Southern Blotting Technique Notes

Southern Blotting Technique

Overview

  • Southern blotting is a technique used to detect specific DNA sequences within DNA samples.

  • It provides information about DNA identity, size, and abundance.

Step 1: DNA Digestion with Restriction Enzymes

  • Restriction Enzymes (Restriction Endonucleases): Enzymes that cleave DNA at specific recognition sites (restriction sites).

  • The DNA samples are digested with an appropriate restriction enzyme.

  • Incubation occurs overnight at 37 degrees Celsius.

  • Restriction enzymes recognize specific nucleotide sequences and make double-stranded cuts in the DNA.

  • This process results in DNA fragments of varying sizes.

Step 2: Gel Electrophoresis

  • DNA fragments, generated by restriction enzyme digestion, are separated using gel electrophoresis.

  • Loading Buffer: Added to DNA samples; functions as a tracking dye to monitor separation progress.

  • Agarose Gel Electrophoresis: Commonly used to separate DNA fragments of different sizes.

  • DNA Ladder (Molecular Weight Size Marker): Used to determine the size of DNA fragments.

    • Loaded into a well at one end of the gel.

  • DNA samples are loaded into wells, and an electric current is applied.

  • DNA molecules migrate based on charge and size.

Principles of Electrophoresis

  • The phosphate backbone of DNA is negatively charged.

  • DNA fragments migrate towards the positively charged anode in an electric field.

  • Smaller fragments move faster through the gel than larger ones, because all DNA fragments have the same amount of charge per mass.

Visualization of DNA Fragments

  • After electrophoresis, molecules in the gel are stained to make them visible.

  • Ethidium Bromide: An intercalating dye used to stain DNA.

  • Under UV light, DNA fragments appear as bands, each representing same-sized DNA fragments.

Step 3: DNA Denaturation

  • Double-stranded DNA fragments are denatured using an alkaline solution (sodium hydroxide).

  • The gel is soaked in the alkaline solution with gentle agitation.

  • At alkaline pH, guanine and thymine are deprotonated.

  • The high concentration of hydroxide ions breaks hydrogen bonds, separating the two DNA strands.

  • After denaturation, the alkaline solution is removed.

Step 4: Neutralization

  • A neutralizing solution is used to neutralize the pH of the gel, which allows for efficient DNA transfer.

Step 5: Southern Transfer Setup

  • Components:

    • Transfer buffer

    • Solid support

    • Blotting paper (as a wick)

  • The wick is placed over the solid support with ends in the transfer buffer.

  • Extra thick blotting paper is placed on top of the wick and wetted with transfer solution.

  • The neutralized gel is placed on the wetted wicking paper.

  • A nylon or nitrocellulose membrane (pre-wetted) is placed on top of the gel.

Step 6: Transfer Process

  • Pre-wetted extra thick blotting paper is placed on top of the membrane.

  • Exposed areas of the wick are covered with plastic wrap to prevent buffer bypass.

  • A dry stack of paper towels is placed on top of the membrane and gel.

  • A glass plate with a weight is added to maintain tight contact.

  • Capillary action transfers DNA from the gel onto the membrane.

    • Buffer moves from high to low water potential.

  • Ion Exchange Interactions: DNA (negatively charged) binds to the membrane (positively charged).

Step 7: Completion of Transfer and DNA Fixation

  • The transfer proceeds overnight.

  • Blotting material and membrane are carefully removed.

  • The membrane is briefly rinsed to remove agarose.

  • UV Radiation: Used to permanently attach the transferred DNA to the membrane.

Step 8: Hybridization with Labeled DNA Probes

  • The membrane is placed in a bottle with a pre-hybridization solution.

    • Pre-hybridization Solution: Reduces nonspecific hybridization with the probe.

  • Incubation occurs in a hybridization oven at 42 degrees Celsius for two hours.

  • The pre-hybridization solution is removed, and hybridization buffer is added.

  • Labeled DNA probes are added to the hybridization solution.

    • DNA Probes: Fragments of DNA with variable length, labeled radioactively or fluorescently.

  • The bottle is incubated overnight at 42 degrees Celsius.

Role of Phosphorus-32

  • DNA contains phosphorus in phosphodiester linkages.

  • Radioactive phosphorus ^{32}P is used to track DNA.

  • ^{32}P labeled DNA probes hybridize to complementary sequences in the DNA fragments.

  • After hybridization, the hybridization solution is removed.

Step 9: Washing

  • Wash buffer is added to the bottle.

  • The membrane is incubated at 52 degrees Celsius for thirty minutes.

  • The washing process is repeated three times to remove unbound probe.

Step 10: Autoradiography

  • Used to identify the location of radioactive DNA on the membrane.

  • The Southern blot filter is placed inside a light-proof cassette box.

  • An X-ray film is laid over the top.

  • The cassette is closed and left for several hours or days.

  • Radioisotope-labeled DNA exposes the film, creating black bands that indicate the positions of labeled DNA.

Applications

  • Determine which gene is present in each sample.

  • Identify the position of each gene in the gel.

  • Isolate specific genes of interest for further analysis (by cutting out bands from the gel after electrophoresis).