Biotechnology Lecture: PCR, Cloning, and GMOs

Interplay of Biology and Technology

• Recursive Relationship: Biology and technology drive each other. Technology facilitates protein purification and a deeper understanding of DNA replication.

• Useful Enzymes: Scientific understanding of DNA replication has led to the discovery and application of specific enzymes:     * Helicase (rep protein): Unwinds the DNA double helix.     * ssDNA-binding protein: Stabilizes single-stranded DNA during replication.     * Primase (Primosome): Synthesizes RNA primers.     * DNA Polymerase III: Involved in synthesizing new DNA strands.     * DNA Polymerase I: Removes RNA primers and fills gaps.     * Ligase: Joins DNA fragments together.

• Technological Milestones: The discovery of these enzymes led to the development of the Polymerase Chain Reaction (PCR) and Recombinant DNA/Cloning, which in turn provide an even deeper understanding of biological processes.

DNA Hybridization and Polymerase Chain Reaction (PCR)

• Mechanism of DNA Hybridization:     1. Denaturation: Double-stranded DNA (dsDNA) is heated to separate into two single strands.     2. Reannealing: DNA strands pair back together upon cooling.

• PCR Components: To perform PCR, the following ingredients are required:     * Source DNA.     * Nucleotides: $ATP$, $GTP$, $TTP$, and $CTP$.     * Polymerase enzyme.     * Buffer solution.     * Primer pairs.

• The PCR Cycle:     1. Heat: To denature the DNA.     2. Cool: To allow primers to anneal (bind).     3. Extend: To allow the polymerase to synthesize new DNA.

• PCR Kinetics and Competition: A key question is why denatured dsDNA doesn't just reanneal to itself instead of binding to primers. Two reasons include:     * Size: Primers are much smaller and therefore faster, reaching binding sites more quickly.     * Concentration: Primers are present in vastly higher concentrations than the source DNA.

• Exponential Growth: Amplification follows an exponential pattern, resulting in approximately $2^{N+1}$ copies for $N$ cycles. For example:     * $1^{st}$ cycle: $4$ copies.     * $2^{nd}$ cycle: $8$ copies.     * $3^{rd}$ cycle: $16$ copies.     * $4^{th}$ cycle: $32$ copies.     * $5^{th}$ cycle: $64$ copies.     * By the $35^{th}$ cycle, there are approximately $68$ billion copies of the DNA.

Applications of PCR in Forensics and Paternity Testing

• General Applications:     * Sequencing specific regions of a patient's or suspect's genome.     * Paternity testing.     * Identifying the presence of a specific species in a sample.     * Broad-spectrum PCR ($16s$) to identify all species in a complex sample.

• Short Tandem Repeats (STRs):     * STRs exist throughout the human genome at approximately $700,000$ unique locations (loci).     * They consist of conserved sequences flanking a variable number of repeats ($n$). Example: $\text{CS1}-(AC)_n-\text{CS2}$. If $n=3$, the sequence is $\text{CS1}-ACACAC-\text{CS2}$.     * The number of alleles observed in a population varies by locus; some loci are highly variable.

• Inheritance Patterns: For each autosomal locus, an individual inherits one copy from the mother and one from the father.     * Example: You may have alleles with $n=4$ and $n=8$, while a sibling might have $n=6$ and $n=10$.     * PCR is used to amplify these loci and measure the length of the products to determine identity or biological relationship.

• Forensic Samples: DNA can be extracted from various sources found at crime scenes, including semen, blood, hair, and saliva on cigarette butts or envelopes.

Restriction Modification Systems and Cloning

• Bacterial Immunity: Bacteria protect themselves from foreign DNA using "Restriction Modification Systems."     * Restriction Enzymes: Selectively degrade foreign DNA at specific "restriction sites."     * Modification Enzymes: Methylate the bacterium's own DNA at the same sites to prevent self-destruction.

• Restriction Enzymes (Type II):     * Hundreds are commercially available; Type II is the most frequently used.     * Most are homodimers that recognize palindromic sequences of $4 \text{ to } 8$ nucleotides ($nt$).     * DNA Palindromes: A sequence that reads the same $5'$ to $3'$ on both complementary strands. Example: $5' \text{ GAATTC } 3'$ paired with $3' \text{ CTTAAG } 5'$.

• Cutting Types:     * Sticky Ends: Leave overhanging single-stranded ends (e.g., $EcoR1$ cutting $5' \dots G \downarrow AATTC \dots 3'$).     * Blunt Ends: Cut straight across both strands with no overhang.

