Chapter 12 — Gene Cloning & DNA Profiling

Biotechnology vs. DNA Technology

  • Biotechnology = manipulation of living systems to create useful or therapeutic products.
    • Classic examples (centuries old): winemaking, cheesemaking, agricultural cloning.
  • DNA technology (a.k.a. genetic engineering / molecular biology) = direct manipulation of DNA itself.
    • Feasible only after discovery that DNA is genetic material ( 1952 ).
    • Accelerates and refines traditional biotech applications.

Gene Cloning: Big-Picture Overview

  • Goal: isolate a gene of interest, insert it into a vector, and mass-produce either the DNA or the encoded protein.
  • Conceptual steps = “molecular cutting + pasting.”
  • Product = recombinant DNA (DNA assembled from at least two different sources).

Essential Tools for Gene Cloning

  • Plasmid Vector
    • Small circular DNA found naturally in bacteria.
    • Key components:
    • Origin of replication ⇒ ensures autonomous copying inside host.
    • Antibiotic-resistance gene ⇒ selectable marker (only transformed cells survive antibiotic).
    • Multiple-cloning site (MCS) ⇒ cluster of unique restriction sites.
  • Restriction Enzymes (Restriction Endonucleases)
    • Isolated from bacteria (natural defense vs. phages).
    • Recognize specific palindromic DNA sequences (typically 6–8 bp).
    • Cut to leave sticky ends (single-stranded overhangs) → facilitate base pairing.
  • DNA Ligase
    • “Molecular glue” that seals sugar-phosphate backbones after sticky ends anneal.

Step-by-Step Construction of Recombinant Plasmid

  1. Cut the genomic source DNA to excise the gene of interest using a chosen restriction enzyme.
  2. Cut the plasmid with the same enzyme ⇒ identical sticky ends.
  3. Mix fragments → complementary sticky ends hydrogen-bond (metaphor: Velcro™ re-closing).
  4. Add DNA ligase → covalently seals nicks → intact recombinant plasmid.

Transformation, Selection, & Clonal Expansion

  • Introduce plasmid into bacteria ( transformation ).
  • Plate on antibiotic; only cells with plasmid survive.
  • Each colony descends from one transformed cell ⇒ a clone (all carry identical plasmid & gene copy).
  • Downstream options:
    • Plasmid amplification: grow liters of culture, purify plasmid, deliver gene to other organisms, site-directed mutagenesis, etc.
    • Protein expression: engineer vector with strong promoter/ribosome-binding site → bacteria transcribe/translate → harvest & purify protein.
  • Rationale for bacterial amplification: a single culture can generate billions of identical gene copies—far more than direct extraction from multicellular tissue.

DNA Profiling (Forensic Genetics)

  • Purpose: compare DNA samples to determine if they originate from the same individual.
  • Principle: although humans are 99.9\% identical, the remaining 0.1\% yields a unique DNA fingerprint.
  • Modern forensic workflow focuses on 13 standardized loci containing Short Tandem Repeats (STRs).

STRs as Genetic Markers

  • STR = short ( 2–5 bp) motif repeated head-to-tail.
  • Number of repeats varies greatly among individuals (e.g., \text{AGAT} repeated 7 vs. 15 times).
  • Probability two unrelated people match at all 13 loci is astronomically low ⇒ high evidentiary power.

Polymerase Chain Reaction (PCR) – DNA Amplification

  • Needed because crime-scene DNA is usually scant.
  • Function: exponentially amplify specific STR loci.
  • One PCR cycle (repeated 30–35 times):
    1. Denaturation (heat) ⇒ strands separate.
    2. Annealing ⇒ locus-specific primers bind flanking each STR.
    3. Extension ⇒ thermostable DNA polymerase synthesizes complementary strands.
  • Doubling each cycle: 1 \rightarrow 2 \rightarrow 4 \rightarrow 8 \rightarrow \dots → after 30 cycles, millions of copies.

Gel Electrophoresis – Fragment Separation

  • Matrix: agarose (purified seaweed carbohydrate) forms molecular sieve (think “DNA racetrack”).
  • Apply electric field: DNA (negatively charged) migrates toward anode.
    • Smaller fragments travel faster/farther, larger fragments lag.
  • Result = series of bands; band pattern constitutes the visual DNA profile.
  • Match criteria: every band from crime scene sample must align exactly with suspect’s bands.

Example Interpretation

  • STR Site 1: crime scene band ( 7 repeats ) matches suspect.
  • STR Site 2: crime scene band ( 10 repeats ) differs from suspect ( >10 repeats ).
  • Conclusion: suspect excluded (no full profile match).

Ethical, Practical & Real-World Implications

  • Biotechnology acceleration raises debates on cloning ethics, GMO crops, gene therapy, and biosecurity.
  • DNA profiling balances public-safety benefits with privacy concerns (e.g., retention of innocent peoples’ DNA in databases).
  • Recombinant protein production (e.g., insulin, growth hormone) showcases humanitarian and commercial potential.

Quick Reference: Key Numbers & Terms

  • Discovery of DNA as genetic material: 1952 (Hershey–Chase).
  • Restriction site length: 6–8 bp (typical).
  • Human genetic similarity: 99.9\%; individuality within 0.1\%.
  • Standard STR loci in U.S. CODIS system: 13.
  • Typical PCR cycles: 30–35; doubling each cycle.