They are chemicals which induce mutations. Most of the mutagens are also carcinogens.
(A) Nitrous acid:
It deaminates cytosine to uracil, guanine to reanthin and adenine to hypoxanthin. This results in replacement of A-T with H-C, C-G with U-A and C-X. Nitrous acid is a common insustrial chemical and preservative of meat products.
(B) Alkylating agents:
They bring about methylation and ethylation of nitrogen bases, e.g. methyl guanine, ethyl gaunine. The alkylated base may slip out of DNA duplex or cause change of base pair, e.g. C-G to A-T. Alkylating agents include ethyl ethane sulphonate (EES), ethyl methane sylphonate (EMS), diethyl sulphate (DES), dimethyl nitrosamine dibromide, griseofulvin, endrin, etc. Ethyl dibromide (EDB) is used in food preservation, Griseofulvin is common antifungal medicine while endrin is an insecticide.
(C) Base Analogous:
e.g., (i) 5-bromouracil (Bu), (ii) 5-iodouracil (Iu), (iii) 5-chlorouracil (Cu), (iv) 5- Flurouracil (Fu)
All these replace thymine and pair with guanine. 2-aminopurine is incorporated in place of adenine with its tautomes pairing with cytosine. Thymine in its enol state pairs with guanine.
Proflavin, acriflavin, euflavin and acridine orange are intercalated in between base pairs leading to their insertion and deletion so as to cause frame-shift or gibberish mutations.