Technique
TOPO vectors are designed in such a way that they carry this specific sequence 5´-(C/T)CCTT-3' at the two linear ends. The linear vector DNA already has the topoisomerase enzyme covalently attached to both of its strands' free 3' ends. This is then mixed with PCR products. When the free 5' ends of the PCR product strands attack the topoisomerase/3' end of each vector strand, the strands are covalently linked by the already bound topoisomerase. This reaction proceeds efficiently when this solution is incubated at room temperature with required salt. Different types of vectors are used for cloning fragments amplified by either Taq or Pfu polymerase as Taq polymerase (unlike Pfu) leaves an extra "A" nucleotide at the 3'end during amplification.
The TA TOPO cloning technique relies on the ability of adenine (A) and thymine (T) (complementary basepairs) on different DNA fragments to hybridize and, in the presence of ligase or topoisomerase, become ligated together. The insert is created by PCR using Taq DNA polymerase, a polymerase that lacks 3' to 5' proofreading activity and with a high probability adds a single, 3'-adenine overhang to each end of the PCR product. It is best if the PCR primers have guanines at the 5' end as this maximizes probability of Taq DNA polymerase adding the terminal adenosine overhang. Thermostable polymerases containing extensive 3´ to 5´ exonuclease activity should not be used as they do not leave the 3´ adenine-overhangs. The target vector is linearized and cut with a blunt-end restriction enzyme. This vector is then tailed with dideoxythymidine triphosphate (ddTTP) using terminal transferase. It is important to use ddTTP to ensure the addition of only one T residue. This tailing leaves the vector with a single 3'-overhanging thymine residue on each blunt end.
Read more about this topic: TOPO Cloning
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