sentences:  These DNA templates from eluted sequences are then amplified via PCR and converted to single stranded DNA, RNA, or modified base oligonucleotides, which are used as the initial input for the next round of selection.One of the most critical steps in the SELEX procedure is obtaining single stranded DNA (ssDNA) after the PCR amplification step.This will serve as input for the next cycle so it is of vital importance that all the DNA is single stranded and as little as possible is lost.