Insert and vector molar ratio for DNA ligation
For double-stranded DNA, molecule count at a fixed mass is inversely proportional to fragment length. The required insert mass therefore scales with vector mass, the insert-to-vector length ratio, and the desired molar ratio.
The result is a mass conversion, not a universal optimal reaction. End compatibility, DNA purity, concentration accuracy, phosphorylation, ligase, buffer, temperature, and construct geometry affect ligation success.
How to calculate insert DNA mass
- Enter vector mass: Use the mass of linear vector added to one reaction.
- Enter fragment lengths: Use positive whole base-pair counts for double-stranded vector and insert.
- Choose a molar ratio: Enter the desired insert:vector molecule ratio, such as 3 for 3:1.
- Calculate and plan: Use the mass result with a validated ligation protocol and reaction-volume constraints.
Formula and variables
The average mass per base pair cancels because both fragments are assumed to be double-stranded DNA.
minsert = mvector × (bpinsert/bpvector) × desired insert:vector molar ratio- minsert — Insert mass
- Calculated DNA mass to add (ng)
- mvector — Vector mass
- Mass of linear vector in the reaction (ng)
- bpinsert — Insert length
- Double-stranded insert length (bp)
- bpvector — Vector length
- Linear vector length (bp)
One-kilobase insert and three-kilobase vector
Use 50 ng of a 3,000 bp vector with a 1,000 bp insert at a 3:1 insert:vector molar ratio.
- Vector
- 50 ng, 3,000 bp
- Insert
- 1,000 bp
- Ratio
- 3:1
- minsert = 50 × (1000/3000) × 3
Result: Calculated insert mass is 50 ng.
Under the double-stranded length approximation, this gives three insert molecules per vector molecule.
Understanding your results
Calculated ratio is a starting condition
Mass and length determine molecule ratio, but they do not determine ligation efficiency.
- Very small inserts may require a different experimental ratio.
- Multiple inserts or assembly methods require another stoichiometric model.
- Concentration uncertainty propagates directly into the achieved ratio.
- Reaction volume and manufacturer recommendations still apply.
Assumptions
- Vector and insert are linear double-stranded DNA.
- Mass measurements represent intact ligatable DNA.
- One insert and one vector species are being compared by molecule count.
Limitations
- Does not assess end compatibility, phosphorylation, insert orientation, vector recircularization, reaction volume, or ligase conditions.
- Does not model multiple-fragment assembly or single-stranded oligonucleotide ligation.
- Does not recommend one universally optimal molar ratio.
Common mistakes
- Entering a vector:insert ratio when the field requests insert:vector.
- Using plasmid map length that excludes or double-counts the linearized construct.
- Mixing kilobases and base pairs without conversion.
- Treating calculated mass as a guarantee of cloning success.
Practical use cases
Restriction cloning preparation
Convert a planned molecule ratio into nanograms of one insert.
Ligation teaching
Demonstrate why shorter DNA fragments require less mass for the same molecule count.
Frequently asked questions
Why does insert length affect required mass?
For double-stranded DNA, longer molecules have more mass per molecule, so achieving the same molecule count requires proportionally more mass.
Is 3:1 always the best ratio?
No. It is a common starting point in some protocols, but appropriate ratios depend on fragment sizes, ends, method, and experimental conditions.
Can I use this for Gibson assembly?
Not as a complete design tool. Multi-fragment assembly typically requires fragment-specific molarity and protocol constraints.
Sources and review
- NEBioCalculator Ligation Calculator — New England Biolabs. Accessed 2026-07-13.
- Ligation Protocol for NEB PCR Cloning Kit — New England Biolabs. Accessed 2026-07-13.
Reviewed 2026-07-13.