Reverse Transcription PCR (RT-PCR) and Western Blotting

Advanced Cell & Molecular Biology - Reverse Transcription PCR (RT-PCR) and Western Blotting

Instructor Contact Information:

Dr. Adrian O'Hara

Email: A.W.OHara@ljmu.ac.uk

Learning Outcomes

• Understand the intricate process of reverse transcription, including enzyme roles and temperature sensitivity.

• Grasp the details of PCR (Polymerase Chain Reaction), emphasizing the specificity and efficiency of the method.

• Comprehend western blotting as a method for studying proteins, focusing on sample preparation, detection techniques, and relevance to disease understanding.

Molecular Biology Techniques

This section involves understanding key concepts in molecular biology and their relevance to genetics and biochemistry. Key questions include:

• What is the difference between alleles A and a? Demonstrates the principle of dominance in inheritance.

• How are different sequences inherited? Through Mendelian genetic principles including dominant and recessive traits.

• Why is allele A dominant? Focuses on the functional expression of the allele and resultant phenotypic characteristics.

• What is the phenotypic effect caused by different proteins? Relates protein function to observable traits in an organism.

• How do changes in DNA sequence affect protein structure and function? Discusses mutations and polymorphisms.

• What mechanisms produce different forms of a protein? Covers alternative splicing and post-translational modifications that enhance protein diversity.

References:

Molecular Biology and Biochemistry Education, Vol. 36, No. 1, pp. 77-84, 2008

Reverse Transcription Process

1. Tissue Preparation:

   • Collect and lyse cells from tissue samples (e.g., brain), ensuring proper sample handling to preserve RNA integrity.

   • Purify mRNA from the lysate using methods such as oligo(dT) or kit-based purification, to isolate high-quality mRNA suitable for downstream applications.

2. mRNA Hybridization:

   • Hybridize the purified mRNA with poly-T primer to facilitate the synthesis of cDNA, allowing accurate transcription of the expressed genes.

3. cDNA Synthesis:

   • Use high-fidelity reverse transcriptase to synthesize complementary DNA (cDNA) from the mRNA template, ensuring that the reverse transcriptase is active and free of contaminants.

   • RNA is degraded using RNAse H, a crucial step for the synthesis of double-stranded cDNA.

   • A second strand of cDNA is synthesized using DNA polymerase, with RNA fragments acting as effective primers, leading to a full cDNA copy ready for PCR amplification.

Polymerase Chain Reaction (PCR)

Overview of PCR

• Definition:
  
  The Polymerase Chain Reaction (PCR) is a powerful method used to amplify specific DNA sequences exponentially using a pair of synthetic oligonucleotide primers, complementary to the termini of the target sequence.

• Purpose:
  
  PCR enables the amplification of a single copy or a few copies of DNA to generate thousands to millions of copies of a desired segment, making it indispensable for various applications in molecular biology.

• Applications:

  • Biomedical Research: Essential for cloning, gene expression analysis, and genetic engineering.

  • Criminal Forensics: Used in DNA fingerprinting and identification.

  • Molecular Archaeology: Assists in the analysis of ancient DNA and other archaeological samples.

PCR Principles

• PCR involves amplifying a specific region of DNA, generally producing fragments between 0.1 to 10 kilo base pairs (kbp), with advanced techniques allowing segments up to 40 kbp, crucial for studying large genes.

• The quantity of the amplified product is dictated by the reaction substrates (DNA template and primers), which become limiting as the amplification progresses, indicating the need for optimization.

Essential Components of PCR

1. DNA Template

• The DNA template must contain the target sequence, which can be genomic (gDNA) or complementary DNA (cDNA), chosen based on experimental design.

• Quantity: Typically, ≤ 1 µg of DNA is required to ensure successful amplification without inhibition.

2. DNA Polymerase

• Enzyme Used:
  
  A thermostable DNA polymerase (e.g., Taq polymerase) catalyzes the template-dependent DNA synthesis, crucial for the high temperatures used in PCR.

• Performance:
  
  Notably, Taq polymerase has a synthesis rate of about 9000 bases in less than 10 seconds but lacks 5′ → 3′ proofreading activity, leading to a misincorporation rate of approximately 1 in 9000 nucleotides, which may necessitate the use of high-fidelity polymerases for critical applications.

3. Primers

• PCR requires a pair of synthetic oligonucleotides that are complementary to the 3' ends of both the sense and anti-sense strands of the target DNA, their length and specificity significantly influencing the success rate of the PCR.

• Primers are essential as DNA polymerase needs double-stranded regions to initiate elongation, and nonspecific binding should be minimized to increase yield and specificity.

4. dNTPs

• Short for deoxynucleoside triphosphates, standard PCR employs equal molar concentrations of dATP, dTTP, dGTP, and dCTP.

• Typical concentration in a reaction using Taq is 200-250 µM for each dNTP, yielding approximately 6 – 6.5 µg of synthesized DNA in a 50 µl reaction, assuming a 1.5 mM concentration of MgCl2.

