In gel electrophoresis the separated DNA fragments can be visualised:

In UV radiation after staining with ethidium bromide

The correct answer is Option (4) - In UV radiation after staining with ethidium bromide

When DNA is cut into fragments by restriction endonucleases, these fragments can be separated using a technique called gel electrophoresis. Because DNA fragments are negatively charged molecules, they can be moved towards the anode (positively charged electrode) when subjected to an electric field within a gel-like matrix. This electric field causes the DNA fragments to migrate through the gel.

To visualize the separated DNA fragments, a specific compound called ethidium bromide is used for staining. After the gel electrophoresis process, the gel is treated with ethidium bromide, and then it is exposed to UV radiation. This step is crucial because pure DNA fragments are not visible to the naked eye under normal visible light and without staining.

When the ethidium bromide-stained gel is exposed to UV light, the DNA fragments appear as bright orange-colored bands. This staining and visualization process enables scientists to analyze and study the size and distribution of DNA fragments accurately, which is essential for various molecular biology and genetic research applications.