Introduction
Molecular cloning — the construction of recombinant DNA — is a foundational process in biological research and discovery. With molecular cloning, scientists can amplify and manipulate genes of interest and then insert them into plasmids for replication and protein expression.
Many methods exist to move pieces of DNA. Oftentimes several approaches will work for any specific cloning project; however, for any given project there is usually an ideal approach. This may be due to speed, cost, availability of starting materials, or simply personal preference. The method you choose determines the sequence constraints you must satisfy, the enzymes and reagents you need, the number of steps and the hands-on time, the fidelity of the final junction, and how easily the product can be re-used or re-purposed in later experiments.
This guide details the most popular cloning methods for the creation of recombinant DNA. For each method we cover the underlying mechanism, the required sequence features, the reaction workflow, the practical advantages, and the limitations you should anticipate.
1. Restriction Enzyme Cloning
Restriction enzyme (endonuclease) molecular cloning is the "classic" cloning method, and it remains one of the most popular today. Restriction enzymes are naturally produced by certain bacteria and archaea, where they function as part of restriction-modification defense systems, and they cleave double-stranded DNA (dsDNA) at specific sequence sites.
Mechanism. In restriction cloning, scientists use specific restriction enzymes to cut the dsDNA of interest into fragments containing precise 5′ or 3′ single-strand overhangs (sticky ends), or no overhang (blunt ends). Two pieces of DNA that have complementary overhangs, or that are both blunt-ended, are then fused during a ligation reaction with T4 DNA ligase. T4 DNA ligase catalyzes the formation of a phosphodiester bond between a 5′ phosphate and a 3′ hydroxyl group; because the ends are compatible and annealed, the ligation efficiently seals the nick and covalently joins the two fragments. Cutting and re-joining in this way leaves a short "scar" — the residual restriction-site bases at the junction — which in most cases is harmless but must be considered if a seamless junction is required (for example, when the reading frame or a functional element spans the junction).
Why it remains popular. Restriction enzyme cloning benefits from the hundreds of available enzymes, many of which are relatively cheap. They cut specific target sequences, which range from four to 13 base pairs, and produce predictable cleaved ends in the DNA fragments. Given its prevalence, the vast majority of plasmids used for DNA cloning and expression contain several popular restriction enzyme sites, usually within the multiple cloning site (MCS).
Subcloning and PCR-added sites. You can easily move (subclone) any piece of DNA that already has restriction sites on either side of it into any plasmid that has the same sites in the same orientation within its MCS. Due to their short length, it is also easy to add restriction sites to any piece of DNA during PCR amplification, allowing for it to then be digested and ligated into your desired plasmid.
Important caveats. Restriction enzyme target sites can be repeated throughout a specific DNA sequence. This can make it difficult at times to identify compatible restriction enzymes that cut your insert or backbone at only the desired location for your cloning project. Additional practical issues include: (i) enzymes differ in their optimal buffers and temperatures, so multi-enzyme digests may require a compromise buffer or sequential digestion; (ii) some enzymes exhibit star activity at high concentration, low ionic strength, or in the presence of non-optimal cofactors, cutting at non-canonical sites; and (iii) 5′ overhangs, 3′ overhangs, and blunt ends ligate with very different efficiencies — 5′ compatible overhangs are typically the most efficient, while blunt-end ligation is inefficient and requires higher ligase and insert concentrations. As noted, restriction enzyme cloning also leaves behind a short scar and can be time-consuming compared to other cloning methods.
2. Gateway Recombination Cloning
Gateway cloning is a recombination-based cloning method. During Gateway, moving a piece of DNA from one plasmid into another is done via a single recombination reaction, drastically simplifying the process and reducing the amount of time required for cloning.
Site requirements. To utilize this approach, the fragment of DNA you would like to clone into a plasmid must already be surrounded by specific recombination sites — similar in spirit to restriction enzyme cloning. To do this, your DNA fragment must first be amplified with specific Gateway attB1 and attB2 sites attached to the 5′ and 3′ ends of the DNA sequence.
