Q.How does one visualise DNA on an agarose gel?

After separating DNA fragments by gel electrophoresis, the DNA needs to be stained because it is colourless. A common stain used for this purpose is ethidium bromide. This compound intercalates into the DNA double helix. When the stained agarose gel is then exposed to ultraviolet (UV) radiation, the ethidium bromide-DNA complex fluoresces. This fluorescence allows the DNA fragments to be visualised as distinct bright o …

Agarose gel electrophoresis is a fundamental technique used to separate DNA fragments based on their size. After DNA is cut by restriction enzymes, the resulting fragments are loaded into wells of an agarose gel, which acts as a molecular sieve. When an electric current is applied, the negatively charged DNA fragments migrate towards the positive electrode. Smaller fragments move faster and further through the gel matrix than larger ones, resulting in a separation of DNA fragments into distinct bands.

However, DNA itself is a colourless molecule. This means that after the electrophoresis process, the separated DNA bands are not directly visible to the naked eye within the translucent agarose gel. To observe these bands and analyse the results of the separation, a specific visualisation method is required.

The standard method for visualising DNA on an agarose gel involves the use of a special staining agent.

• Staining with Ethidium Bromide (EtBr): The most common and effective way to visualise DNA is by staining the agarose gel with Ethidium Bromide (EtBr).
  • EtBr is a fluorescent dye that has the unique property of intercalating, or inserting itself, between the base pairs of the DNA double helix.
  • Once bound to DNA, EtBr undergoes a conformational change that allows it to fluoresce intensely when exposed to ultraviolet (UV) light.

Method 2 -- reason from the general mechanism of intercalating dyes, not just the specific protocol steps

Rather than listing the protocol steps in order, start from why any dye can make DNA visible at all: DNA itself absorbs UV light only weakly and does not fluoresce, so a separate reporter molecule is needed that (i) binds specifically and efficiently to DNA and (ii) fluoresces strongly when excited by UV light.

Ethidium bromide satisfies both requirements by intercalation -- its flat, ring-shaped structure slides in and stacks between adjacent base pairs of the double helix, much like a card slotting into the pages of a stacked deck. Once wedged between base pairs, illumination with UV light causes it to fluoresce a bright orange-pink -- far more brightly than the free dye in solution, so background (unbound dye in the gel) stays comparatively di …