Restriction Enzyme Calculators

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Restriction enzymes (restriction endonucleases) are bacterial enzymes that cut double-stranded DNA at specific recognition sequences, typically 4–8 base pairs long. They are the molecular scissors of molecular biology — essential tools for cloning, constructing recombinant DNA, DNA fingerprinting, and restriction fragment length polymorphism (RFLP) analysis. Type II restriction enzymes are used in the lab because they cut at defined, predictable positions within or near their recognition sequence. Common examples include EcoRI (cuts G↓AATTC), HindIII (A↓AGCTT), and BamHI (G↓GATCC).

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How Restriction Enzymes Work

Restriction enzymes recognize a specific palindromic DNA sequence (reads the same on both strands 5'→3') and cut both strands. The cut may be staggered (creating sticky ends with single-stranded overhangs) or blunt (creating flush ends).

Sticky vs. Blunt Ends

  • 5' overhang (sticky end): Most Type II enzymes; EcoRI cuts G↓AATTC leaving 5'-AATT overhangs. Sticky ends can base-pair with complementary overhangs from the same enzyme → efficient ligation
  • 3' overhang (sticky end): Some enzymes leave 3' extensions; KpnI, PstI
  • Blunt end: EcoRV, SmaI cut flush; efficient for some cloning strategies; ligate less efficiently

Common Type II Restriction Enzymes

  • EcoRI: G↓AATTC → 5'-AATT overhangs
  • HindIII: A↓AGCTT → 5'-AGCT overhangs
  • BamHI: G↓GATCC → 5'-GATC overhangs
  • NcoI: C↓CATGG → 5'-CATG overhangs (contains ATG start codon)
  • NotI: GC↓GGCCGC → 8-bp recognition; rare cutter

Restriction Digest Protocol

Use 1–10 units enzyme per μg DNA; incubate at optimal temperature (usually 37°C, some 60–65°C); use manufacturer-supplied buffer; digest 1–2 hours; inactivate at 65–80°C. Analyze products by agarose gel electrophoresis.

Glossary

Restriction Enzyme (Type II)
A bacterial endonuclease that recognizes a specific palindromic DNA sequence and cuts both strands; produces sticky or blunt ends; essential tool for DNA cloning and restriction mapping.
Sticky Ends
Single-stranded DNA overhangs produced by staggered restriction enzyme cuts; can base-pair with complementary overhangs from the same enzyme; enable efficient, directional ligation in cloning.
Recognition Sequence
The specific DNA sequence (typically 4–8 bp palindrome) bound and cut by a restriction enzyme; EcoRI = GAATTC; HindIII = AAGCTT; BamHI = GGATCC.

Frequently Asked Questions

Restriction enzymes are bacterial endonucleases that recognize specific DNA sequences (typically 4–8 bp palindromes) and cut both strands of the double helix. They evolved in bacteria to destroy foreign DNA (bacteriophage); the bacteria's own DNA is protected by methylation of the recognition sequence. In the lab, Type II restriction enzymes are used as molecular scissors — they cut predictably at their recognition sequence, generating defined DNA fragments. Cut products are analyzed by agarose gel electrophoresis (restriction mapping) or used directly for cloning (ligation into vectors).

Sticky ends (cohesive ends): most Type II enzymes make staggered cuts in the palindrome, leaving single-stranded overhangs. 5' overhang: EcoRI cuts G↓AATTC; leaves 5'-AATT single-stranded tails. 3' overhang: PstI cuts CTGCA↓G; leaves 3'-ACGT tails. Sticky ends can base-pair with complementary sequences from the same enzyme, allowing efficient ligation. Blunt ends: some enzymes (EcoRV, SmaI) cut exactly in the center of the palindrome; no overhang; any two blunt ends can be ligated (versatile but inefficient). For cloning, sticky ends are preferred because they require specific enzyme compatibility and ligate much more efficiently than blunt ends.

Cloning with restriction enzymes: (1) Cut the target DNA (insert) and the vector plasmid with the same restriction enzyme(s). Both produce complementary sticky ends. (2) Mix insert and linearized vector; add T4 DNA ligase to seal the phosphodiester bonds. (3) Transform the ligation mixture into competent E. coli. (4) Select transformants on antibiotic plates. (5) Screen colonies for correct insert by colony PCR, restriction digest, or sequencing. For directional cloning (correct orientation), use two different enzymes with incompatible sticky ends — the insert can only ligate in one orientation. This is the most common molecular cloning strategy.

A restriction map shows the positions and sizes of restriction enzyme recognition sites along a DNA molecule. Construction: (1) Digest DNA with single enzymes and measure band sizes on agarose gel. (2) Digest with pairs of enzymes and compare band sizes. (3) Deduce relative positions of sites from fragment sizes. Example: single digest with EcoRI gives 3 kb + 7 kb = 10 kb total. Single BamHI digest gives 4 kb + 6 kb. Double digest EcoRI + BamHI gives 1 kb + 2 kb + 3 kb + 4 kb. By comparing single and double digest patterns, positions of each site are mapped. Restriction maps are used for clone characterization and genome organization analysis.