Homopolymer OR T/A Tailing- This method uses the ability of annealing of complementary strands or sequences. Suppose a vector has an oligo( dA) sequence at the 3 -OH end and the insert has an oligo(dT) sequence at its 3 -OH end. Then when both the molecules are mixed, the molecules are held by hydrogen bond or can anneal until the ligase joins them by phosphodiester bond.

The important component in this method is terminal deoxynucleotidyl transferase. This enzyme adds nucleotides at the 3 -OH end of DNA without any complementary sequence. It can add up to 10-40 homopolymer residues at the end.

Commonly, the annealed circles with nicks and mismatching are used directly for transformation as these mistakes are repaired after the recombinant molecule enters into the host.