Phage, Restriction Enzymes, and Recombinant DNA Technology
Lab Logistics and Schedule
Last lab session: Thursday night.
Lab reports due: May 8th (two weeks from this Thursday). - This allows completion before finals. Ensure that reports include detailed methodology, results, and thoughtful discussion. Properly cite all sources and adhere to the guidelines posted on Canvas.
Final exam: May 15th (one week after classes end). The exam will cover all lecture and lab material. Review lecture notes, lab protocols, and key concepts to prepare.
A group can use the same figures (gel photos), but each student writes their own unique report. Each report should offer a unique interpretation of the data and demonstrate individual understanding of the experiments.
Lab report guidelines are posted on Canvas. Review these guidelines carefully to ensure that all requirements are met. Pay close attention to formatting, citation style, and content expectations.
Lecture on Phage
Phage and Bacterial Interaction
Hershey-Chase experiment: Phage have a protein coat (outside cell) and a DNA genome (enters cell). This experiment demonstrated that DNA, not protein, is the genetic material that carries hereditary information. Radioactive isotopes were used to label DNA and protein, tracking their roles during phage infection.
Phage infection: Phage injects DNA into the bacterial host cell. This injection initiates a complex process that reprograms the host cell to produce more phage particles.
This process:
Shuts down host cell metabolism. The phage redirects the host cell's energy and resources to viral replication.
Degrades the host genome into pieces. Phage enzymes break down the host DNA to prevent competition with phage DNA replication.
Hijacks cellular machinery for phage synthesis (DNA replication, transcription, etc.). The phage utilizes host enzymes and ribosomes to synthesize new phage genomes and proteins.
Packages new viral particles called virions. Newly synthesized DNA is packaged into protein coats to form infectious virions.
Causes lysis (rupture) of the cell, releasing new phages. Lysis releases a burst of new phage particles that can infect other bacterial cells.
This cycle is called the lytic cycle. The lytic cycle is characterized by rapid phage replication and host cell destruction.
Generalized Transduction
During the lytic cycle, host genome fragments can be mistakenly incorporated into new phages. - These phage particles can transfer fragments of the original host genome to a new recipient cell. This occurs when phage enzymes mistakenly package bacterial DNA fragments into phage particles.
This process is called generalized transduction, a mechanism of horizontal gene transfer. Generalized transduction allows for the transfer of any part of the bacterial genome to a new host.
The transferred DNA fragment, although linear, can be incorporated into the recipient's genome via homologous recombination, potentially complementing lost functions. The transferred DNA must be integrated into the host chromosome to be stably inherited. Homologous recombination requires significant sequence similarity between the transferred DNA and the host genome.
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Bacterial Defense Mechanisms: Restriction Enzymes
Bacteria can defend against phage by degrading the viral genome. This defense mechanism is crucial for bacterial survival against phage infections.
Discovery (1940s-1960s): Scientists observed that some bacteria strains were resistant to phage infection. - Resistant bacteria chop the phage genome into pieces. This resistance is due to the presence of restriction enzymes that cleave foreign DNA.
Arber's Hypothesis: Phage resistant bacteria also have an enzyme that prevents degradation of their own DNA. - He proposed restriction enzymes (endonucleases) cut phage DNA and modification enzymes (methyltransferases) protect bacterial DNA by acting on the same recognition sequence. This hypothesis explained how bacteria can distinguish between their own DNA and foreign DNA.
Restriction and Modification Enzymes
Restriction enzymes (endonucleases): Restrict phage infection by cutting DNA into discrete fragments. These enzymes recognize specific DNA sequences and cleave the DNA at or near those sites.
Modification enzymes (methyltransferases): Protect bacterial DNA from restriction enzymes by methylating a specific recognition sequence. Methylation alters the DNA sequence to prevent restriction enzyme binding.
The restriction enzyme and modification enzyme act on the same recognition sequence. This ensures that the bacterial DNA is protected from its own restriction enzymes.
