Determination of the starting point of transcription

One way to identify an active gene is to determine
the starting point of transcription. This
can be done by comparing the RNA formed and
the DNA template. After transcription, the RNA
formed (single-stranded) is hybridized to a
complementary single strand of DNA (RNA/
DNA hybridization). An endonuclease (S1 nuclease)
that cleaves only single-stranded DNA
degrades the nonhybridized single strand of
DNA, while the hybridized strand is protected
(RNA protection assay). Subsequently, the RNA
can be removed and the transcribed DNA segment
analyzed (e.g., its size or sequence determined).