Cells or nuclei are incubated with BrUTP in the presence of sarkosyl, which prevents the attachment of RNA polymerase to the DNA. Therefore only RNA polymerase that are already on the DNA before the addition of sarkosyl will produce new transcripts that will be labeled with BrU.
To generate new transcripts, cells or nuclei are treated with BrUTP in the presence of sarkosyl, which prevents RNA polymerase from adhering to DNA. Consequently, only RNA polymerase that were already attached to DNA prior to adding sarkosyl will produce labeled transcripts.