Recombinant DNA Techniques

Overview of Recombinant DNA Technology

  • Bacteria utilize restriction enzymes to protect against viral DNA invasions.

    • Each bacterium produces specific restriction enzymes, each recognizing distinct DNA sequences.

Role of Restriction Enzymes

  • When isolated in a test tube, these enzymes can cut any DNA submitted, not just viral DNA.

  • This capability has sparked the development of recombinant DNA technologies.

Definition of Recombinant DNA Molecule

  • Involves cutting DNA from two or more organisms with the same restriction enzyme and reconnecting fragments.

    • Visual Representation:

    • Example of DNA pieces in purple and green being cut and reconnected.

    • Result: A recombinant DNA molecule that has never existed in nature before.

Applications in Medicine

Insulin Production

  • People with diabetes require insulin to manage sugar levels, as their insulin may not function correctly.

    • Traditional sources of insulin include:

    • Human insulin harvested from deceased donors (rare).

    • Insulin extracted from pigs (many are allergic).

  • Today, approximately $99.9 ext{%}$ of human insulin is produced via recombinant DNA technology using bacteria.

Process of Bacterial Insulin Production

  1. Plasmid Isolation:

    • Removal of plasmids from bacteria (extrachromosomal DNA).

  2. Gene Cutting:

    • Isolation of the insulin gene from the human genome using a restriction enzyme.

  3. Insertion:

    • Inserting the gene into the plasmid to create hybrid plasmid DNA.

  4. Transformation:

    • Inserting the recombined plasmid back into bacteria which express the human insulin gene.

  5. Protein Production:

    • The bacteria transcribe and translate the gene to produce human insulin, which is then purified for medical use.

Plasmids in Recombinant DNA

  • Definition:

    • Plasmids are small circular DNA separate from chromosomal DNA in bacteria, encoding non-essential but beneficial traits.

Components of a Plasmid

  1. Origin of Replication:

    • Site where DNA replication commences.

  2. Drug Resistance Site:

    • Confirms the presence of the plasmid.

    • Example: Plasmid permits bacterial growth in the presence of ampicillin.

  3. Multiple Cloning Site (MCS):

    • Contains unique restriction sites for inserting foreign DNA.

    • Enzymes like EcoR1, BamH1, and SalI can be used to cut the plasmid only once to introduce foreign DNA.

Genomic Library Construction

  • Definition:

    • A genomic library contains a collection of cloned DNA fragments from an organism's genome.

Process of Creating a Genomic Library

  1. DNA Extraction:

    • Isolate total DNA from organisms.

  2. Restriction Enzyme Digestion:

    • Cut DNA into fragments of varying sizes using restriction enzymes.

  3. Fragment Insertion:

    • Insert fragments into plasmids; ideally one fragment per plasmid.

  4. Transformation:

    • Introduce plasmids back into bacteria and culture them, resulting in a library of bacterial colonies, each harboring unique DNA fragments.

Limitations of Genomic Libraries

  • Insulin production from a genomic library is impractical because it contains both introns and exons.

    • Bacteria lack mechanisms to remove introns, leading to unreadable sequences.

cDNA Libraries

Overview

  • A cDNA library is made from complementary DNA synthesized from mature mRNA (which excludes introns).

  • Importance:

    • Essential for producing proteins of interest like insulin without the complication of intronic sequences.

cDNA Library Construction Process

  1. mRNA Isolation:

    • Extract mature mRNA from cells (no introns).

  2. Reverse Transcription:

    • Use reverse transcriptase to synthesize cDNA from mRNA.

  3. Fragment Insertion:

    • Insert cDNA into plasmids.

  4. Transformation:

    • Introduce plasmids back into bacteria for protein expression.

Comparison of Genomic Libraries and cDNA Libraries

  • Genomic Library:

    • Contains all genetic material (including introns); used for studying gene regulation and expression.

  • cDNA Library:

    • Contains only expressed genes (exons); used for protein production.

    • For insulin production, cDNA libraries are preferred due to the absence of introns.

Hybridization Techniques

Definition

  • Hybridization involves the joining of complementary single-stranded DNA strands from different sources.

Purpose

  • Determines genetic similarities or relationships between different species through hybridization assays.

Example Study

  • History of Evolutionary Studies on Humans and Chimpanzees:

    • Earlier studies claimed a 1% difference in DNA; modern analysis suggests an actual similarity of 80-85%, highlighting a deeper evolutionary divergence than previously thought.

Other Applications of Recombinant DNA Technology

Cancer Research using Animal Models

  • Exploring mouse genes linked to cancer requires determining analogous human genes.

  • Colony Block Hybridization:

    • Identifies similar human genes through labor-intensive processes, often taking years.

  • Gel Electrophoresis as a Faster Method:

    • Allows rapid testing for gene presence.

Southern Blotting Technique

  • Developed by Ed Southern for DNA analysis; involves transferring DNA from a gel to a membrane for hybridization studies.

PCR Technique (Polymerase Chain Reaction)

  • A rapid method to amplify DNA segments.

Outline of the PCR Process

  1. Steps:

    • Heat: DNA denaturation at 95^{ ext{o}} ext{C} to separate strands.

    • Anneal: Primers bind to template DNA.

    • Extend: DNA polymerase synthesizes new DNA strands.

  2. Repetition: Cycles of heating, annealing, and extending amplify the target DNA.

Components Required for PCR

  • Template DNA, DNA primers, nucleotides (NTPs), and DNA polymerase (preferably from thermophilic bacteria).

  • Magnesium ions as a necessary cofactor for polymerase activity.

Sanger Sequencing Method

Overview

  • Named after Frederick Sanger; the method employs dideoxynucleotides to terminate DNA strand elongation, enabling sequence determination.

Explanation of dideoxy Regulation

  • Dideoxynucleotides lack the 3'-OH group needed for elongation, thus halting synthesis upon incorporation into a replicating strand of DNA.

Sequencing Execution

  • Sanger's approach allows determination of DNA sequences using the relative ratios of dideoxy to standard nucleotides.

  • Results are visualized via polyacrylamide gel electrophoresis.

Closing Thoughts

  • Modern genetic analysis reveals that what was deemed "junk DNA" plays roles in genome regulation and expression.

  • Ethical implications concerning genetic testing highlight the necessity of proper protocols and awareness regarding data privacy.