Methylation of th e recognition site protects the bacterial DNA from being cut by its own restriction enzyme. The methyltransferase modifies the bacterial DNA, preventing self-cleavage.
The restriction endonuclease must act faster than the methyltransferase on the incoming phage DNA to be effective. If the restriction enzyme acts quickly enough, it can cleave the phage DNA before it is methylated.
Some phages also express methyltransferases, providing resistance to bacterial defense mechanisms. This is a counter-defense mechanism employed by phages to evade bacterial restriction enzymes.
Characterization of Restriction Sites
1970: Hamilton Smith characterized the first recognition sequence for a restriction site: . Y stands for pyrimidine, R stands for purine. This sequence is a palindrome, meaning it reads the same forward and backward on opposite strands.
The enzyme, HindII, cuts in the center of this six-base pair palindromic sequence. The precise cleavage site is within the palindromic sequence.
The sequence is small; Therefore, it can happen often. The short recognition sequence means that HindII will cut DNA frequently.
HindII cuts the unmethylated sequence in phage DNA, but not the methylated sequence in the Haemophilus influenzae genome. This specificity protects the bacterial genome from self-cleavage.
Gel Electrophoresis
Dan Nathan and Kathleen Dannath used HindII to cut the SV40 mammalian virus genome and ran the fragments on gel electrophoresis in 1971. Gel electrophoresis is a fundamental technique in molecular biology for separating DNA fragments based on size.
Electrophoresis: Separates molecules in an electric field based on charge and size. Charged molecules migrate through the gel matrix when an electric field is applied.
Gel electrophoresis uses a gel (polyacrylamide or agarose) as a sieve. The gel matrix provides resistance to the movement of DNA fragments, allowing for separation based on size.
DNA fragments have the same negative charge due to phosphate groups, so separation is based on size. The phosphate backbone of DNA gives it a consistent negative charge.
Smaller fragments migrate faster through the gel. Smaller fragments experience less resistance from the gel matrix and migrate more quickly.
The SV40 genome was cut into 11 different sized fragments, creating a restriction digest. The unique pattern of fragments produced by restriction enzyme digestion is characteristic of the DNA molecule.
Restriction Enzymes: Revolutionizing Molecular Biology
Restriction enzymes allow scientists to manipulate DNA by cutting and reattaching different DNA sequences. This capability is essential for many molecular biology techniques, including gene cloning and recombinant DNA technology.
Digesting two different DNA molecules (e.g., bacterial plasmid and Drosophila genome) with the same restriction enzyme generates compatible sticky ends that can be ligated together using DNA ligase. Sticky ends are short, single-stranded overhangs that can base-pair with complementary sequences.
This is the basis of recombinant DNA technology. Recombinant DNA technology involves combining DNA from different sources to create novel genetic constructs.
Paul Berg first accomplished it in 1972 and won the Nobel Prize in 1980 for this method. Berg's pioneering work laid the foundation for modern genetic engineering.
Gene Cloning
Scientists can isolate a piece of DNA carrying a gene of interest by cutting the genome with a restriction enzyme. This allows for the targeted isolation of specific genes from complex genomes.
A bacterial plasmid can be cut with the same enzyme, allowing the insertion of the eukaryotic gene into the plasmid. The plasmid serves as a vector for carrying the gene of interest into bacterial cells.
The recombinant plasmid is transformed into bacterial cells, which are then cloned. Transformation introduces the recombinant plasmid into bacterial cells, which then replicate the plasmid.
Clones are replicates of cells with the eukaryotic gene. Each bacterial cell containing the plasmid produces numerous copies of the eukaryotic gene.
The plasmid replicates every time the bacterial cells divide. This ensures that the gene of interest is stably maintained and amplified.
The plasmid can be purified away from the rest of the genomic DNA from the bacteria. Plasmid purification allows for the isolation of the cloned gene from the bacterial genome.
