Why should the scalpel be clean while cutting band in gel extraction?

Correct answer: To prevent DNA contamination

Gel extraction aims to recover a selected DNA fragment with as much purity and integrity as possible for a later step such as ligation, cloning, or amplification. A scalpel that has contacted another sample, old gel, dust, or biological material can transfer unwanted DNA into the excised gel slice. It may also introduce nucleases or other contaminants that damage the target DNA or interfere with downstream enzymes. Using a clean, suitable cutting tool and avoiding unnecessary gel mass reduces this risk and improves the reliability of the recovered fragment. The scalpel does not sweeten the gel, activate the restriction enzyme, or prepare buffer. Therefore preventing DNA and nuclease contamination makes A correct.