The TA vector contains single 3’-dU overhangs that are compatible with direct ligation of the PCR product without the need for intermediate reactions. Following transformation, the dU residues are replaced with dT residues as the bacteria replicate the plasmid. The ease of such a technique was the reason behind choosing pCR II TOPO TA cloning vector. In addition, pCR II TOPO TA vector contains ampicillin resistance marker which permits only transformed bacterial cells to grow in ampicillin containing media, this strategy used for cloning.