Refined Cloning Techniques
Using two different restriction enzymes allows for directional cloning, controlling the orientation of the inserted DNA fragment. Directional cloning ensures that the gene of interest is inserted into the plasmid in the correct orientation for expression.
PCR can be used to amplify a piece of the eukaryotic genome before cloning, with engineered restriction sites on the PCR primers. PCR amplification increases the amount of DNA available for cloning and allows for the introduction of specific restriction sites.
Bioethical Concerns and Applications
Initial bioethical concerns arose with the development of recombinant DNA technology. These concerns included the potential for unintended consequences and the misuse of genetic engineering technologies.
Recombinant DNA technology has led to thousands of clinical and agricultural developments in the last 50 years. The benefits of recombinant DNA technology have been widespread and transformative.
Examples:
1982: Eli Lilly produced the first recombinant human insulin. This was a major breakthrough for treating diabetes.
Most drugs and vaccines are now made using this method. Recombinant DNA technology has revolutionized the pharmaceutical industry.
FDA-approved recombinant pharmaceuticals treat hundreds of conditions. These treatments have significantly improved human health.
Recombinant Protein Production
Recombinant DNA technology allows for the production of recombinant proteins. This enables the large-scale production of proteins for therapeutic, diagnostic, and industrial applications.
Bacterial plasmids can be engineered to promote transcription, allowing bacterial cells to produce large quantities of the recombinant protein. Strong promoters are used to drive high levels of gene expression in bacterial cells.
Agricultural applications: recombinant bovine somatotropin (rBST) used in milk production. rBST increases milk yield in dairy cows.
Eukaryotic cell systems (yeast, insect, or mammalian cells) can also be used for recombinant protein expression. Eukaryotic cells are often used for producing complex proteins that require post-translational modifications.
Physical Mapping and Genome Sequencing
Needham's lab showed that digesting the SV40 genome with multiple combinations of restriction enzymes could be used to deduce a physical map of the genome. Physical maps provide a framework for understanding the organization of the genome.
This led to the creation of physical maps of many different species' genomes, paving the way for the genomics era. Physical maps were essential for early genome sequencing efforts.
Arber, Smith, and Nathans were jointly awarded the Nobel Prize in 1978 for this discovery. This Nobel Prize recognized the transformative impact of restriction enzymes on molecular biology.
Even once genome sequencing started, genomes were prepared using restriction enzymes to digest the genomes into small chunks that were then cloned into bacterial plasmids and purified for sequencing. Restriction enzymes facilitated the fragmentation of genomes for sequencing.
Southern Blotting
Ed Southern developed Southern blotting to detect specific DNA fragments in restriction digests of large genomes. Southern blotting allows for the identification of specific DNA sequences within complex mixtures.
DNA fragments are transferred from an agarose gel onto nitrocellulose filter paper and then exposed to a labeled DNA probe. The probe hybridizes to complementary DNA fragments, which are then visualized. This process allows for the detection of specific DNA fragments based on their sequence.
Radioactive, fluorescent, or chemical tags are used to label the probe. These labels enable the detection of the hybridized probe.
This technique was used to discover oncogenes and develop DNA fingerprinting. Southern blotting played a crucial role in early cancer research and forensic science.
Modern Applications and Transgenesis
Southern blotting has largely been replaced by PCR, but the underlying principles are still relevant. PCR is a more rapid and sensitive technique for detecting specific DNA sequences.
Western blots: detect proteins.
Northern blots: detect RNAs. (same idea as Southern blots). These blotting techniques are used to analyze protein and RNA expression.
Inserting plasmids into eukaryotic cells creates transgenic organisms. Transgenesis allows for the introduction of new genes into eukaryotic organisms.
Transgenesis is used in lab animals (Drosophila, mice, etc.) and crop plants (genetically modified organisms or GMOs). Transgenic animals and plants are used for research and agricultural purposes.
GMOs are engineered with genes from different species to impart novel traits. GMOs can exhibit enhanced nutritional content, pest resistance, or herbicide tolerance.