Southern Blotting (Technical Review) Lim Wei Chun Southern Blot • Professor Sir Edwin Mellor Southern (1975): Detection of specific sequences among DNA fragments in a large, complex sample of DNA separated by gel electrophoresis. Function of Southern Blot • determine the molecular weight of a restriction fragment and to measure relative amounts in different samples • detect the presence of a particular bit of DNA in a sample • analyze the genetic patterns which appear in a person's DNA. • analyze restriction digestion fragmentation of DNA or a biological sample • definitive test to ensure that a particular section of DNA of known genetic sequence has been successfully incorporated into the genome of the host organism • detecting large gene rearrangements/deletions and large trinucleotide repeat expansions Overview of Southern Blotting Type of Membrane Nitrocellulose Nitrocellulose with Reinforcing Fleece FEATURE:Superior for chemiluminescence Suitable for repeated probing. detection using horseradish peroxidase. MEMBRANE TYPE: Nitrocellulose, 100% pure Nitrocellulose, reinforced Western, Southern, Northern blotting Western, Southern, Northern blotting 75-110 µg/cm2 0.45 µm 0.2 µm 0.1 µm APPLICATIONS: BINDING: PORE SIZES: TRANSFER METHODS Semi-dry Blotting Tank Blotting Vacuum Blotting Capillary Blotting Alkaline Method IMMOBILIZATION UV-crosslinking, DNA, RNA Baking (80 °C), DNA, RNA Drying, DNA, RNA Drying, Protein DETECTION METHODS Colorimetric Chemiluminescent Isotopic Fluorescent REPROBING ++ Recommended + Satisfactory Nylon Membrane Highly Positive charged Nylon Membrane For nucleic acid applications. Easier For nucleic acid applications. to block than highly positively charge Highest binding capacity available. membrane Nylon, moderately positively charged Southern, Northern blotting Nylon, highly positively charged Southern, Northern blotting 75-90 µg/cm2 0.45 µm 0.2 µm - >400 µg/cm2 0.45 µm 0.2 µm - >600 µg/cm2 0.45 µm - ++ ++ ++ ++ not recommended ++ ++ ++ ++ not recommended ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ + + ++ ++ + - + - ++ ++ ++ ++ limited ++ ++ ++ ++ + ++ ++ ++ + ++ ++ ++ Comparison of membrane • Nitrocellulose – high ionic strength – usually 10x SSC • Nylon membrane - able to bind DNA under a variety of conditions (acid, neutral, alkaline, high or low ionic strength), but a high-salt buffer such as 20× or 10×SSC appears to be beneficial • Positively charged nylon membrane – alkaline buffer enable covalent bonding with membrane but will provide high background if use chemiluminescent detection system. Not work as well with uncharged nylon membrane Handling of Membrane • Wear non-powdered gloved to avoid contamination • Cut membranes only with clean blunt-ended forceps Types of Southern Blotting Capillary blotting -Upward -Downward Semi-Dry blotting / Electroblotting Vacuum Blotting Capillary Blotting Upward capillary transfer Backward capillary transfer Vacuum Blotting (Example from Amersham) Semi-dry blotting An exploded view of the Trans-Blot SD cell: 1, safety lid; 2, cathode assembly with latches; 3, filter paper; 4, gel; 5, membrane; 6, filter paper; 7, spring-loaded anode platform, mounted on four guide posts; 8, power cables; 9, base. (Example from biorad) Comparison of Southern blot transfer method Capillary Transfer Advantage - High sensitivity - Economical Vacuum Blotting Semidry Blotting / Electroblotting Disadvantage - Consumes a relatively large amount of time (up to 12 hrs), buffer and blotting paper - Cannot transfer polyacrylamide gel - Less time and fewer solutions - Cannot transfer polyacrylamide are required as compared to the gel capillary method for blotting. - Extensive prudence has to be taken to avoid vacuum leak -Fastest to complete - Requires special care to prevent crushing or melting - Efficient method for of the agarose gel polyacrylamide gel transfer - Less sensitive Chemiluminescence Southern Blot Specimen Preparation Salt Extraction 5µg DNA restrict with restiction enzyme for overnight at 370C Restriction Endonuclease Digestion of genomic DNA Probe synthesis by normal PCR and labeling of probe with DIG using PCR Agarose Gel Electrophoresis Gel Denaturation, Neutralization, and Transfer Gel treated with HCl, blotted to Nylon membrane and baked at 800C DIG Labeling of the Probe Assessment Gel