making cDNA

 Reverse Transcriptase (RT)
     
an enzyme found in retroviruses (RNA viruses)
      that catalyzes the synthesis of a
      single-stranded DNA molecule
      from a single-stranded RNA (
mRNA) template.

       Using RT & DNA polymerase one can then
       make a complimentary DNA strand and
      thus make a DNA gene that contains
no introns,
          i.e., one that is complimentary to a mRNA,
                thus
cDNA.

                Remember that genomic DNA
                contains introns & exons.