Molecular Biology Techniques: Restriction Enzymes, Gel Electrophoresis, and DNA Fingerprinting

Restriction Enzymes

  • Definition: Restriction enzymes (restriction endonucleases) cut DNA at specific recognition sequences.
  • Origin: Found in bacteria; protect against viruses.
  • Key feature: Recognize palindromic DNA sequences and cleave within the site.
  • Nomenclature (examples):
    • Escherichia coli -> EcoR I
    • Haemophilus influenzae -> HindIII
    • Format: Genus species, strain, order/identity indicators (e.g., EcoR I, HindIII).
  • Outcome: Produce DNA fragments; ends can be sticky (cohesive) or blunt.

Palindromic sequences

  • Palindrome: sequence reads the same 5′→3′ on both strands.
  • Many restriction enzyme recognition sites are palindromic.
  • Example concept: Recognizes a palindromic sequence and cuts within it.

DNA Digest (Restriction Enzyme Digestion)

  • Mechanism: Breaks phosphodiester bonds on both DNA strands at specific sites to yield fragments.
  • Ends:
    • Sticky (cohesive) ends
    • Blunt ends
  • Multiple sites: A linear DNA molecule can have multiple restriction sites, yielding multiple fragments.
  • Linear vs Circular DNA:
    • Linear DNA: Cut at multiple sites to generate several fragments (e.g., HindIII and EcoRI cuts).
    • Circular DNA: Number of fragments for a given set of cuts may differ from linear DNA (same enzymes can yield a different fragment count).

Gel Electrophoresis

  • Principle: Apply an electric field to move negatively charged DNA toward the positive electrode through an agarose gel.
  • Separation: Fragments separated by size; larger fragments migrate slower, smaller fragments faster.
  • Visualization: After running, fragments are visualized by staining.
  • DNA ladder: A set of DNA fragments of known lengths used to estimate sizes; length in base pairs can be read from the ladder alongside samples.
    • Note: 1  kb=1000 bp1\,\,\text{kb} = 1000\,\text{bp}
  • Buffers and protection:
    • Buffers provide electrical conductivity and maintain pH.
    • EDTA chelates divalent cations (e.g., (\text{Ca}^{2+}, \text{Mg}^{2+})) to inhibit nucleases.
    • Common buffers: TAE and TBE.
  • Loading dye:
    • Components: Tris-HCl (pH ~7.6), Glycerol (density), EDTA, tracking dyes (e.g., Xylene cyanol FF, bromophenol blue, Orange G).
    • Purpose: Density for sinking into well, pH stability, tracking progression.
  • Visualization options:
    • Ethidium bromide (EB) traditional stain; UV visualization; EB is a carcinogen.
    • SYBR Safe as a safer alternative.

DNA fingerprinting (Gel pattern interpretation)

  • Concept: The pattern of DNA fragments (bands) after restriction digestion and electrophoresis forms a DNA fingerprint.
  • Relatedness: DNA fingerprints of related individuals show many similarly sized fragments.
  • Inheritance: A child’s banding pattern is a composite of both parents (one chromosome set from each parent).
  • Application: Compare suspect DNA profiles to a reference (e.g., a victim) to assess likelihood of relatedness or identity.

Practical workflow overview

  • Steps to perform gel-based DNA analysis:
    • Obtain DNA samples; digest with restriction enzymes.
    • Prepare agarose gel and cast it.
    • Load digested samples with loading dye into gel wells.
    • Run electrophoresis to separate fragments.
    • Visualize bands using a DNA stain under UV or blue/green light.
    • Compare banding patterns to interpret identity or relatedness.

Quick takeaways for exam review

  • REs recognize and cut at short, specific palindromic sequences.
  • Digestion produces DNA fragments; ends may be sticky or blunt.
  • Fragment number depends on number and location of restriction sites; circular DNA behaves differently from linear DNA.
  • Gel electrophoresis separates fragments by size; larger fragments migrate slower; use a DNA ladder to estimate sizes.
  • Buffers (TAE/TBE) provide conductivity and maintain pH; EDTA protects DNA from nucleases.
  • Loading dye includes density and tracking dyes; sample visualization requires staining (EB vs SYBR Safe).
  • DNA fingerprinting allows assessment of identity and relatedness; a child’s fingerprint is a composite of both parents.
  • In practical cases, compare the DNA fingerprints of suspects to the reference sample to identify the biological parent.