How can we pick a fragment of DNA from EtBr stained gel,  How would you pick the fragment you are interested in?
(a) By digesting with an enzyme, which will give you fragments of size that you are interested in
(b) By probing the gel with a piece of the sample DNA       (c)By digesting with multiple enzymes
(d)By performing a PCR

The correct answer is:

(a) By digesting with an enzyme, which will give you fragments of the size that you are interested in.

Explanation:

1. Restriction enzyme digestion is used to cut DNA at specific sequences, producing fragments of known sizes.
2. After digestion, agarose gel electrophoresis is performed to separate the fragments based on size.
3. The gel is stained with ethidium bromide (EtBr) (or other intercalating dyes like SYBR Safe) to visualize DNA under UV light.
4. To isolate a specific fragment, we:
   • Identify the band corresponding to the desired size.
   • Cut the gel at that location.
   • Extract the DNA using a gel extraction kit or other purification methods.

Why Not the Other Options?

• (b) Probing the gel with a piece of sample DNA → This is used in Southern blotting, not for picking a DNA fragment from a gel.
• (c) Digesting with multiple enzymes → This can help refine fragment selection but is not the method for physically picking a fragment from a gel.
• (d) Performing a PCR → PCR amplifies DNA but does not help in picking a DNA fragment from a gel.

Thus, option (a) is the correct answer.