Step by Step
RE
What restriction enzymes are
Restriction enzymes (restriction endonucleases) are bacterial proteins that recognize specific short DNA sequences (recognition sites) and cut both strands of DNA at or near that site. Bacteria use them as a defense against foreign DNA — they cut invading viral DNA while protecting their own DNA through methylation.
Memory trick: Restriction enzyme = bouncer at a DNA club. Only cuts DNA with the wrong "ID" (unmethylated foreign DNA).
Pal
Palindromic recognition sequences
Most restriction enzymes recognize palindromic sequences — sequences that read the same on both strands in the 5'→3' direction. Example: EcoRI recognizes 5'-GAATTC-3' / 3'-CTTAAG-5'. Both strands read GAATTC in the 5'→3' direction. Recognition sites are typically 4–8 base pairs long.
Memory trick: DNA palindrome = like "racecar" — reads the same forward and backward on each strand.
SE
Sticky ends vs blunt ends
Staggered cuts produce sticky ends (short single-stranded overhangs). Example: EcoRI cuts between G and A on each strand, leaving 5' overhangs (AATT). These sticky ends can base-pair with complementary sticky ends from other DNA cut with the same enzyme — making them ideal for cloning. Blunt-end cutters cut both strands at the same position — no overhang, harder to ligate.
Memory trick: Sticky ends = Velcro. They stick to matching sticky ends. Blunt ends = smooth — need more force (ligase + special conditions) to join.
Rec
Recombinant DNA — the application
To clone a gene: cut both the gene of interest and a plasmid vector with the same restriction enzyme → compatible sticky ends form → mix and add DNA ligase → ligase seals the nicks → recombinant plasmid. This is the foundation of genetic engineering, insulin production, and gene therapy vectors.
Applied Walkthrough
1
A researcher wants to insert the human insulin gene into a bacterial plasmid for expression.
2
Both the insulin gene DNA and the plasmid are cut with EcoRI — generating compatible AATT sticky ends on both.
3
The insulin gene and linearized plasmid are mixed — complementary sticky ends base-pair (anneal).
4
DNA ligase seals the phosphodiester bonds — recombinant plasmid formed. Bacteria take it up and produce human insulin. This is how commercial insulin has been made since the 1980s.
Exam Application
Exams test recognition sequences (palindromic), the difference between sticky and blunt ends, the role of DNA ligase in cloning, and the overall recombinant DNA process. EcoRI is the classic example — know its recognition sequence (GAATTC) and that it produces 5' AATT overhangs. Gel electrophoresis is used to verify cut fragments by size.
⚠ Common Trap
Students confuse restriction enzymes with DNA ligase — restriction enzymes CUT DNA, ligase JOINS it. Also: sticky ends from one restriction enzyme can only join with compatible sticky ends from the SAME enzyme (or one producing the same overhang). Using two different enzymes on insert and vector produces incompatible ends and no ligation.
✓ Quick Self-Check
1. What is a restriction enzyme and where do they come from?
Restriction endonucleases are bacterial enzymes that cut DNA at specific recognition sequences — originally a bacterial defense against viral DNA.
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2. What does "palindromic" mean in the context of restriction sites?
The recognition sequence reads the same on both strands in the 5'→3' direction — like a DNA palindrome.
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3. What are sticky ends and why are they useful?
Short single-stranded overhangs produced by staggered cuts. They can base-pair with complementary sticky ends from other DNA cut with the same enzyme — essential for cloning.
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4. What enzyme joins DNA fragments after restriction enzyme cutting?
DNA ligase — it seals the phosphodiester bonds between the annealed sticky ends.
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5. What is recombinant DNA?
DNA that has been artificially created by combining DNA from two different sources — for example, inserting a human gene into a bacterial plasmid.
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