Restriction enzyme digestion is a technique used to cut DNA at specific, predetermined sites. Restriction enzymes—also called restriction endonucleases—are molecular scissors that recognize short, palindromic DNA sequences and cleave the DNA at or near these sites.

How Restriction Enzyme Digestion Works

1. Choosing the Enzyme

Each restriction enzyme recognizes a specific DNA sequence, typically 4–8 base pairs long. Common examples include EcoRI (GAATTC) and HindIII (AAGCTT). Scientists choose enzymes based on where they cut relative to the DNA region of interest.

2. Setting Up the Reaction

The purified DNA is mixed with the restriction enzyme, a buffer specific to that enzyme, and water. The buffer provides the optimal salt concentration and pH for the enzyme to function. The mixture is incubated at the enzyme’s optimal temperature, usually 37°C.

3. Incubation

During incubation, the enzyme scans the DNA for its recognition sequence. Upon finding a match, it cuts the DNA backbone at precise locations. If the DNA contains multiple recognition sites, it will be cut into multiple fragments of varying sizes.

4. Heat Inactivation

After sufficient incubation time, the reaction is often heated to 65–80°C to denature and inactivate the enzyme, stopping the digestion. The resulting DNA fragments can then be analyzed by agarose gel electrophoresis or used in downstream applications like cloning.

Practical Restriction Digest Protocol

Set up a 20–50 µL reaction in a microcentrifuge tube. For a single digest, combine 1 µg of DNA, 2 µL of 10× restriction buffer (select the buffer recommended by the manufacturer), 10 U of restriction enzyme (typically 1 µL), and nuclease-free water to the final volume. Mix gently by pipetting and incubate at the enzyme’s optimal temperature (usually 37°C) for 1 hour. For double digests, use a buffer compatible with both enzymes — most manufacturers provide a compatibility chart. If no single buffer provides >75% activity for both enzymes, perform sequential digests: first with the enzyme requiring lower salt, clean up the DNA, then digest with the second enzyme. Always include a control reaction without enzyme to confirm the DNA is not degraded. After incubation, heat-inactivate at 65–80°C for 20 minutes (if the enzyme is heat-labile) or purify the digested DNA using a column cleanup kit. Analyze 5–10 µL by agarose gel electrophoresis. Troubleshoot incomplete digestion by extending incubation time to 2–4 hours, adding fresh enzyme after 1 hour, or using 2–5 U per µg of DNA. If the DNA is supercoiled plasmid, it may require 2–3 hours for complete linearization. For genomic DNA, use 2–5 U per µg and incubate overnight.

Real-World Application

In cloning a human gene (2 kb) into pUC19, both the PCR product and vector are digested with EcoRI and HindIII. A double digest in CutSmart buffer yields compatible sticky ends. After 1 hour at 37°C, gel purification isolates the linearized vector (2.7 kb) and insert (2 kb). Ligation and transformation into E. coli produces colonies, 80% of which contain the correct insert by colony PCR.