The BP reaction. This fragment can then be cloned into a Gateway donor plasmid, which contains compatible attP sites, via a proprietary BP clonase, creating an entry clone. The entry clone now has recombined attL sites flanking your DNA fragment of interest. The BP reaction is a site-specific recombination event between attB and attP sites and is catalyzed by the BP clonase enzyme mix (a combination of the phage lambda Int and IHF proteins). The donor backbone contributes a ccdB counterselection cassette, so that non-recombined donor plasmids kill the host and only true entry clones survive.
The LR reaction. Now that you have made an entry vector containing your DNA of interest (or obtained one of the thousands of entry vectors deposited with Addgene), it can be rapidly shuttled into any compatible Gateway destination vector, which contains attR sites, via LR clonase enzymes. The LR reaction is the reverse recombination event, between attL and attR sites, catalyzed by LR clonase (phage lambda Int, Xis, and IHF). Thus, you can clone your gene of interest one time into a donor plasmid, or acquire one that already has your gene in it, and then use bacterial recombination to easily move it into any destination plasmid that fits your experimental goal.
Trade-offs. Although Gateway cloning is a simple and efficient cloning method, Gateway vectors and recombination enzymes can be quite expensive. In addition, it is quite difficult to switch to another cloning method, such as restriction cloning, once your project has been completed via Gateway cloning, because the entry clone is locked into the att-site framework. Addgene's collection contains thousands of Gateway donor, Gateway entry, and Gateway destination vectors with different promoters, tags, selection markers, and fluorescent proteins to fit a variety of experimental conditions.
3. TOPO Cloning (TA Cloning)
Topoisomerase-based cloning, often called TOPO cloning or TA cloning, is a method that relies on the hybridization of the complementary base pairs adenine (A) and thymine (T).
Mechanism. TOPO cloning utilizes the Taq polymerase, which naturally leaves a single A overhang on the 3′ end of PCR products (a non-template-dependent terminal transferase activity). The complementary T comes from a pre-cut, linear, cloning-ready TOPO vector that has a DNA topoisomerase I covalently bound to the phosphate group on the free 3′ T. The topoisomerase acts as a ligase that joins the A and T compatible ends together. Concretely, topoisomerase I from Vaccinia virus recognizes the duplex sequence 5′-(C/T)CCTT-3′; the enzyme cleaves the phosphodiester backbone at the 3′ end of this site and forms a transient covalent tyrosyl-phosphate intermediate, storing the bond energy that is later used to ligate the incoming A-overhang. This chemistry means TOPO cloning needs neither restriction enzymes nor an exogenous ligase, providing an incredibly quick and easy way to clone a fresh PCR product into a plasmid — typically a 5-minute bench-top reaction, or as little as 30 seconds for some kits.
Limitations. The major disadvantage of TOPO cloning is that very few plasmid backbones are available TOPO-ready, and it is not feasible to create a TOPO vector yourself. Additionally, the efficiency can vary depending on the polymerase used (proofreading polymerases that generate blunt ends are unsuitable, and the A-overhang must be generated deliberately), and the single A overhangs degrade over time, further reducing ligation efficiency. TOPO-ready Gateway entry plasmids are also available, allowing for rapid cloning of PCR products into donor plasmids without the need for restriction enzyme cloning.
4. Gibson Assembly (Isothermal Assembly Reaction)
Isothermal cloning, more commonly known as Gibson assembly, takes advantage of the properties of three common molecular biology enzymes: a 5′ exonuclease, a polymerase, and a ligase.
Mechanism (three enzymatic steps in one tube). In Gibson assembly, DNA fragments with 20–40 base-pair homology at their ends can be easily ligated together in one isothermal reaction (typically 50 °C). First, the 5′ exonuclease chews back the 5′ ends of your DNA fragments, generating long overhangs that anneal to each other due to their homology. Second, DNA polymerase then closes the gap created by the 5′ exonuclease, extending the annealed 3′ ends using the complementary strand as template. Finally, DNA ligase seals the nicks in the DNA to create one piece of dsDNA. The coordinated activity of all three enzymes at a single temperature is what makes the reaction "isothermal": the exonuclease, polymerase, and ligase are balanced so that chew-back, gap-fill, and sealing occur in the same tube without buffer changes or temperature shifts.
