Q.Which of the given statements is correct in the context of visualizing DNA molecules separated by agarose gel electrophoresis?

The key here is understanding that DNA itself is colourless and transparent — you cannot see it directly in visible light, whether stained or unstained. Ethidium bromide is a dye that intercalates between the DNA bases, but it does not make DNA visible under ordinary light. Instead, ethidium bromide is a fluorescent dye: when you shine ultraviolet (UV) light on it, it absorbs that energy and emits visible orange-red light, which is what you actually see as glowing bands.

So, the correct statement is the one that pairs ethidium bromide staining with UV exposure.

• (A) and (B) are wrong because unstained DNA is invisible in any kind of light.
• (C) is wrong because even stained DNA does not show up in visible light — the dye needs UV excitation to fluoresce.
• (D) is exactly what happens in the lab: ethidium bromide stained DNA is visualised under UV light.

To understand why only one of these statements is correct, you need to picture what happens during agarose gel electrophoresis. DNA molecules are negatively charged, so when an electric current is passed through the gel, they move toward the positive electrode. The gel acts like a sieve — smaller fragments travel faster and farther, while larger ones lag behind. After the run, the DNA is separated into distinct bands, but those bands are completely invisible to the naked eye. The gel looks like a blank, translucent slab.

This is where staining comes in. The most common stain used in molecular biology labs is ethidium bromide (EtBr). This molecule intercalates — slides itself between — the stacked bases of the DNA double helix. Once bound, it becomes a fluorescent dye. But here is the crucial point: fluorescence only happens when the dye is excited by light of a specific wavelength. Ethidium bromide absorbs ultraviolet (UV) light (around 300 nm) and then emits visible orange-red light. So if you shine a UV lamp or a UV transilluminator on the stained gel, the DNA bands glow brightly against a dark background.

Now look at the options one by one.

Option (B) says "DNA can be seen without staining in visible light." That is also false, for the same reason. Even if you have a gel loaded with a lot of DNA, it remains invisible until stained.

Option (C) says "Ethidium bromide stained DNA can be seen in visible light." This is a common misunderstanding. Ethidium bromide itself is a reddish-orange powder, but when it binds to DNA, it does not become coloured in the usual sense. It only becomes fluorescent. Under a normal room light or a desk lamp, the stained DNA bands are still invisible. You need the UV source to make them glow.

Option (D) says "Ethidium bromide stained DNA can be seen under exposure to UV light." This is exactly correct. In every standard molecular biology lab, after electrophoresis, the gel is placed on a UV transilluminator, and the DNA bands appear as bright orange-red bands. This is the standard method described in NCERT textbooks for visualizing DNA fragments.

Alternative Approach: Reason From the Underlying Physical Principle, Not Memorised Facts

Rather than recalling "the answer is D" as a fact, derive it from one physical principle

that applies to any similar question about stains/dyes.

Step 1: State the principle.

A fluorescent dye does not simply "add colour" the way a visible-light dye (like

ink) does. It absorbs light of one specific (often invisible) wavelength and re-emits

light at a different, longer wavelength. If you never supply the correct excitation

wavelength, a fluorescent dye stays completely dark to the human eye, however much of it

is present.

Step 2: Apply this principle to ethidium bromide.

Ethidium bromide intercalates between DNA base pairs and is a fluorescent dye: it

absorbs ultraviolet (UV) light and emits visible orange-red light. Ordinary room light

or a desk lamp does not supply the UV excitation it needs, so stained DNA under normal

light looks exactly as invisible as unstained DNA.

Step 3: Test each option against the principle.

(a)/(b) "DNA seen in visible light" (stained or not) -- fails; DNA itself is

colourless/transparent regardless.

(c) "Stained DNA seen in visible light" -- fails the excitation-wavelength requirement;

ordinary light cannot trigger fluorescence.

(d) "Stained DNA seen under UV" -- matches the principle exactly: UV supplies the

correct excitation wavelength, and the dye fluoresces visibly.

Key Takeaway:

Whenever a question involves a fluorescent stain (ethidium bromide, GFP, DAPI, etc.),

check what wavelength of light it needs to be excited by before assuming it will just

"show up" under any light source.