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Restriction Enzymes & Cloning

Restriction Enzymes & Cloning

4 min read Updated Apr 18, 2026

Restriction enzymes are bacterial proteins that cut DNA at specific sequences. Bacteria evolved them to slice up invading viral DNA. Molecular biologists hijack them to cut and paste DNA in the lab - the foundation of modern genetic engineering.

Which technique answers which question

Biotechnology
Read the verb in the question. It names the technique. PCR
“I need millions of copies of one sequence”
Denature at 95°, anneal primers at about 55°, extend with heat-stable Taq polymerase at 72°. Repeat: copies double every cycle.
Needs primers, so you must already know the flanking sequence.
 Gel electrophoresis
“I need to sort fragments by size”
DNA is uniformly negative, so it always runs toward the positive electrode. Small fragments thread through the gel faster.
Smallest travels furthest. Always.
 Restriction cloning
“I need this gene inside a cell”
A restriction enzyme cuts at its palindromic site, leaving sticky ends. Ligase joins gene to plasmid, and bacteria take it up.
Cut the insert and the vector with the same enzyme.
 Southern · Northern · Western
“I need to detect one specific molecule”
Run a gel, blot onto a membrane, then probe. Southern finds DNA, Northern finds RNA, Western finds protein using an antibody.
SNoW DRoP: Southern-DNA, Northern-RNA, Western-Protein.
 Sanger sequencing
“I need to read the actual bases”
Chain-terminating dideoxynucleotides stop synthesis at random points, giving a ladder of every possible length.
Each terminator is a different color, so the ladder spells the sequence.
 CRISPR-Cas9
“I need to edit a sequence in place”
A guide RNA takes Cas9 to a matching sequence, Cas9 cuts both strands, and the cell's own repair machinery does the editing.
Bacterial immune system repurposed. The specificity is in the RNA, not the protein.
 One PCR cycle 95° denature 55° anneal primers 72° Taq extends repeat · copies double every cycle
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Why restriction sites are palindromesA restriction site such as GAATTC reads the same 5' to 3' on both strands. That is what lets one enzyme, working as a dimer, cut both strands at equivalent positions and leave matching single-stranded overhangs on each side.

Why PCR needed a hot-spring bacteriumEach cycle starts by heating to 95° to separate the strands, which would destroy an ordinary polymerase. Taq, from a thermophile, survives it, and that single fact is what made the technique practical.

cDNA and why it mattersReverse transcriptase turns mRNA into cDNA, which has no introns because the message was already spliced. That is how a human gene is expressed in bacteria, which cannot splice: you give them the edited version.

Match the verb in the question to the technique. Amplify means PCR. Separate means a gel. Detect means a blot. Read means sequencing. Insert means cloning. Edit means CRISPR. Almost every question on this topic is that translation.

Recognition Sequences

Restriction enzymes recognize and cut palindromic sequences. A palindrome in DNA reads the same 5’ to 3’ on both strands. For example, EcoRI recognizes:

5'-G A A T T C-3'
3'-C T T A A G-5'

Read the top strand 5’ to 3’ (GAATTC), then read the bottom strand 5’ to 3’ (from right to left on the page, GAATTC). Same sequence. That is a palindrome.

Sticky Ends and Blunt Ends

EcoRI cuts between G and A on each strand, leaving overhangs:

5'-G            A A T T C-3'
3'-C T T A A            G-5'

These single-stranded overhangs (sticky ends) can base-pair with any other EcoRI-cut DNA. This is what makes cloning work - you cut a gene of interest with EcoRI, cut your plasmid vector with EcoRI, and the sticky ends recombine. Ligase seals the nicks.

Some enzymes (like EcoRV, SmaI) cut straight across, producing blunt ends. Blunt ends are less selective in ligation but still usable.

Plasmid Vectors

A plasmid is a small circular DNA molecule found naturally in bacteria. Cloning vectors are engineered plasmids with:

  • An origin of replication (ori) so the plasmid replicates in bacterial hosts.
  • A multiple cloning site (MCS) - a stretch of unique restriction sites where you can insert your gene.
  • A selectable marker (usually an antibiotic resistance gene) so bacteria that contain the plasmid can be selected.

To clone a gene:

  1. Cut both the gene and the plasmid with the same restriction enzyme.
  2. Mix fragments with DNA ligase.
  3. Transform bacteria with the ligated product (e.g., by heat shock or electroporation).
  4. Grow bacteria on a selective medium (with antibiotic) - only transformed bacteria with the plasmid survive.
  5. Confirmed bacterial colonies now carry millions of copies of your insert.

Applications

Recombinant DNA technology enables:

  • Protein production: human insulin, growth hormone, and many other therapeutics are made by bacteria engineered to express the human gene.
  • Gene analysis: any gene can be cloned and studied - its sequence, expression, and function.
  • Genetic engineering: cloning plus modern tools (CRISPR, transgenic organisms) has built modern biotechnology.
What is a palindromic DNA sequence, and why do restriction enzymes recognize them?
Click to reveal answer
A palindrome reads the same 5’ to 3’ on both strands (e.g., GAATTC). Restriction enzymes are typically homodimers that bind as two copies arranged in opposite orientations. The palindromic sequence allows the two subunits to make identical contacts on each strand, enabling specific, symmetric cleavage.
Why are sticky ends preferred over blunt ends for cloning?
Click to reveal answer
Sticky ends have short single-stranded overhangs with known sequences that complementary-base-pair with any other DNA cut by the same enzyme. This positions the two fragments correctly before ligase seals the nicks, giving efficient and directional ligation. Blunt ends ligate less efficiently and in either orientation.
What three essential features does a cloning plasmid contain?
Click to reveal answer
(1) An origin of replication so the plasmid replicates in the bacterial host. (2) A multiple cloning site with unique restriction enzyme recognition sequences for inserting the gene of interest. (3) A selectable marker (usually antibiotic resistance) so bacteria carrying the plasmid can be identified on selective media.
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