DNA ligation and cloning is a set of techniques used to join DNA fragments together and insert them into a vector—a carrier DNA molecule—for replication inside a host organism, typically bacteria. This allows scientists to produce large quantities of a specific DNA sequence.
How DNA Ligation and Cloning Works
- Preparing the Insert and Vector
Both the DNA fragment of interest (the insert) and the vector (usually a plasmid) are cut with the same restriction enzymes. This creates complementary sticky ends—short, single-stranded overhangs that can base-pair with each other.
- Ligation
The cut insert and vector are mixed together with DNA ligase, an enzyme that seals the sugar-phosphate backbone of DNA. The complementary sticky ends align, and the ligase forms covalent bonds, joining the insert permanently into the vector.
- Transformation
The ligated plasmid is introduced into competent bacterial cells through a process called transformation. This is often achieved by heat shock or electroporation, which makes the bacterial membrane temporarily permeable to DNA.
- Selection
The transformed bacteria are plated on agar containing an antibiotic. The plasmid carries an antibiotic resistance gene, so only bacteria that took up the plasmid survive. This creates colonies, each descended from a single cell containing the cloned DNA.
- Screening
Colonies are screened by colony PCR or restriction digestion to confirm they contain the correct insert. Positive colonies are grown in liquid culture to produce large amounts of the desired plasmid.
Practical Ligation Protocol
Calculate the optimal insert:vector molar ratio using the formula: mass of insert (ng) = (mass of vector (ng) × insert size (kb) / vector size (kb)) × desired ratio. For standard cloning, use a 3:1 insert:vector molar ratio. Set up a 10 µL ligation reaction: 50 ng of linearized vector, the calculated mass of insert, 1 µL of 10× T4 DNA ligase buffer (containing ATP), 1 µL of T4 DNA ligase (400 U/µL), and water to 10 µL. Include two controls: a vector-only ligation (no insert) to measure background from self-ligation, and a no-ligase control. Incubate at 16°C for 1–2 hours or at 4°C overnight for maximum efficiency. For blunt-end ligations, use 1 µL of ligase and incubate at 16°C for 16 hours. After ligation, transform 2–5 µL into 50 µL of competent E. coli cells by heat shock: incubate on ice for 30 minutes, heat at 42°C for 45 seconds, return to ice for 2 minutes, add 950 µL of SOC medium, and incubate at 37°C for 1 hour with shaking. Plate 100 µL on LB-agar with the appropriate antibiotic and, if using blue-white screening, add 40 µL of X-gal (20 mg/mL) and 40 µL of IPTG (100 mM). White colonies indicate successful insertion (disrupted lacZ gene), while blue colonies contain self-ligated vector. A good ligation should yield 10–100× more colonies than the vector-only control, with >70% white colonies.
Real-World Application
When cloning a 0.8 kb GFP gene into pUC19 (2.7 kb) with a 3:1 molar ratio, the ligation produces ~200 colonies on ampicillin-X-gal plates. Of these, 165 (82%) are white. Colony PCR on 10 white colonies confirms the GFP insert in all 10, demonstrating efficient ligation and screening.
