and pairs with adenine. But sometimes, Tautomer of 5-BU pairs with guanine in place of adenine . Thus during DNA replication, 5-BU may become incorporated in place of cytosine. At the next round of DNA replication, it will normally pair with adenine. This will finally produce an A-T base pair in the place of a GC pair.

8. Alternatively , Tautomer of 5-BU paired with Guanine , is converted to its normal form in next round of DNA replication. This results in pairing between 5-BU and adenine instead of guanine. In upcoming round of DNA replication Adenine pairs with Thymine. Thus 5-BU produces AT → GC as well as GC → AT base substitutions. Clearly , 5-BU is capable of inducing reversion of the mutations it produces . 2-AP is believed to induce mutations much in the same way as 5-BU. But it specifically induces AT → GC base substitutions

9. Alkylating Agents Several chemicals have reactive alkyl (e.g., methyl, -CH3; and ethyl,-CH2-CH3) groups. They transfer these groups to DNA bases and to phosphate groups of DNA.These chemicals are called Alkylating agents . Examples: Ethyl Methane Sulphonate -most commonly used in plants N-methyl-N-nitro-N- nitrosoguanidine (NTG)-magic bullet,Highly powerful mutagen. Methyl Methane Sulphonate Ethyl Ethane Sulphonate Mustard gas

10. (1)The transfer of alkyl groups to the phosphate residue of DNA produces phosphate triesters . This may interfere with DNA replication as well as cause breakage in the sugar- phosphate backbone . ( 2) Alkyl groups are readily transferred to the 6-oxygen and 7-nitrogen of DNA bases. Of these, 7-ethyl guanine is believed to base-pair with thymine in place of cytosine. This ultimately leads to GC→AT base substitution . (3) During the repair of alkylated bases, errors may occasionally occur producing base substitutions or even frame-shift mutations. (4) Some alkylating agents cross-link DNA strands and/or DNA molecules. Such cross-links prevent DNA replication, and induce gene mutations and chromosome breakage resulting in structural chromosomal aberrations. (5) 7-Ethyl guanine often dissociates from deoxyribose and is removed from the DNA strand ( depuriation ). This may lead to the production of mutations.

11. Acridine dyes Acridine dyes, eg , A criflavin , P roflavin , Ethidium bromide (potent mutagen for plasma genes and used to induce male sterility) etc ., are very powerful mutagens, particularly in prokaryotes. Since acridines are positively charged, they insert themselves between DNA. This is known as intercalation, Intercalation increases the rigidity of DNA double helix as well as disturbs its configuration. This insertion causes a "stretching" of the DNA duplex and the DNA polymerase is "fooled" into inserting an extra base opposite an intercalated molecule. Replication of intercalated DNA molecules often results in addition or deletion of one to few (usually one) base pairs, which produces frame-shift mutations.

12. Deamination Agents Nitrous acid is a potent mutagen, which acts both on replicating and non replicating DNA . It leads to oxidative deamination of Cytosine , A denine and Guanine , which yields Uracil , Hypoxanthine and Xanthine , respectively. Uracil base-pairs with Adenine leading to G-C→ A-U substitution.

13. Oxidative deamination of Adenine produces H ypoxanthine, pairs with cytosine producing AT→GC substitution. But X anthine pairs with cytosine (the same as Guanine ), hence deamination of guanine does not yield mutation.