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Restriction Digestion of DNA

Cleaving DNA at Specific Recognition Sequences ("Molecular Scissors")

1 Aim

To cleave DNA at specific recognition sites using restriction endonucleases and analyze the resulting fragments using agarose gel electrophoresis.

2 Principle

Restriction Digestion is a fundamental molecular biology technique used to cut DNA molecules at highly specific sequences using enzymes known as Restriction Endonucleases. These enzymes act as "molecular scissors."

Restriction enzymes naturally evolved in bacteria as a defense mechanism to chop up invading viral (bacteriophage) DNA. They recognize short palindromic DNA sequences (sequences that read the same forward and backward on complementary strands) and cleave the DNA backbone.

Types of Cleavage:

• Sticky Ends (Cohesive Ends): The enzyme makes a staggered cut, leaving short, single-stranded DNA overhangs. These are highly desirable for DNA cloning because they easily pair with complementary sticky ends. (e.g., EcoRI, HindIII)
• Blunt Ends: The enzyme cuts straight down the middle of the recognition sequence, leaving no overhangs. (e.g., SmaI, HaeIII)

Fig 1: EcoRI recognizes the palindromic sequence GAATTC and makes a staggered cut, generating 5' sticky ends.

3 Materials Required

Chemicals and Reagents

• DNA sample (Plasmid, Lambda phage, or Genomic DNA)
• Restriction enzyme (e.g., EcoRI, HindIII, BamHI)
• Specific 10X Restriction Enzyme Buffer
• Nuclease-free water
• 6X DNA Loading Dye

Equipment

• Dry bath incubator or Water bath (set to 37°C)
• Microcentrifuge
• Micropipettes and sterile tips
• Agarose gel electrophoresis apparatus
• UV Transilluminator / Gel Doc

4 Preparation of Reaction Mixture

All reagents must be kept on ice during preparation to prevent premature enzymatic activity. Typical reaction volume is 20 µl.

Component	Volume (20 µl Reaction)
DNA sample (~1 µg)	5 µl
10X Restriction Buffer	2 µl
Restriction Enzyme (e.g., EcoRI)	1 µl (10 Units)
Nuclease-free water	12 µl

*Rule of thumb: The volume of the enzyme should never exceed 10% of the total reaction volume to prevent Star Activity from glycerol.

5 Procedure

1. Label sterile microcentrifuge tubes properly (Control, EcoRI digest, etc.).
2. Add nuclease-free water, followed by the 10X buffer, and then the DNA sample.
3. Add the restriction enzyme last. Always use a fresh tip to avoid contaminating the enzyme stock.
4. Mix gently by flicking the tube or pipetting up and down slowly. Do not vortex heavily, as it may denature the enzyme.
5. Centrifuge briefly to collect all contents at the bottom.
6. Incubate the reaction mixture at 37°C for 1 hour in a water bath or dry bath incubator. (Note: Some enzymes require different temperatures, always check the manufacturer protocol).
7. Terminate the reaction by heat inactivation (e.g., 65°C for 20 mins) or by adding EDTA/Loading Dye.

6. Analysis & Result

To confirm that the DNA was successfully cleaved, we run the products on a gel.

1. Prepare a 1% agarose gel containing Ethidium Bromide.
2. Add 6X Loading Dye to your digested samples.
3. Load an Undigested Control DNA in Lane 1, the DNA Ladder in Lane 2, and the Digested Samples in subsequent lanes.
4. Run electrophoresis at 80–100 V until the dye front is 75% down the gel.
5. Visualize under a UV Transilluminator.

Result: The Undigested control will show a single high-molecular-weight band (or multiple bands if it is a supercoiled plasmid). The Digested sample will show distinct, separate bands corresponding to the cleaved fragments. The size of these fragments can be estimated using the DNA ladder.

7. Troubleshooting: Common Digestion Errors

Observation	Likely Cause & Solution
Partial Digestion (Extra unexpected larger bands)	The enzyme didn't cut all the sites. Caused by too little enzyme, too much DNA, insufficient incubation time, or inactive enzyme. Increase time or enzyme units.
Star Activity (Unexpected smaller fragments)	The enzyme lost its specificity and cut at incorrect sites. Caused by too much glycerol (>5%), wrong buffer, or incubating for too long.
Smeared DNA on gel	DNA degradation caused by nuclease contamination. Ensure tips and water are strictly nuclease-free.

8. Advantages & Applications

Key Advantages

• Highly specific and predictable DNA cleavage.
• Essential first step for Recombinant DNA Technology.
• Simple, reproducible protocol.

Applications

• DNA Cloning: Splicing genes into plasmids.
• Restriction Fragment Length Polymorphism (RFLP): Used in genetic fingerprinting and paternity testing.
• Gene Mapping: Creating restriction maps of unknown genomes.

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Important Viva Questions

Q1. What is a "Palindromic Sequence"?

A palindromic sequence in DNA reads the same in the 5' to 3' direction on one strand as it does in the 5' to 3' direction on the complementary strand. Examples include GAATTC (EcoRI) and AAGCTT (HindIII).

Q2. What is "Star Activity"?

Star activity is the relaxation or alteration of the specificity of a restriction enzyme. Instead of cutting at its exact recognition sequence, it cuts at similar, incorrect sequences, resulting in a smear or unexpected extra bands. It occurs under sub-optimal conditions like high glycerol concentration (>5%), incorrect pH, or over-incubation.

Q3. Why are restriction enzymes shipped and stored in 50% Glycerol, and why must they be kept on ice?

Glycerol acts as a cryoprotectant, preventing the enzyme from freezing solid and denaturing at -20°C storage. They must be kept on ice on the benchtop because they are highly temperature-sensitive proteins; leaving them at room temperature degrades their enzymatic activity quickly.

Q4. What are Isoschizomers?

Isoschizomers are pairs of restriction enzymes isolated from different bacterial strains that recognize and cut at the exact same recognition sequence.

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🧠 Interactive Concept Quiz

Test your knowledge! Click on the questions below to reveal the correct answers.

1. What does the "Eco" and "R" stand for in the enzyme EcoRI?

✅ Answer: Escherichia coli, strain RY13

Restriction enzymes are named after the bacteria they are isolated from. "Eco" represents the genus and species (Escherichia coli), "R" represents the strain, and "I" indicates it was the first enzyme isolated from that strain.

2. Why doesn't a bacteria's own restriction enzyme cut its own DNA?

✅ Answer: DNA Methylation

Bacteria protect their own DNA by adding methyl groups (-CH3) to specific bases within the recognition sequence. The restriction enzyme cannot bind to or cut methylated DNA.

3. Why must you keep the volume of the restriction enzyme under 10% of the total reaction volume?

✅ Answer: To prevent Star Activity caused by Glycerol.

Enzymes are stored in 50% glycerol. If the enzyme makes up more than 10% of your reaction volume, the glycerol concentration in the tube will exceed 5%, which alters the enzyme's structure and causes it to cut the DNA randomly.