Advantages. A major benefit of Gibson cloning is that it allows for the assembly of multiple fragments of DNA in the chosen orientation at a time, and without the need for any unwanted sequence at the junctions (such as a restriction enzyme or Gateway recombination sites). Any dsDNA fragments can be used, so if properly designed, any insert fragment (PCR product or synthesized oligo) with appropriate overhangs can be efficiently ligated into any plasmid backbone. Because the design is purely sequence-homology based, junctions can be placed anywhere in the sequence, giving complete freedom over the final construct architecture.
Limitations. A drawback of this system is that Gibson assembly works best when combining DNA fragments over 200 base pairs. Anything shorter than 200 base pairs has the potential to be completely degraded by the 5′ exonuclease, because there is not enough duplex length for the polymerase and ligase to "rescue" the fragment before chew-back consumes it. Additional considerations: highly repetitive sequences can cause mis-annealing, high GC content can impede exonuclease processivity, and the reaction requires careful primer design so that the 20–40 bp homology arms match the intended assembly.
5. Golden Gate and MoClo
Golden Gate and Modular Cloning (MoClo) take advantage of the unique properties of type IIS restriction endonucleases.
The key property of type IIS enzymes. These endonucleases cut dsDNA at a specified distance away from the recognition sequence. Cutting distal to their recognition site allows for the creation of custom overhangs, which is not possible with traditional restriction enzyme cloning. Scientists have utilized this approach to create compatible custom overhangs that can then be efficiently assembled together.
Two-fold advantage. First, the entire cloning step (digestion and ligation) can be carried out in one reaction with a single type IIS restriction enzyme, since the resulting overhangs will be distinct and preserve the directionality of the cloning reaction. Second, the restriction site is encoded on both the insert and plasmid in such a way that all recognition sequences are removed from the final product, with no undesired sequence (a "scar") retained. In practice, the enzyme (e.g., BsaI, BsmBI/Esp3I, BbsI, or SapI) is added together with T4 DNA ligase; the correct assembly removes the enzyme's own recognition sites, so the product becomes "locked" and can no longer be re-cut, while incorrect or unassembled products are re-cut and cannot survive. This one-pot, scarless, directional assembly makes Golden Gate ideal for building standardized parts (promoters, tags, terminators) into modular constructs.
Limitations. A disadvantage of type IIS assembly cloning systems is that, like restriction enzyme sites, type IIS sites can be found throughout endogenous DNA sequences. Thus, it is important to confirm that there are no additional sites present within the fragments you want to assemble before you get started. Additional caution: overhang design must avoid palindromic or self-complementary overhangs that would allow mis-assembly, and the choice of enzyme must be compatible with the internal sequences of all parts.
6. Ligation Independent Cloning (LIC/SLIC)
Ligation Independent Cloning (LIC) relies on the 3′ to 5′ exonuclease activity of T4 DNA polymerase.
Mechanism. In LIC, the T4 DNA polymerase's exonuclease activity creates "chewed-back" overhangs of 10–12 base pairs on the 5′ end of both the vector and insert. These overhangs can easily anneal, creating a circular product with four nicks that bacteria repair after transformation. LIC does not require site-specific recombination or a ligation step, making it an easy, cheap, and rapid cloning method.
The single-dNTP trick. LIC depends on the addition of only one free dNTP to the reaction. In the presence of a single free dNTP, T4 polymerase will continue to function as an exonuclease until a base is exposed on the single-strand overhang that is complementary to the free nucleotide. T4 will then resume its polymerase activity, add back the free base, and become stuck at this point, with no other free bases to add. This controlled chew-back is what generates the defined single-stranded overhangs required for annealing.
