Q.While isolating DNA from bacteria, which of the following enzymes is not required?

When isolating DNA from bacteria, you need to break open the cell wall and membrane, remove unwanted macromolecules, and preserve the DNA itself.

Lysozyme digests the bacterial cell wall (peptidoglycan), releasing the cell contents. Protease breaks down proteins that bind to DNA or form part of cellular structures. Ribonuclease degrades RNA, which would otherwise contaminate your DNA sample—all three are essential.

Deoxyribonuclease (DNase), however, is the enzyme that cuts DNA into fragments. Adding it during isolation would destroy the very molecule you're trying to extract. You want to avoid DNase contamination, not introduce it deliberately. …

When we isolate DNA from bacterial cells, we're essentially trying to break open the cell, remove everything that isn't DNA, and keep the DNA intact. The process requires careful dismantling of cellular structures and selective removal of contaminants, which is where enzymes come into play.

Bacteria have a tough peptidoglycan cell wall that protects them. Lysozyme is the enzyme that attacks this wall, breaking the glycosidic bonds in peptidoglycan and allowing us to breach the bacterial fortress. Without lysozyme, we can't even get inside the cell to access the DNA. This enzyme is absolutely essential in the first step of bacterial DNA isolation.

Once the cell is open, we face a mixture of DNA, RNA, and proteins all tangled together in the cellular lysate. Ribonuclease (RNase) becomes crucial here because RNA is a major contaminant that can interfere with downstream applications. RNase specifically degrades RNA molecules into smaller nucleotides, clearing them away while leaving DNA untouched. This selective action makes it indispensable for obtaining pure DNA. …

Alternative Approach: Match Each Enzyme to Its Step in the Isolation Protocol

Instead of listing "what each enzyme does," walk through the actual DNA-isolation

protocol in order and slot each named enzyme into its stage -- this makes the "odd one

out" obvious rather than something to memorise separately.

Step 1 -- Breaching the cell wall: bacterial cells are protected by a peptidoglycan

wall. Lysozyme digests this wall, letting the cell's contents spill out. This stage

needs lysozyme; there is no substitute among the options.

Step 2 -- Clearing RNA contamination: the resulting lysate is a mixture of DNA, RNA,

and protein. Ribonuclease (RNase) selectively degrades the RNA, leaving DNA intact,

so it does not contaminate the final DNA preparation.

Step 3 -- Clearing protein contamination: DNA-binding proteins (e.g. histones,

structural proteins) are still tangled with the DNA. Protease digests these

proteins into amino acids, freeing the DNA from protein complexes.

Step 4 -- Check the fourth option against the protocol's actual GOAL.

The stated goal of every step so far is to strip away everything except DNA, while …