ORFs can often be confirmed by matching their sequences to a database of known genes or EST's . The reaction is initiated using a pair of short primer sequences which match the ends of the sequence to be copied. Thereafter, each cycle of the reaction copies the sequence between the primers. Primers can bind to the copies as well as the original sequence, so the total number of copies increases exponentially with time. The phenotype results from the expression of a subset of a cell's genome .
Polymerases work from the DNA complement of the sequence to be built.
DNA polymerases copy DNA to DNA to replicate the genome before mitosis , while RNA polymerases copy DNA to RNA as the first step in gene transcription . For comparative transcription studies, two cDNA probes are prepared from the total mRNA of two different kinds of cells and labeled with two different reporters . Fluorescent reporters are used to detect the presence of cDNA on the spots of a microarray. The enzymes that perform this function are called reverse transcriptases . Reverse transcription is is used naturally by retroviruses to insert themselves into an organism's genome.