Primer design. Complementary overhangs are built into the PCR primers for the insert, based on the destination vector sequence and choice of restriction site. Because of the relatively long stretches of base pairing in the annealed product, ligation is rendered unnecessary. The product may be transformed directly into E. coli, where the normal replication process will repair the nicks.
Limitations. It is important to note that LIC has difficulty assembling DNA fragments with repetitive sequences and DNA that ends in sequences that form complex secondary structures. Because LIC depends on exonuclease chew-back, secondary structure or repeats in the overhang region can lead to mis-annealing or incomplete overhangs. The related method SLIC (Sequence and Ligation Independent Cloning) uses a similar T4-polymerase chewing approach and can additionally be combined with RecA-mediated annealing for longer, more complex assemblies.
7. Yeast-Mediated Cloning and Oligonucleotide Stitching
Yeast-mediated cloning is very similar in principle to Gibson cloning, but instead of an in vitro reaction with purified enzymes, it takes advantage of the powerful recombination abilities of yeast.
Mechanism. By simply transforming into yeast two (or more) fragments of dsDNA that have 30 or more bases of homologous ends, the endogenous DNA repair pathways will produce the fused product. Yeast is highly efficient at homologous recombination, so the cell's own machinery performs the assembly that purified enzymes perform in Gibson assembly.
Capacity advantage. One major advantage is that much larger final products can be generated (up to 100 kb) compared to other cloning methods that utilize bacteria, where it becomes progressively more difficult to clone plasmids larger than 10 kb. Yeast also tolerates repetitive and structurally complex DNA that would be unstable or prone to rearrangement in E. coli.
Oligonucleotide stitching. Another advantage is the ability to perform oligonucleotide stitching, in which pieces of DNA that share no homology ends can still be fused in a seamless manner. To accomplish this, you can transform into yeast the fragments of DNA to be fused along with custom synthesized DNA oligos that span each junction. These oligos should be 60–80 bp long, with 30–40 bp of homology to each of the larger fragments. The fragments should have complementary ends with 30–40 base pairs of homology. The bridging oligo therefore acts as a "splint" that provides the homology needed for recombination across an otherwise non-homologous junction.
Limitations. While this approach doesn't work in other cell types, it can be a major time- and cost-saver for labs working with yeast. Yeast transformation is slower and less efficient than bacterial transformation, and yeast-based assembly requires that the host be a suitable yeast strain with robust homologous recombination. Screening of correct clones may also require colony PCR or phenotypic assays before the construct is moved into its final host.
Choosing a Method: A Practical Comparison
Because several methods can succeed for a given project, the choice is usually driven by the specific constraints below.
- Classic and inexpensive, well-supported by existing plasmids: restriction enzyme cloning — at the cost of a scar and limited junction flexibility.
- One-time cloning then rapid shuttling between many vectors: Gateway recombination — at the cost of expensive enzymes and vector lock-in.
- Fastest route for a single fresh PCR product: TOPO/TA cloning — at the cost of few available backbones and A-overhang stability.
- Seamless multi-fragment assembly with defined junctions: Gibson assembly — best for fragments over 200 bp.
- Scarless, one-pot, standardized modular assembly: Golden Gate/MoClo — requires internal type IIS site screening.
- Cheap, ligation-free cloning with defined overhangs: LIC/SLIC — limited by repeats and secondary structure.
- Very large or repetitive constructs and seamless stitching: yeast-mediated cloning — at the cost of slower yeast workflows.
In practice, many laboratories combine methods — for example, using PCR to add homology arms and then Gibson or Golden Gate to assemble, or using Gateway for standardized entry clones and restriction cloning for final validation. The "ideal" method is therefore best selected against your specific requirements for fidelity, junction sequence, fragment number and size, cost, and turnaround time.
Figures referenced in the original guide were created with BioRender.com. This page is a rewritten, in-depth technical summary of widely used molecular cloning methods.