The tas will amplify a gene of interest using pcr and supply you with the amplified. Ligation of the pcr product into the topo vector. For ligation, we will use the topo ta cloning kit by invitrogen. It allows for efficient insertion of pcr products into a plasmid vector in a single step, at room temperature, in 5 min. It relies on the phenomenon that taq polymerase has a nontemplate-dependent terminal transferase activity that adds a single deoxyadenosine (a) to the 3" ends of pcr products. The linearized vector supplied with the kit has single, overhanging 3" deoxythymidine (t) residues that allow pcr inserts to ligate efficiently with the vector. Be sure to include a vector only control: a ligation reaction with no fragment dna. This will give you an idea of the background ligation (the vector religating without insert). This could be prevented by dephosphorylation of the linearized vector but doesn"t always work completely.
