Q.How are DNA fragments visualised during gel-electrophoresis ? What is elution ?

Concept: Visualisation of DNA fragments using a staining dye, followed by extraction (elution) from the gel.

DNA fragments separated by gel electrophoresis are not visible to the naked eye. To visualise them, the gel is stained with a fluorescent dye such as ethidium bromide (EtBr). This dye intercalates between the DNA base pairs and fluoresces bright orange-pink when exposed to ultraviolet (UV) light. The stained bands are then photographed under a UV transilluminator. …

The Concept: Why We Need to See and Then Extract DNA

Gel electrophoresis separates DNA fragments by size — smaller fragments move faster through the gel, larger ones lag behind. After the electric field has run for the right amount of time, you end up with a ladder of invisible bands. You cannot see DNA with the naked eye, so you need a way to make those bands visible. That is visualisation.

Once you have identified the band you want (say, a specific gene you have cut out with restriction enzymes), you need to get that pure DNA out of the gel so you can use it for further experiments — cloning, sequencing, or PCR. That physical removal and purification is elution.

Step-by-Step: Visualisation

1. Staining the gel with ethidium bromide

Ethidium bromide (EtBr) is a fluorescent dye that intercalates — it slides itself between the stacked base pairs of the DNA double helix. Once bound, it becomes much more fluorescent than when it is free in solution.

2. Exposing the gel to UV light

After staining (either by soaking the gel in an EtBr solution or by adding EtBr to the gel before it sets), you place the gel on a UV transilluminator. The UV light (typically 302 nm or 254 nm) is absorbed by the DNA-bound EtBr and re-emitted as visible orange-red light (around 590 nm).

3. Viewing and photographing the bands

You see bright orange bands against a dark background. Each band corresponds to a group of DNA fragments of the same size. A standard DNA ladder (a mixture of fragments of known sizes) is run alongside your samples so you can estimate the size of your unknown fragments.

Step-by-Step: Elution

Elution is the recovery of a specific DNA fragment from the gel. Here is how it is done in practice:

1. Cutting out the band

Under UV light, use a clean, sharp scalpel or a gel cutter to excise the gel slice containing your desired DNA band. Cut as close to the band as possible to minimise the amount of excess agarose.

2. Dissolving the gel slice …