9.1 Genetic Engineering Techniques

Creating Recombinant DNA

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Recombinant DNA Technology

Recombinant DNA is a DNA molecule assembled from pieces originating in two or more different sources. The whole point is to move a gene of interest from one organism into another so you can study it, mass-produce its protein, or alter an organism's traits.

Gene cloning is the process of isolating a specific gene and making many identical copies of it. The standard approach uses plasmids, which are small, circular DNA molecules found naturally in bacteria. Plasmids act as vectors, carrying foreign DNA into a host cell (usually E. coli) where it can be replicated every time the bacterium divides.

A classic real-world example: the gene for human insulin was inserted into a bacterial plasmid, allowing bacteria to produce insulin at scale. Before this, insulin for diabetic patients had to be extracted from pig or cow pancreases.

Restriction Enzymes and Plasmids

Here's how a recombinant plasmid is actually built, step by step:

1. Choose a restriction enzyme. Restriction enzymes are proteins that recognize specific short DNA sequences (usually 4–8 base pairs long) and cut the double helix at those sites. For example, EcoRI recognizes the sequence GAATTC.
2. Cut both the plasmid and the donor DNA with the same restriction enzyme. This produces matching sticky ends, which are short single-stranded overhangs that can base-pair with complementary sequences.
3. Mix the cut plasmid and the DNA fragment together. The complementary sticky ends on the plasmid and the insert will hydrogen-bond to each other, forming a circular recombinant molecule.
4. Seal the backbone with DNA ligase. DNA ligase forms covalent bonds across the nicks in the sugar-phosphate backbone, producing a stable, continuous recombinant plasmid.
5. Transform the plasmid into bacteria. The host bacteria take up the recombinant plasmid and begin replicating it along with their own DNA.

DNA Amplification and Analysis

Polymerase Chain Reaction (PCR)

PCR lets you take a tiny amount of DNA and amplify a specific target sequence into billions of copies within a few hours. It's used in forensics, disease diagnosis, paternity testing, and research whenever you need more DNA than you started with.

What you need for PCR:

• A DNA template (the sample containing your target sequence)
• Two primers (short synthetic DNA sequences complementary to the flanking regions of the target)
• Taq DNA polymerase (a heat-stable DNA polymerase originally isolated from the thermophilic bacterium Thermus aquaticus)
• Free nucleotides (dNTPs) to build the new strands

The three steps of each PCR cycle:

1. Denaturation (~95°C): Heat separates the two DNA strands by breaking hydrogen bonds.
2. Annealing (~55–65°C): The temperature drops so primers can bind to their complementary sequences on the template strands.
3. Extension (~72°C): Taq polymerase synthesizes new DNA strands by adding nucleotides to the primers, moving 5' to 3'.

These three steps repeat 25–35 times in a thermal cycler. Because each cycle doubles the target DNA, the amount increases exponentially: after 30 cycles, you have roughly $2^{30}$ (about 1 billion) copies.

Gel Electrophoresis and DNA Sequencing

Gel electrophoresis separates DNA fragments by size so you can visualize and compare them.

1. DNA samples are loaded into wells at one end of an agarose gel.
2. An electric current is applied. DNA is negatively charged (due to its phosphate groups), so fragments migrate toward the positive electrode.
3. Smaller fragments move faster through the gel's pores; larger fragments lag behind.
4. The result is a pattern of bands. You estimate fragment sizes by running a DNA ladder (a set of fragments with known sizes) alongside your samples.

Gel electrophoresis is the basis of DNA fingerprinting, where restriction enzymes cut an individual's DNA at variable sites, producing a unique banding pattern.

DNA Sequencing determines the exact order of nucleotides (A, T, G, C) in a DNA molecule.

• Sanger sequencing uses modified nucleotides called dideoxynucleotides (ddNTPs) that lack the 3'-OH group needed to add the next nucleotide. When a ddNTP is incorporated, the growing chain terminates. Running the reaction with all four ddNTPs (each labeled with a different fluorescent dye) produces fragments of every possible length. These fragments are separated by capillary electrophoresis, and a detector reads the fluorescent color of each terminal base to reconstruct the sequence.
• Next-generation sequencing (NGS) technologies (such as Illumina) can sequence millions of fragments simultaneously, making whole-genome sequencing far faster and cheaper than Sanger methods. This is what made projects like the Human Genome Project practical to complete.

Gene Editing and Applications

CRISPR-Cas9 Gene Editing

CRISPR-Cas9 is a gene-editing system borrowed from bacteria, where it originally served as an adaptive immune defense against viruses. It has become the most widely used gene-editing tool because it's relatively fast, cheap, and precise compared to older methods.

How CRISPR-Cas9 works:

1. A guide RNA (gRNA) is designed to be complementary to the target DNA sequence you want to edit.
2. The gRNA binds to the Cas9 protein (an endonuclease) and directs it to the matching location in the genome.
3. Cas9 creates a double-strand break (DSB) at the target site.
4. The cell's own repair machinery fixes the break through one of two pathways:
   • Non-homologous end joining (NHEJ): The broken ends are glued back together, but the process is error-prone and often introduces small insertions or deletions (indels). This can disrupt or knock out the target gene.
   • Homology-directed repair (HDR): If you supply a DNA template with the desired edit, the cell can use it as a blueprint to make a precise change, such as correcting a single-base mutation.

Applications include studying gene function by knocking out specific genes, creating animal models of human diseases, and developing potential therapies for genetic disorders like sickle cell disease.

Gene Therapy and Transgenic Organisms

Gene therapy aims to treat disease by delivering a functional copy of a gene into a patient's cells to compensate for a defective version.

• Viral vectors are the most common delivery method. Viruses like adenoviruses and retroviruses are modified so they can't cause disease but can still insert DNA into host cells.
• Conditions being targeted include sickle cell anemia (caused by a single-base mutation in the hemoglobin gene) and cystic fibrosis (caused by mutations in the CFTR gene that affect chloride ion transport).
• In 2023, the FDA approved the first CRISPR-based gene therapy (Casgevy) for sickle cell disease, marking a major milestone.

Transgenic organisms contain DNA from another species integrated into their genome.

• Transgenic plants: Bt corn carries a gene from the bacterium Bacillus thuringiensis that produces a protein toxic to certain insect pests, reducing the need for chemical pesticides. Golden Rice was engineered to produce beta-carotene (a vitamin A precursor) to address vitamin A deficiency in developing countries.
• Transgenic animals: The Harvard Oncomouse was engineered to carry an activated oncogene, making it prone to cancer and useful as a research model. Other transgenic animals have been developed to produce human proteins (like clotting factors) in their milk, a process called pharming.