Chapter 3: Cloning Vectors and Restriction Enzymes Study Guide

Discovery and Foundations of Restriction Endonucleases

The Cohen and Boyer Experiment (1973–1976): * Known as the "Copy & Paste" era of molecular biology. * In 1973, Stanley Cohen and Herbert Boyer perfected genetic engineering techniques to cut and paste DNA using restriction enzymes. * The process involved removing a plasmid from a bacterium, treating it with the restriction enzyme EcoRI to cut the DNA, and leaving "sticky ends." * 1976 marked the first successful expression of a human gene within a bacteria.
Definition and Biochemical Activity: * Restriction enzymes are classified as endonucleases, meaning they cut within the DNA molecule. * Their biochemical activity is the hydrolysis ("digestion") of the phosphodiester backbone at specific sites in a DNA sequence. * The term "specific" indicates that an enzyme will only digest a DNA molecule after locating a particular recognition sequence.
Historical Role and Origin: * Discovered in the late 1960s by Linn and Arber in E. coli (specifically named EcoB). * The natural biological role of these enzymes is to prevent invasion by foreign DNA, such as viruses, by cutting the invading DNA into pieces. * Unlike exonucleases that chew DNA from the ends, endonucleases cut at sites within the foreign DNA. * They are described as "finely honed molecular knives."

Nomenclature and Classification of Restriction Endonucleases

Naming Conventions: * The names are derived from the Latin name of the source microorganism, using the first three letters. * First Letter: Derived from the Genus (e.g., "H" from Haemophilus). * Next Two Letters: The first two letters of the Species name (e.g., "in" from influenzae). * Strain Designation: Sometimes included (e.g., "d" from strain Rd). * Roman Numerals: Indicate the order of discovery if a microorganism produces multiple enzymes. If only one is produced, it ends with "I" (e.g., Hind I). If more are produced, they are numbered sequentially (II, III, IV, etc.).
Examples of Nomenclature: * EcoRI: Derived from Escherichia coli. * BamHI: Derived from Bacillus amyloliquifaciens. * Sma I: Derived from Serratia marcescens. * Mlu I: Derived from Micrococcus luteus. * Hpa I: Derived from Haemophilus parainfluenzae. * Taq Polymerase: Specifically from Thermophilus aquaticus or Thermus aquaticus.

DNA Recognition Sequences and End Products

Specificity and Palindromes: * Restriction endonucleases recognize specific palindromic sequences (sequences that read the same in forward and reverse directions). * They typically recognize 4-bp, 6-bp, or 8-bp sequences. * Cutting Frequency: * Four-base cutters occur more frequently: $4^4 = 256\,\text{bp}$ . * Six-base cutters: $4^6 = 4096\,\text{bp}$ . * Eight-base cutters (e.g., NotI): $4^8 \approx 65000\,\text{bp}$ . * The frequency of cuts decreases as the length of the recognition sequence increases.
Types of Produced Ends: * Cohesive Ends ("Sticky Ends"): Staggered cuts leave single-stranded overhangs that can hydrogen bond to compatible complementary strands. * 5' Overhang: The 5' phosphate groups are exposed/protruding, and the 3' hydroxyl groups are recessed. Example: BamHI (G^GATCC). * 3' Overhang: The 3' hydroxyl groups are protruding. Example: SacI (GAGCT^C). * Blunt Ends ("Flush Ends"): DNA ends that line up evenly with no overhangs. Example: SmaI (CCC^GGG).
EcoRI Specifics: * Cleaves double-stranded DNA at the hexameric sequence GAATTC: * 5' …NNNNGAATTCNNNN… 3' * 3' …NNNNCTTAAGNNNN… 5' * Cleavage occurs at the G residue on each strand, leaving 5'-overhanging sticky ends.

The Restriction-Modification (R-M) System

Host DNA Protection: * Bacteria protect their own DNA from digestion by their restriction enzymes using methylases. * Methylases recognize and methylate the same specific sites targeted by the restriction enzymes. * The paired enzyme system (Restriction Endonuclease + Methylase) is known as the R-M system. * Methylation protects the DNA; after replication, the parental strand remains methylated, ensuring immediate protection.

Cloning Vectors: Types and Requirements

Fundamental Requirements: * Must be capable of replicating within a host cell. * Must have convenient restriction enzyme (RE) sites for inserting DNA of interest. * Must possess a selectable marker (e.g., antibiotic resistance) to identify host cells that received the recombinant DNA. * Should be small and easy to isolate.
Vector Classification by Insert Capacity: * Plasmids: Usually accommodate inserts < 10000\,\text{bp} ( $10\,\text{kb}$ ). * Lambda (\lambda) Phage: $10 - 15\,\text{kb}$ inserts. * Cosmids: Combination of plasmids and phage; around $45\,\text{kb}$ inserts. * Bacterial Artificial Chromosomes (BACs): Up to $300000\,\text{bp}$ ( $300\,\text{kb}$ ). * Yeast Artificial Chromosomes (YACs): Up to $1000000\,\text{bp}$ ( $1000\,\text{kb}$ ). * Human Artificial Chromosomes (HACs): Up to $20000000\,\text{bp}$ ( $20000\,\text{kb}$ ). * Expression Vectors: Specialized for expressing proteins.