5. Buffer Solution

• A buffer maintains the pH level of the reaction, typically Tris-HCl at pH 8.3-8.8, critical for optimal enzyme activity throughout the amplification cycles.

• The pH may drop to around 7.2 at 72°C during incubation, which can affect the performance of the polymerase.

6. Divalent Cations

• Essential for all thermostable DNA polymerases, particularly magnesium (Mg²⁺) or manganese (Mn²⁺), which play pivotal roles in enzymatic activity and stability.

• Their functions include:

  • Formation of complexes with dNTPs that act as substrates for synthesis.

  • Stabilization of primer-template complexes to enhance fidelity.

• Typical magnesium concentration is around 1.5 mM, while optimized higher concentrations (4.5 – 6 mM) may help to minimize non-specific priming, which is advantageous in high efficiency PCR protocols.

7. Monovalent Cations

• Standard PCR buffers contain 50 mM KCl, which is optimal for amplifying segments over approximately 500 base pairs, with increasing KCl concentration up to 70-100 mM enhancing yields of shorter fragments, aiding in both specificity and efficiency of amplification.

PCR Procedure

• The PCR process comprises several stages:

  • Denaturation: Heating to 94-96°C to ensure complete separation of the DNA strands, an essential first step for effective amplification.

  • Annealing: Cooling to around 68°C allows primers to efficiently bind to the targeted DNA sequence, the annealing temperature should be optimized based on primers' melting temperature.

  • Extension/Elongation: Raising the temperature to about 72°C for optimal DNA polymerase activity to synthesize the new complementary strand, duration is critical for amplifying longer segments.

  • Final Extension: This concluding step and subsequent hold reinstate enzymatic activity for any remaining single strands, generally repeating these cycles between 20 to 40 times ensures sufficient amplification.

• The formula to determine the number of target DNA copies formed after a specific number of cycles is given by $2^n$, where $n$ is the number of cycles. For instance, with 30 cycles: $2^{30} = 1,073,741,824$ copies of the original double-stranded DNA, exemplifying exponential amplification capabilities.

Protein Analysis - Western Blotting

Overview of Proteins

• Proteins are macromolecules composed of one or more long chains of amino acids, folded into specific structures that determine their functional roles in biological processes.

• Most proteins adopt unique three-dimensional conformations essential for their functionality, emphasizing the role of molecular chaperones in protein folding.

Methods for Studying Proteins

• The structure and activity of proteins can be assessed through several methodologies:

  • In vitro: Controlled experiments with purified proteins allow detailed mechanistic studies.

  • In vivo: Studies within living organisms enable the observation of physiological roles and interactions in their native environments.

  • In silico: Computational approaches provide insights into protein structures and dynamics, allowing predictions of function based on sequence data.

Protein Purification Techniques

• Proteins are typically fractionated through column chromatography, employing diverse strategies for separation based on:

  • Charge (ion-exchange chromatography).

  • Hydrophobicity (hydrophobic chromatography).

  • Size (gel-filtration chromatography).

  • Affinity for specific molecules (affinity chromatography), allowing for targeted isolation of proteins of interest.

Analyzing Proteins with SDS-PAGE and Western Blotting

• SDS-PAGE: Utilizes an electric field to separate proteins based on size and charge. Proteins are treated with sodium dodecyl sulfate (SDS), which denatures them and imparts a negative charge, allowing their migration through the gel matrix, where smaller proteins travel faster than larger ones.

• Western Blotting: This powerful method detects specific proteins after their separation via SDS-PAGE. The proteins are transferred onto a membrane, then probed with specific antibodies for visualization, which is critical for assessing protein expression levels and modifications in different conditions.

Western Blotting Workflow:

1. Sample Preparation: Ensure protein stability before analysis, adjusting concentrations for optimal detection.

2. Electrophoresis: Carry out SDS-PAGE for effective separation based on size.

3. Electrotransfer to membrane: Accurately transfer separated proteins to a nitrocellulose or PVDF membrane.

4. Probing and Washing: Includes a blocking step to prevent non-specific binding, using buffer solutions to enhance sensitivity.

5. Detection Methods: Employ a variety of detection modalities, including colorimetric, chemiluminescence, or fluorescence; secondary antibodies can enhance specificity and signal detection.

Example of Western Blot Results:

• Detection of proteins such as NFκB1 and GAPDH is noted, with varying molecular weights identified in control versus treated samples, displaying the importance of controls and replicates in experimental validation.

Summary

• Reverse transcription generates a cDNA copy from mRNA, which can be utilized for PCR to amplify genetic sequences of interest, central to gene expression studies.

• Key components for PCR include:

  • DNA Template

  • Primers

  • DNA Polymerase

  • dNTPs

  • Buffer solutions

  • Monovalent and Divalent Cations

• The PCR procedure involves cycles of denaturation, annealing, and extension, optimally repeated 20 to 40 times to achieve desired amplification levels.

• Proteins can be analyzed through advanced techniques like SDS-PAGE and Western blotting, utilizing properties of charge and size for effective isolation and characterization, essential for understanding biological functions and disease mechanisms.