Gene Cloning and Restriction Enzyme

Objective: To stably introduce and express the Huntington gene in bacterial cells after its amplification.

The Huntington gene has been amplified approximately one billion times.
Linear DNA cannot be directly inserted into bacterial cells due to degradation; it must be stably integrated for inheritance.

Plasmids are small, circular DNA molecules found in bacteria, which are utilized for cloning genes.
Vectors: A term often used interchangeably with plasmids in biotechnology, specifically referring to plasmids engineered for gene cloning.
- Cloning vector or cloning plasmid are common terms.
The goal is to clone the Huntington gene into a plasmid to facilitate its introduction into bacteria.

PUC19: A specific plasmid used for cloning genes.
- Size: Tens of thousands of bases (much smaller than a bacterial chromosome).
Plasmid map includes:
  - AMP R: Ampr gene for antibiotic resistance, discussed later.
  - Multiple Cloning Site (MCS): Located in red; contains various restriction sites for inserting genes.
  - Restriction Sites: Specific sites labeled with enzymes like HIND III, BAM HI, and ECO RI.
  - Origin of Replication: A region that allows for self-replication of the plasmid within the bacterial cells.

Mechanisms of DNA Acquisition: Bacteria obtain DNA via three known methods of horizontal gene transfer:
  1. Conjugation: Direct transfer through mating bridges between donor and recipient cells.
  2. Transformation: Uptake of naked DNA from the environment.
  3. Transduction: DNA transfer mediated by bacteriophages (viruses infecting bacteria).
Preferred Method: Transformation is often used in biotechnology due to its simplicity.
- Conjugation and transduction are more complex and less efficient as methods for gene transfer.

Linear Fragment: The Huntington gene is isolated and amplified as a linear fragment via PCR (Polymerase Chain Reaction).
Inserting Gene into Vector: A break must be induced in the circular plasmid to allow insertion of the linear gene fragment.
- Multiple Cloning Site (MCS): Has multiple restriction sites to create breaks for gene insertion.
Restriction Enzymes: Specific enzymes that cut double-stranded DNA at unique sequences, enabling gene insertion.
- Function: Protect bacteria from phage infections by cleaving unmethylated viral DNA, leaving bacterial DNA intact due to methylation.

Example of Restriction Enzyme (EcoRI):
- Recognizes the sequence GAA TTC and cuts to generate sticky ends, facilitating the recombination of DNA from different sources.
- Allows for the specific combination of the desired Huntington gene with the plasmid vector through complementary base pairing.

After cutting the vector and the Huntington gene with restriction enzymes:
- Ligation: The enzyme ligase is employed to seal the nicks in the sugar-phosphate backbone of the DNA, creating a complete, circular plasmid with the inserted gene.

Sequencing: A method used to confirm the accurate insertion of the Huntington gene into the plasmid.
- Dideoxy Sequencing: Uses dideoxynucleotides that terminate DNA strand elongation during PCR, producing fragments of different lengths that can be analyzed to read the sequence.
Comparison of the sequenced DNA against the known Huntington gene sequence is crucial to ensure no mutations have occurred during cloning.

Once verified, the cloned Huntington gene in the plasmid is introduced into E. Coli using the transformation method.

The next section will address methods for validating the presence of the Huntington gene in bacterial cells and alternatives to E. Coli for gene cloning in biotechnology.