Plasmid Vectors: pBR322 and pUC Series

General Plasmid Characteristics: * Small, circular, double-stranded extrachromosomal DNA. * Provide antibiotic resistance genes for selection. * Contain an origin of replication (ori), the site where DNA replication begins.
pBR322: * One of the first widely used E. coli cloning vectors, developed by Boyer and Cohen. * Size: $4361\,\text{bp}$ or $4362\,\text{bp}$ . * Features: Contains an origin of replication and genes for Ampicillin ( $amp^r$ ) and Tetracycline ( $tet^r$ ) resistance. * Cloning into specific sites (e.g., BamHI) can split the $tet^r$ gene, leading to insertional inactivation.
pUC Series: * Modern improvement where 40% of the DNA from pBR has been deleted. * Multiple Cloning Site (MCS): A cluster of restriction sites in one area, allowing directional cloning. * Blue-White Screening: * Uses the lacZ gene to produce $\beta$ -galactosidase. * Active $\beta$ -galactosidase cleaves X-gal in the media to produce blue colonies. * Inserting foreign DNA into the MCS disrupts the lacZ gene, preventing $\beta$ -galactosidase production. * Recombinant colonies (with inserts) appear white.

Bacteriophage and Hybrid Vectors

Lambda (\lambda) Phage Vectors: * Bacterial viruses that infect E. coli. * Life Cycles: Lytic (cell lysis and phage release) or Lysogenic (integration into host genome). * Cloning Process: The middle "stuffer" region of the phage genome is removed and replaced with foreign DNA ( $12 - 20\,\text{kb}$ ). * Efficiency: Phage vectors infect cells much more efficiently than plasmids transform them. * Plaques: Identifiable as clearings in a bacterial lawn where phage have killed the host bacteria.
Cosmids: * Hybrids of phages and plasmids. * Contain "cos" sites (cohesive ends of phage DNA) for packaging into $\lambda$ phage heads. * Contain a plasmid origin of replication. * Can accept large inserts ( $40 - 50\,\text{kb}$ ) because almost all $\lambda$ genes are removed.
M13 Phage Vectors: * Filamentous phage that produces single-stranded DNA (ssDNA). * Contains a Multiple Cloning Site and $\beta$ -galactosidase fragment. * Useful for DNA sequencing and site-directed mutagenesis.
Phagemids (e.g., pBluescript): * Vectors that combine features of plasmids and phages. * Contains the f1 origin of replication for ssDNA recovery. * Features two phage RNA polymerase promoters (T7 and T3) on either side of the MCS for efficient transcription.

Methodological Challenges and Solutions in Cloning

Vector Religation: * Problem: The vector closes on itself without the insert. * Solution: Treat the vector with alkaline phosphatase. This removes the 5' phosphates necessary for ligation, preventing the vector from self-ligating while allowing the insert (which has phosphates) to join.
Reverse Orientation: * Problem: The insert is cloned in the opposite direction than intended. * Solution: Directional Cloning. Use two different restriction enzymes in the MCS to ensure the insert only fits in one orientation.
Ligation Mechanism: * Phage T4 DNA ligase catalyzes the ATP-dependent joining of DNA fragments. * Byproducts include AMP and PPi.

Five Basic Steps in Gene Cloning

Isolation: Obtain the genomic DNA containing the target gene.
Digestion: Cut the target gene and the vector using restriction endonucleases to produce compatible ends.
Ligation: Join the target gene and vector using DNA ligase to create a recombinant vector.
Transformation: Introduce the recombinant vector into a host cell (e.g., E. coli DH5 $\alpha$ for sequencing or BL21DE3 for expression).
Screening: Identify positive transformants that contain the target gene.

Questions & Discussion

Practice Questions (AS/DEC 2018): * a) Construct a plasmid vector by highlighting characteristics that make them a good cloning vector (4 marks). * b) Compare the advancements of pUC series plasmids compared to pBR plasmids (4 marks). * c) Differentiate between cosmid and phagemid in relation to their plasmid and phage characteristics (4 marks).
Homework Assignment: * Search and compare information on BAC versus YAC. * Differentiate the characteristics of BAC (Bacterial Artificial Chromosomes) versus YAC (Yeast Artificial Chromosomes).