DNA Cloning Steps and Vectors

• Cloning Process:     1. Obtain the foreign DNA of interest.     2. Obtain a "vector" (e.g., a plasmid).     3. Cut both the vector and foreign DNA, usually with the same restriction enzyme.     4. Ligate (join) the vector and foreign DNA together using ligase.     5. Transform the vector into a viable host cell (e.g., bacteria).     6. Select for cells containing the recombinant vector.

• Vector Definition: A DNA molecule into which foreign DNA can be inserted and which replicates in a host. They are often based on naturally occurring DNA pieces.

• Essential Vector Features:     1. Multiple Cloning Site (MCS): A region with several restriction sites for DNA insertion.     2. Origin of Replication: Features needed for replication in the host.     3. Selectable Markers: Features used to identify successful transformation and insertion.

• Selection Process:     * Antibiotic Resistance: (e.g., ampicillin) Eliminates cells that did not take up the vector. Non-transformed cells are killed.     * Screening (e.g., X-gal): Used to determine if the vector is "empty" or contains the DNA insert. This helps distinguish recombinant DNA from original vector sequences.

Types and Uses of Vectors

• Common Vector Types:     * Plasmids: Replicated in bacteria (typically $E. coli$); hold foreign DNA up to $20\text{ kb}$.     * Bacterial Artificial Chromosomes (BACs): Typical capacity of $100\text{ to } 200\text{ kb}$; can hold up to $300\text{ kb}$.     * Yeast Artificial Chromosomes (YACs): Typical capacity of $200\text{ kb to } 1\text{ Mbp}$; can hold up to $5\text{ Mbp}$.     * Others: Mammalian artificial chromosomes, expression vectors (for protein production), and fusion protein vectors.

• Selection Criteria for Vectors:     * Ease of use (plasmids are easiest).     * Size of the desired insert.     * Copy number required per host cell.     * Need for protein expression or eukaryotic modifications (e.g., post-translational modifications).

The Biotechnology Industry and Cloned Products

• Historical Timeline:     * 1973: First recombinant DNA research.     * 1976: Genentech founded.     * 1980: The U.S. Supreme Court approves patenting life (oil-eating bacteria).     * 1982: Humulin (Human Insulin) becomes the first recombinant DNA drug approved for market.

• Notable Cloned Products:     * Human Insulin: (Humalog and Humulin by Eli Lilly).     * Recombinant Hepatitis B Vaccine: (Engerix-B by SmithKline Beecham).     * Epoetin alfa: (Epogen by Amgen).     * Blood-clotting Factor VIII: For Hemophilia A (Recombinate by Baxter Healthcare).

Understanding GMOs and Genetic Engineering

• Selective Breeding vs. Genetic Engineering:     * Selective Breeding: Humans have manipulated genetics for centuries (e.g., modern dog breeds, varieties of cabbage/broccoli/kale from wild mustard).     * Genetically Modified Organism (GMO): Broadly refers to anything changed by breeding, but specifically "natural" products like the coyote vs. domesticated dogs (e.g., Winston) highlight the distinction.     * Genetically Engineered (GE): Specifically refers to organisms containing proteins from a completely different species, something impossible via breeding.

• Nature's Engineer: Agrobacterium tumefaciens:     * Causes "crown gall" tumors in plants.     * Uses a Ti (Tumor Inducing) plasmid to inject T-DNA into plant cells via a pilus (type IV secretion system).     * T-DNA integrates into the plant genome to produce opines (food for the bacteria).     * Scientists use this by replacing opine genes with desired genes for plant transfer.

• Examples of GE Plants:     * Bt Crops: (Tobacco, corn, soy) Express a toxin from Bacillus thuringiensis that acts as an insecticide for moths/butterflies.     * Roundup Ready Crops: resistant to glyphosate (herbicide) which normally interferes with the ESPS synthase enzyme required for amino acid synthesis (tyrosine, tryptophan, phenylalanine).     * Golden Rice: Engineered to biosynthesize beta-carotene (Vitamin A precursor) in edible parts.

• Controversies and Risk Analysis:     * Concerns: Toxicity to humans, "off-target" hits, insect/plant resistance, allergic reactions, and patenting/ownership issues.     * Alternate Considerations: Safety of alternative pest controls, comparative yields, and environmental costs of herbicides vs. engineered solutions.     * Decision Making: Requires a complex "cost-risk-benefit analysis" considering all alternatives; there are no simple answers.