Lecture 8: recombinant dna ii: functional complementation, sequencing and. The favorite cell type is picked, and the mrna is isolated and collected. mrna cannot be cloned, so an oligo-dt primer is hybridized to the poly(a) tail (all ts). Reverse transcription of rna to form cdna can occur using reverse transcriptase. The rna can be removed off the dna with the addition of alkali as dna is more stable, and rna is subject to auto-hydrolysis. The problem now lies in making the single strand of. The 3" end after the removal of the rna will have the end of the base sequence, which is not xed amongst different cdna molecules, and would require different primers to copy back the other direction. Thus a poly(dg) tail is added by tdt (terminal deoxynuleotidal transferase) enzyme. The enzyme adds g nucleotides to the 3" end without a template. This strand is then hybridized with oligo-dc primer.