The Sister Chromatid Exchange Test

Mutagenic carcinogens such as alkylating agents, or physical mutagens such as ultra-violet light, induce lesions in DNA. When DNA containing such lesions replicates during S-phase, exchanges often occur between the two daughter molecules (Wolff et al, 1974). At the cytological level this becomes manifest as exchanges between the two sister chromatids of a metaphase chromosome. To visualize these exchanges, however, the cells must be treated so that the two sister chromatids will be different from one another. This can be accomplished by exploiting the fact that DNA is a double molecule that replicates semiconservatively, i.e., it has two complementary polynucleotide strands that separate from one another, and each acts as a template for the synthesis of new complementary strands. This means that each newly synthesized DNA molecule consists of one old polynucleotide strand that is conserved and a brand new strand.