Ligation failures are usually not the enzyme's fault. They're the ratio, and specifically the fact that DNA is quantified by mass but reacts by molecule count. Here's the distinction and why it matters.
Molar, not mass
Your Nanodrop tells you ng/µL. Your ligation cares about how many molecules are in the tube. Those aren't proportional, because a long fragment weighs more per molecule than a short one.
100 ng of a 500 bp insert contains roughly six times as many molecules as 100 ng of a 3,000 bp insert. Set up both at "equal mass" and you've run two completely different experiments.
For 50 ng of a 5 kb vector, a 1 kb insert, at 3:1:
(50 × 1 ÷ 5) × 3 = 30 ng of insert
Thirty nanograms against fifty feels wrong the first time — you're adding less insert than vector by mass, yet running a threefold molar excess. That's the length effect doing its work. Our ligation calculator takes vector mass, both lengths and your concentrations, and returns the volumes to pipette.
Which ratio to use
| Situation | Molar ratio (insert:vector) |
|---|---|
| Sticky / cohesive ends | 3:1 |
| Blunt ends | 5:1 or higher |
| Very short insert (<500 bp) | 5:1 – 10:1 |
| Large insert (>5 kb) | 1:1 – 2:1 |
| Multi-fragment assembly | 2:1 per fragment |
Blunt ends need more insert because blunt ligation is dramatically less efficient — there's no base pairing to hold the ends together while ligase works, so the collision has to do all the work.
The two failure modes
Too little insert → the vector finds its own ends first and self-ligates. You get a lawn of colonies, all empty. This is the classic result and it's why the empty-vector control matters.
Too much insert → inserts ligate to each other, giving concatemers, or you get multiple inserts per vector. Fewer colonies, and the ones you get are wrong.
Total DNA matters too
Ratio isn't the only variable — concentration is. Keep total DNA around 50–100 ng in a 10–20 µL reaction.
Too dilute and the ends never meet. Too concentrated and everything ligates to everything. There's a subtlety here: low concentration actually favours the intramolecular reaction (circularisation), which is what you want for the final closing step — which is exactly why a 20 µL reaction usually beats a 50 µL one.
Controls tell you what went wrong
Two controls, every time, and they're worth the tubes:
Vector only, with ligase. Colonies here mean self-ligation — your digest was incomplete or you skipped the phosphatase.
Vector only, no ligase. Colonies here mean uncut vector survived the digest. No ratio fixes this; go back to the restriction digest.
Without these, a plate of empty colonies is uninterpretable and you'll spend a week adjusting the wrong variable.
Quick troubleshooting
Many colonies, all empty → self-ligation. Dephosphorylate, check the digest went to completion, gel-purify the vector.
No colonies at all → check ligase activity and buffer (ATP degrades with freeze-thaw), check transformation efficiency with a control plasmid, check the insert is really there.
Colonies with wrong inserts → incomplete digestion or star activity. Gel-purify both fragments.
Works sometimes → usually ATP in an old buffer aliquot. Ligase buffer is more fragile than people expect; use fresh single-use aliquots.
Try the free calculator
Skip the manual math — get instant numbers for your own project:
Frequently Asked Questions
What insert to vector ratio should I use for ligation?
A 3:1 molar ratio of insert to vector is standard for sticky ends, and 5:1 or higher for blunt ends, which ligate far less efficiently.
Why is ligation ratio molar and not mass?
Because ligation depends on the number of molecules, not weight. A 500 bp fragment contains roughly six times more molecules per nanogram than a 3,000 bp fragment.
How do you calculate ng of insert for a ligation?
ng insert = (ng vector × kb insert ÷ kb vector) × molar ratio. For 50 ng of a 5 kb vector with a 1 kb insert at 3:1, that's 30 ng.
Why do I only get empty vector colonies?
Usually vector self-ligation. Treat the vector with alkaline phosphatase, confirm the digest went to completion, and gel-purify before ligating.