Molecular Cloning and Recombinant DNA

Molecular Cloning and Recombinant DNA

  • Learning Outcomes: Understand the principles and applications of recombinant DNA technology and the key steps involved in molecular cloning.

    • Explain the principles and applications of recombinant DNA technology.

    • Outline the key steps involved in molecular cloning.

    • Describe the mechanism, specificity, and experimental uses of restriction enzymes.

    • Explain the structural features of plasmids and their use as cloning vectors.

    • Recognize that molecular cloning can generate genomic and cDNA libraries, understanding the basic differences.

    • Differentiate plasmid vectors for various purposes: cloning plasmids for DNA amplification, expression plasmids for protein production.

    • Explain nucleic acid hybridization principles and detection methods using DNA probes.

Recombinant DNA Technology

  • Definition: Recombinant DNA technology manipulates sections of DNA, such as genes, allowing isolation and multiple copying (gene cloning) of each unique section.

  • Since multiple identical copies (clones) are produced from isolated sections or DNA fragments, this process is vital for further analysis and study.

DNA Cloning

  • Need and Methods:

    • Cut DNA into pieces with restriction enzymes to create fragments for cloning.

    • Copy DNA by inserting it into rapidly replicating organisms (e.g., bacteria, yeast).

Purpose of DNA Cloning

  • DNA serves as genetic material. Its isolation and manipulation allow for various applications:

    • Isolate and modify genes to understand their function.

    • Transition from mutant phenotype to gene sequence.

    • Manufacture proteins (e.g., insulin, factor VIII).

    • Generate vaccines.

    • Create transgenic plants or organisms.

    • Diagnose genetic diseases.

    • Develop gene therapy.

Molecular Cloning Process

  • Create large amounts of DNA fragments post-genomic reduction:

    • Insert DNA fragments (coding sequences) into a cloning vector (e.g., bacterial plasmid) to create a recombinant dna molecule.

    • gloning vector is a dna molecule that can be introduced into a simple/single cell organism and can self-replicate

    • Recombinant DNA molecules are introduced into biological systems for replication to make large numbers of copies of the dna molecule .

    • Identical copies produced are termed DNA or molecular clones.

Restriction Enzymes (RE)

  • Function: Cut DNA like molecular scissors, recognizing specific nucleotide sequences in DNA and cleaving the sugar-phosphate backbone.

  • Identification: First discovered in bacteria to defend against viral DNA.

Characteristics of Restriction Enzymes

  • each one recognises a specific dna sequence (recognition sequence)

  • Specific enzyme types recognize different nucleotide sequences, typically of length 4, 6, or 8 bases.

  • Recognition sequences are often palindromic — the same in both strands over a 5’→3’ direction.

  • Many enzymes create staggered cuts, producing single-strand cohesive ends, instrumental for ligation to matching ends.

Creating Recombinant DNA with Restriction Enzymes

  • DNA cut with the same restriction enzyme produces identical cohesive ends, allowing annealing via complementary base pairing (A=T, C≡G).

  • Following annealing, covalent joining occurs through DNA ligase.

  • restriction enzymes can be used to cute genomes to produce large numbers of DNA fragments, number and size of fragments depends on which restriction enzyme, size of genome and its composition (GC content)

Example Use of EcoRI

  • Recognition site underlies the cutting frequency in human genome (approximately every 4Kbp), creating roughly 780,000 fragments.

  • Enzymes with shorter recognition sites (e.g., AluI) cut more frequently, but genomic non-randomness affects cut frequency—higher GC content impacts some enzymes’ specificity.

Genomic Digest and Southern Blotting

  • Analysis of DNA fragments using agarose gel electrophoresis reveals fragmentation patterns, often appearing as a smear due to numerous cut-size variations.

  • Restriction enzyme mapping involves:

    • Cutting DNA with specific enzymes.

    • Size-separated DNA fragment analysis on agarose gel.

    • Estimating sizes by comparison against fragments of known sized (size marker) to deduce cut locations.

Summary of Restriction Enzymes

  • Act as molecular scissors recognizing and cutting specific nucleic acid sequences.

  • Essential for manipulating DNA, stemming from their bacterial protective roles.