Direct dot Blot Assay Optimize the probegeneration protocol Probing Membranes with DIG-Labeled Probe Hybridize using DIG Easy Hyb Posthybridization Washes Serial dilution of matrix were spotted on membrane and incubate in Color Substrate Solution in dark. Assay development to optimize probe sensitivity Band Visualization Membrane treated with anti-DIG-Fab conjugated with alkaline phosphatase. Hydrolysis of CDPStar. Results are recorded by exposing the membrane to XAR Film. Agarose Gel Electrophoresis Agarose gel electrophoresis shows the separation of the DNA sample after incubated with restriction enzymes. Because of the high molecular weight, the gel look smear. Finishing Electrophoresis Depurination – done by soaking the gel in acid to cleavage the DNA (will improve the transfer of large pieces of DNA) Denaturation - Done by soaking the gel in base solution to denature the DNA rendering it single stranded and in a form suitable to hybridize Neutralization - Done by soaking in high salt solution or tris pH 7. Its function to raise the pH to enable the DNA to bind to the membrane. Duration of transfer -Depends on methods, DNA length and transfer buffer. -With a high-salt buffer, it takes about 18 hr to obtain acceptable transfer of a 15-kb molecule from a 5-mmthick 0.7% agarose gel; with the same gel 90% of the 1-kb molecules will be transferred in 2 hr. - After transfer is complete the DNA is permanently immobilized on the membrane support by drying at ~80°C or exposing to UV irradiation Radioactive probe The Random Hexamer Labeling Process produces a radioactive singlestranded DNA copy of both strands of the template for use as a probe. Nonradioactive probe – DIG probe This probe is easier to achieve because it just need double PCR to synthesis it with the usage of DIG-11-dUTP How probe works Assessment gel Direct dot blot assay 7µL DIG DNA 3:7 1:10 1:8 1:6 1:4 1:2 6µL 5µL 4µL 3µL 2µL Hybridization – The labeled probe is added to the blocked membrane in buffer and incubated for several hours to allow the probe molecules to find their targets. Washing – Excess probe will have bound nonspecifically to the membrane despite the blocking reagents. – Blot is incubated with wash buffers containing NaCl and detergent to wash away excess probe and reduce background. Blocking – Buffer binds to areas on the blot not occupied by patient DNA. – Blocks the empty sites from being bound during hybridization. Detection - labeled probes enable detection on film. Chemiluminescent detection – three step process First step membranes are treated with Blocking reagent to prevent nonspecific attraction of antibody to the membrane. Second step membranes are incubated with a dilution of anti-digoxigenin Fab fragments, which are conjugated to alkaline phosphatase. detection of digoxigenin labeled Third step compounds The membrane carrying the hybridized probe and bound antibody conjugate is reacted with CDP Star and exposed to X-ray film to record the chemiluminescent signal. Precaution on handling the exposure time • Make sure the dark room is free from any light source. • Make sure the working place is always dry in condition. Analysis Striping membrane Important precaution: Membranes should never be allowed to dry before stripping. Once dried, the membrane cannot be stripped and reprobed Troubleshooting Poor Signal - Probe specific activity too low Spotty background - Not enough target DNA - Probe concentration too low -- Unincorporated nucleotides Patchy or time generally Hybridization too short not removes from labeled probe high background -Insufficient blocking agents -Part of the membrane allowed to dry out during hybridization or washing -Not enough wash solution -Probe concentration too high -Probe not denatured Extra bands - Probe contains nonspecific Nonspecific sequences background in one - Target not completely or DNA moreistracks restriction digested Cannot remove probe -Probe too short after hybridization - Insufficient blocking agents - Membrane dried out after hybridization "Before PCR and cheap fast sequencing changed our view of the universe that is genetics, the Southern Blot was a universal workhorse. It is still a useful tool today and you need to know about it so that you can interpret historical data."
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