Gibson Assembly Overlap and Primer Annealing Designer
Plan isothermal Gibson assembly junctions by separating the 3′ template-annealing primer region from the 5′ homology overlap tail on the cited pUC19 cloning vector. The workspace opens the assembly panel in Gibson mode with three pUC19 arcs that share 25-base overlaps, while the article keeps PCR primer annealing calculations distinct from full chimeric oligonucleotide lengths.
The shipped worked Gibson example: the 2,686 bp pUC19 sequence (GenBank L09137.2) split into three arcs, each carrying the next arc's first 25 bases as a shared overlap, joined by the native engine. The panel opens with the product topology set to Linear: two junctions give a 2,711 bp linear product (2,761 fragment bases less two overlaps). Choosing Circular joins the third junction too and returns the 2,686 bp circular plasmid. The overlaps are a construction device, not a designed primer set.
Separate 3′ PCR template priming from 5′ isothermal junction homology
A Gibson assembly primer is a bipartite chimeric oligonucleotide. Its 3′ region anneals to the source template during PCR amplification, while its 5′ tail appends a run of bases identical to the end of the neighboring DNA fragment in the planned construct. Conflating these two segments is one of the most common design errors in seamless cloning: if you compute PCR melting temperature across the full 45–60 nt chimeric oligo, the resulting high figure can make the initial PCR cycles inefficient because only the 18–22 nt 3′ core actually pairs with the template initially.
Conversely, the 5′ overlap region must be evaluated under the conditions of the 50 °C isothermal assembly reaction. As T5 exonuclease degrades 5′ strands inward, the exposed 3′ single-stranded overhangs of adjacent fragments hybridize. Protocols suggest overlap lengths and an overlap Tm near the reaction temperature; those are outside rules of thumb, and the workspace applies neither. It reports the exact shared overlap at each junction and withholds a product whose overlap is missing, repeated or below its homology floor.
Verify junction coordinate accounting and secondary-structure clearance
When linearizing the 2,686 bp pUC19 reference vector by inverse PCR or restriction digestion, trace the exact terminal base coordinates at both ends of the vector backbone. Every base retained in the linearized vector plus every base of the amplified insert, minus the shared overlap length at each junction, determines the final circular coordinate map and reading-frame phase across fusion tags.
Before finalizing primer sequences, inspect the junction window for inverted repeats and extreme GC skew. Because the isothermal reaction stays at a fixed 50 °C without a high-temperature denaturation step, an overlap that forms a stable intramolecular hairpin at 50 °C will sequester the 3′ overhang and prevent inter-fragment hybridization. Shift the junction boundary by a few codons or adjust synonymous wobble bases to clear secondary structure while preserving the encoded polypeptide.
Equations and variables
Assembled circular construct length with n shared overlaps
Lassembled = Σi Lfragment,i − Σj Loverlap,j
- Lassembled
- Final covalently sealed circular plasmid length (bp)
- Lfragment,i
- Length of each linear input DNA fragment including terminal homology tails (bp)
- Loverlap,j
- Length of each shared junction sequence, found as the exact shared terminal sequence; the workspace recommends no length (bp)
Chimeric Gibson primer length partition
Lprimer = L5prime-overlap + L3prime-anneal
- Lprimer
- Total synthesized oligonucleotide length (nt)
- L5prime-overlap
- 5′ tail homologous to the adjacent assembly fragment (unpaired in cycle 1) (nt)
- L3prime-anneal
- 3′ core complementary to the amplification template (governs cycle-1 PCR Tm) (nt)
Scope and limitations
- In the initial cycles of PCR, only the 3′ template-complementary segment (L3prime-anneal) hybridizes to the template; calculating PCR Tm across the entire chimeric primer overestimates the annealing temperature.
- T5 exonuclease chew-back at 50 °C requires exposed 5′ ends; circular uncut vector or fragments shorter than ~150 bp that get completely chewed through will fail to assemble.
- Palindromic sequences, tandem repeats, or stable hairpins inside an overlap can fold unimolecularly at 50 °C and block inter-fragment annealing; the workspace checks repeated homology but does not model secondary structure or melting temperature.
Sources and provenance
- GenBank pUC19 record L09137.2Existing source notice: NCBI public domain
Versioned sequence and SHA-256 are pinned in the shipped example.
- Gibson et al. 2009 enzymatic assembly of DNA molecules up to several hundred kilobasesArticle citation; one-step isothermal chew-back assembly method
- Plasmid assembly methods and verification documentationExisting project method and fixture notices
Technical questions
Should I calculate my PCR annealing temperature using the whole Gibson primer or only the 3′ binding core?
Use only the 3′ template-binding core (typically 18–25 nt) for the initial PCR annealing temperature. During the first two amplification cycles, the 5′ homology tail hangs off as an unpaired flap and does not contribute to duplex stability on the template.
Does the workspace check an overlap melting temperature or recommend an overlap length?
No. It finds each overlap as the exact shared terminal sequence of consecutive fragments, uses no melting temperature, and withholds the product when an overlap is missing, contradictory, repeated elsewhere in the fragments or shorter than its homology floor, which is a limit of applicability rather than a recommended length. Published practice uses overlaps of roughly 15 to 40 bases, and some protocols aim for an overlap Tm near the 50 °C reaction temperature; both are outside rules of thumb the tool does not apply.
Why is the overlap length subtracted when calculating the final plasmid size?
Each junction overlap sequence is present on the 3′ end of one linear fragment and duplicated on the 5′ end of the neighboring fragment. During exonuclease chew-back and annealing, those two identical terminal copies merge into a single double-stranded junction segment in the circular product.
Connected workflows
- Vector and insert molar-ratio calculation
Calculate vector and insert nanogram-to-picomole molar ratios.
- Primer-pair Tm and template matching
Evaluate the 3′ template-annealing core of your amplification primers.
- Type IIS restriction-site inspection
Compare homology-overlap assembly with Type IIS 4-bp overhang cloning.