Cloning with Plasmid Vectors

  • Definition of Plasmids: Small, circular DNA molecules in bacteria, capable of self-replication independent of the bacterial chromosome, often hosting beneficial genes like antibiotic resistance.

  • Used as vectors to introduce DNA fragments into host cells.

Features of Plasmid Vectors

  1. Origin of Replication (ori): Where DNA replication begins, allowing plasmids to replicate in host cells.

  2. Antibiotic Resistance Genes: Enable survival of bacteria with the plasmid in antibiotic presence (selective growth).

  3. Multiple Cloning Site (MCS): A specific area for inserting DNA fragments.

Cloning Mechanism

  • Insert DNA fragments into the MCS of plasmid vectors to create recombinant DNA molecules through:

    • Enzymatic digestion using restriction enzymes.

    • Ligation with DNA ligase to form recombinant DNA.

Amplification in Bacteria

  • Transformation allows E. coli to uptake individual recombinant plasmids.

  • With the origin of replication, multiple copies (100-200) of each plasmid are produced per bacterial cell during division.

  • Bacterial colonies retain and can amplify the original DNA fragment, enabling further analysis.

Identifying Recombinant Plasmids

  • Some plasmids disrupt lacZ genes within MCS; successful DNA insertion yields color changes during colony growth (e.g., white colonies for inserts vs. blue for non-recombinants, known as blue-white selection).

Applications for Cloned DNA

  • Mapping and sequencing specific genes.

  • Isolating genes for deeper studies.

  • Investigating gene structure and functions (regulatory regions, introns).

  • Genetic engineering across species.

  • Creating substantial quantities for experimental use.

  • Developing cDNA libraries or specific probes.

Genomic Libraries and Cloning

  • Genomic Libraries: Overlapping DNA fragments that represent entire genomes facilitate mapping, identification, and sequencing tasks.

Summary of DNA Cloning

  • Cloning enables producing identical DNA fragments by inserting them into rapidly replicating organisms, using plasmids as vectors with essential features for replication and insertion.

Gene Expression through Cloning

In Vitro Gene Expression

  • Cloning genes serves to produce proteins synthetically in labs for varied applications:

    • Therapeutic protein production (e.g., insulin).

    • Investigating protein functionalities.

    • Manufacturing large protein quantities.

    • Understanding mutation effects by expressing gene variants.

Expression Plasmids

  • Specialized vectors for producing protein from cloned genes, differing from basic vectors in additional regulatory sequences:

    1. Bacterial Expression Plasmids: Allow gene expression in bacteria.

    2. Eukaryotic Expression Plasmids: Enable expression in eukaryotic cells with required regulatory elements.

Comparison: Cloning Vector vs. Expression Vector

  • Cloning Vector: Maintained in bacteria, primarily for replication.

  • Expression Vector: Facilitates both replication and gene product extraction, including all the elements needed for regulating DNA expression during processing.

cDNA for Prokaryotic Expression

  • Purpose: Eukaryotic proteins expression in prokaryotes involves using mRNA converted to cDNA since bacteria cannot process introns from large eukaryotic genes.

  • Process of cDNA Creation:

    • Reverse transcription from mRNA to synthesize cDNA for cloning.

Tissue-Centric Gene Expression Analysis

Gene Expression in Multicellular Organisms

  • Different tissues express unique gene sets:

    • Heart vs. kidney cells have distinct gene expression patterns.

  • Collecting mRNA from tissues reveals differing gene expressions.

Making a cDNA Library

  • Isolate tissue mRNA, converting to cDNA and ligating into plasmids for bacterial transformation, enhancing gene representation diversity.

Library Composition and Differences

cDNA vs Genomic Libraries

  • Genomic Libraries: Comprehensive collections necessitating assembly of fragments.

  • cDNA Libraries: Focus only on expressed genes, reflecting tissue-specific profiles.

Applications in Therapeutics and Industry

  • DNA Cloning for Drug Production: Essential for creating therapeutic proteins like insulin, growth hormones, and various factors necessary for diseases.

    • Examples include using E. coli or CHO cells for producing various drugs, assessing treatment efficacy for conditions such as diabetes and cancers.