Soluble microbial products (SMPs) are an important component in wastewater effluents, and consist of proteins, polysaccharides, humic acids, and DNA, etc. Pyrimidines and purines bases, the nitrogenous components of DNA, could act as the precursors of disinfection by-products (DBPs) during wastewater chlorination. This study investigated DBPs formation and the genotoxicity of pyrimidines bases (cytosine, uracil and thymine) and purines bases (adenine and guanine) during chlorination. Among the five bases, pyrimidines bases generated higher levels of DBPs than purines. Uracil generated the highest level of DBPs, while cytosine produced the highest level of nitrogenous DBPs (N-DBPs) including dichloroacetonitrile (DCAN), trichloroacetonitrile (TCAN) and trichloronitromethane (TCNM). None of N-nitrosamines were detected during chlorination of the five bases. The genotoxicity of chlorinated pyrimidines bases was higher than that of purines bases, and the genotoxicity of chlorinated uracil and cytosine was higher than that of glycine (the most common amino acid in wastewater). Formation pathways of three N-DBPs (DCAN, TCAN and TCNM) and two haloacetic acids (dichloroacetic acid and trichloroacetic acid) were tentatively proposed during chlorination of cytosine. The chemical structures of new intermediates generated during chlorination of cytosine were identified using an electrospray time-of-flight mass spectrometer, and the reaction site of intermediates was calculated by the frontier electron density (FED). The content of the formed intermediates under different chlorine dosages was also investigated to further illustrate the reasonability of the hypothesized reaction pathways. (C) 2016 Elsevier B.V. All rights reserved.