Hong Life CalculatorsUnderstanding the ligation insert:vector ratio calculator — concept and principle
In DNA cloning, an insert fragment is joined to a cut vector by DNA ligase. Success depends strongly on how many insert molecules are present for each vector molecule, not on the mass of each. This calculator tells you how many nanograms of insert to add to a given amount of vector to reach a chosen molar ratio.
How it is calculated
Insert (ng) = vector (ng) × (insert length ÷ vector length) × ratio
- Vector amount is the mass of linearised vector in the reaction, in ng.
- Vector and insert lengths are in base pairs.
- The ratio is the desired number of insert molecules per vector molecule, for example 3 for 3 : 1.
The principle
For double-stranded DNA, molar mass is almost proportional to length. The number of moles in a sample is mass ÷ molar mass, so moles of vector ∝ vector ng ÷ vector bp and moles of insert ∝ insert ng ÷ insert bp. Setting insert moles equal to ratio × vector moles and solving gives the formula above; the constant (about 618 g/mol per bp) cancels. Intuitively, a fragment one-third the length weighs one-third as much per molecule, so the same number of molecules needs one-third of the mass. An excess of insert makes it more likely that each opened vector meets an insert before it re-closes on itself or meets another vector.
Worked example
- Inputs: vector 50 ng, vector 3000 bp, insert 1000 bp, ratio 3 : 1.
- Length ratio = 1000 ÷ 3000 = 0.3333.
- Insert for 1 : 1 = 50 × 0.3333 = 16.67 ng.
- Insert for 3 : 1 = 16.67 × 3 = 50 ng.
| Vector length (bp) | Insert needed (ng) |
|---|---|
| 750 | 200 |
| 1,500 | 100 |
| 2,250 | 66.66666667 |
| 3,000 | 50 |
| 3,750 | 40 |
| 4,500 | 33.33333333 |
| 6,000 | 25 |
How to read the result
A 3 : 1 insert : vector ratio is a common starting point for sticky-end ligations. Many protocols suggest trying a range, for example 1 : 1 to 10 : 1, and small inserts or blunt ends often benefit from a larger excess. If the computed mass is tiny (a fraction of a nanogram), dilute the insert so you can pipette it accurately. If it is very large, reduce the vector amount instead of exceeding the reaction's total DNA capacity recommended by the ligase supplier.
Common mistakes
- Using the uncut plasmid length when the vector has lost a stuffer fragment after digestion.
- Inverting the ratio and adding less insert than vector.
- Trusting concentrations measured before gel purification or after heavy losses.
Frequently asked questions
Does the formula work for multi-fragment assembly?
Yes. Calculate each fragment separately against the vector; assembly kits usually specify equimolar amounts or a modest excess of each insert.
What ratio should I use for very short inserts such as annealed oligos?
Much larger excesses are often used because short duplexes are small and numerous; follow your kit's guidance and test a range.
Can I use the vector in pmol instead?
Yes. Convert with pmol = ng × 1000 ÷ (bp × 618) and multiply by the ratio to get insert pmol.
Sources: New England Biolabs, NEBioCalculator, ligation calculator and T4 DNA ligase protocols · Sambrook, J. and Russell, D. W., Molecular Cloning: A Laboratory Manual (Cold Spring Harbor Laboratory Press)