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TA cloning inserts a PCR product into a plasmid vector by pairing the single 3′ A overhang that Taq polymerase adds to PCR products with a linearized vector carrying single 3′ T overhangs. TOPO cloning is Thermo Fisher’s version, in which vaccinia topoisomerase I, already bound to the vector, does the joining instead of DNA ligase. The vendor describes the TOPO TA reaction as a five-minute room-temperature step before transformation.
Short answer
TOPO TA cloning is a way to put a Taq-amplified PCR product into a plasmid without a separate ligase step. The usual failure point is not the vector but the PCR product: it needs a 3′ A overhang, and a proofreading polymerase removes it. The checklist below, taken from the manufacturer’s user guide, shows which PCR products qualify and what to do when they do not.
Definition and scope: TA cloning versus TOPO cloning
“TA cloning” names the principle. Thermo Fisher’s user guide explains that Taq polymerase has a nontemplate-dependent terminal transferase activity that adds a single deoxyadenosine (A) to the 3′ ends of PCR products, and the linearized vector supplied in the kit has single, overhanging 3′ deoxythymidine (T) residues, which allows PCR inserts to ligate efficiently with the vector. “TOPO cloning” and “TOPO TA cloning” name the Invitrogen/Thermo Fisher kit format that uses topoisomerase I in place of ligase. The kit described in the user guide is labelled for research use only, not for diagnostic procedures, and is offered with the vectors pCR2.1-TOPO or pCRII-TOPO.
This page covers the concept and the kit chemistry. It is not a bench protocol; follow the current manufacturer’s instructions for quantities and conditions. For the broader picture of moving a DNA fragment into a vector, see gene cloning.
How TOPO TA cloning works
The activated vector
The vector is supplied linearized, with single 3′ T overhangs and with topoisomerase I covalently bound to it. The manual calls this the “activated” vector. The enzyme comes from vaccinia virus; it binds duplex DNA at specific sites and cleaves the phosphodiester backbone after 5′-CCCTT in one strand (Shuman, 1991, cited in the manual). The energy of the broken bond is kept as a covalent bond between the 3′ phosphate of the cleaved strand and a tyrosyl residue (Tyr-274) of the enzyme. The 5′ hydroxyl of the original cleaved strand can attack that phospho-tyrosyl bond, which reverses the reaction and releases the enzyme (Shuman, 1994, cited in the manual). That reversible bond is what lets the enzyme-loaded vector join the insert without DNA ligase.
The workflow in outline
According to the manual, the sequence is: produce the PCR product, mix it with the TOPO vector, incubate for 5 minutes at room temperature, then transform competent E. coli. See competent cells and bacterial transformation for the downstream steps.
Salt solution and incubation time
The kit recommends a salt solution in the reaction (a 1.2 M NaCl and 0.06 M MgCl2 stock, giving 200 mM NaCl and 10 mM MgCl2 in the reaction). The manual says that with salt, longer incubations are possible because salt prevents topoisomerase I from rebinding and nicking the DNA after ligation, which the manual links to better efficiency for larger or difficult inserts.
Which PCR products can be TA cloned: a polymerase checklist
The table below summarizes the user guide.
| PCR product made with | 3′ A overhang present? | What the manual says |
|---|---|---|
| Taq polymerase alone | Yes | Taq adds a single 3′ A, so the product suits TA cloning. |
| Taq plus a proofreading polymerase | Only if Taq dominates | Taq must be used in excess of a 10:1 ratio to ensure the presence of 3′ A-overhangs. |
| Proofreading polymerase alone | No | Proofreading polymerases remove the 3′ A-overhangs necessary for TA cloning; add A-overhangs after amplification using Taq polymerase (manual section “Adding 3′ A-overhangs post-amplification”). |
| Blunt product | No | Add 3′ A-overhangs, or use the Zero Blunt PCR Cloning Kit to clone blunt PCR products. |
The same guide lists causes of few colonies: vector 3′ T overhangs that have been removed, the wrong polymerase, and too much or too little PCR product. If your amplicon came from a high-fidelity master mix, check its composition first; see master mix in PCR.
TOPO TA, conventional TA ligation and restriction cloning
Thermo Fisher’s TOPO TA product page compares ligation time: 5 minutes for TOPO, 1 hour for TA/UA cloning, and 2 to 23 hours for restriction enzyme cloning. This is the vendor’s own comparison. The page also reports up to 95% recombinant clones, a vendor claim, and lists variants including TOPO TA for subcloning, for in vitro transcription, for sequencing, and TOPO TA Fast supplied with Mach1 competent cells. For other routes to the same end, see restriction enzyme digest setup and Gibson assembly.
Where it matters for lab work
Two practical points follow from the manual. First, the manual advises confirming the orientation of the insert, so clones need checking. It describes restriction analysis with an enzyme that cuts once in the vector and once in the insert, to confirm the presence and correct orientation of the insert, or sequencing with the supplied M13 Forward (-20) primer 5′-GTAAAACGACGGCCAG-3′ and M13 Reverse primer 5′-CAGGAAACAGCTATGAC-3′. See Sanger sequencing. Second, the polymerase choice made at the PCR stage decides whether TA cloning works at all, so it is worth settling before amplifying. Vector features such as the multiple cloning site are covered in the plasmid vector guide.
Common misunderstandings
- “TOPO TA cloning gives the insert in a fixed orientation.” The manual advises confirming the orientation of the insert in the clones, by restriction analysis or sequencing.
- “Any PCR product works.” Products from proofreading polymerases lack the A overhang and need A-tailing first.
- “Up to 95% recombinant” is a guarantee. It is Thermo Fisher’s own claim on its product page.
- “Longer incubation always lowers colonies.” The manual ties longer incubation to the salt solution, which prevents the enzyme from rebinding after ligation.
FAQ
What is the difference between TA cloning and TOPO cloning?
TA cloning is the general approach of pairing A-tailed PCR products with a T-overhang vector. TOPO cloning is Thermo Fisher’s kit format of it, which uses vaccinia topoisomerase I bound to the vector instead of DNA ligase.
Can I use a proofreading polymerase product for TOPO TA cloning?
Not directly. The manual says proofreading polymerases remove the 3′ A overhangs; add them afterwards with Taq polymerase, or use a Taq-dominant mix (Taq in excess of 10:1).
Is TOPO cloning for clinical use?
No. The kit in the user guide is for research use only, not for diagnostic procedures.
Sources
- Thermo Fisher Scientific / Invitrogen, TOPO TA Cloning Kit user guide, Pub. No. MAN0000047 (topota_man.pdf), checked 2026-10-29. Cites Shuman 1991 and 1994.
- Thermo Fisher Scientific, TOPO TA Cloning product page (thermofisher.com), checked 2026-10-29.
