Q.Which of the following statements does not hold true for restriction enzyme?

Restriction enzymes are bacterial defence tools, not viral. Let's check each statement:

• Palindromic sequence — True. Restriction enzymes recognise specific palindromic nucleotide sequences (read the same on both strands in the 5'→3' direction).
• Endonuclease — True. They cut DNA at specific sites within the strand, not from the ends (that would be an exonuclease).
• Isolated from viruses — False. Restriction enzymes are isolated from bacteria, where they protect the cell by cutting foreign (viral) DNA at these recognition sites. The bacterium's own DNA is protected by methylation. …

Restriction enzymes are one of the most elegant tools in molecular biology, and understanding them requires a clear picture of where they come from and what they actually do. The name itself gives a clue: these enzymes "restrict" the growth of invading viruses (bacteriophages) by chopping up their DNA. Bacteria produce them as a defence mechanism, and each restriction enzyme recognises a very specific, short DNA sequence — usually 4 to 8 base pairs long.

Now, what kind of sequence do they recognise? This is where the idea of a palindrome comes in. In everyday language, a palindrome is a word that reads the same forwards and backwards, like "MALAYALAM". In DNA, a palindromic sequence is one where the sequence on one strand reads the same as its complementary strand when both are read in the 5' to 3' direction. For example, the sequence GAATTC on one strand will have CTTAAG on the complementary strand — read 5' to 3', both are GAATTC. Restriction enzymes bind precisely to such sequences. So statement (A) is correct.

What about the type of enzyme? Restriction enzymes cut DNA at specific points within the DNA molecule, not at the ends. That makes them endonucleases — they break phosphodiester bonds internally. This is different from exonucleases, which nibble away at the ends. So statement (B) is also correct.

Now, statement (C) says restriction enzymes are isolated from viruses. This is where the trap lies. Restriction enzymes are produced by bacteria, not viruses. In fact, they are part of the bacterial restriction-modification system that protects the bacterial cell from viral DNA. The virus is the target, not the source. So this statement is false.

Method 2 -- define each key term first, then test the four statements against the definitions

Before touching the options, pin down two definitions:

• Endonuclease: cuts a nucleic acid strand within the molecule, at internal points.
• Palindromic sequence: a double-stranded DNA sequence that reads identically on both strands when each is read 5' to 3'.

Now run each option past these definitions plus one further fact (source organism):

• (a) "Recognises a palindromic sequence" -- matches the definition exactly. TRUE.
• (b) "It is an endonuclease" -- matches the definition exactly (cuts internally, not from the ends). TRUE.
• (c) "Isolated from viruses" -- restriction enzymes are a bacterial defence system against invading viruses; the bacterium is the source, the virus is the target. FALSE. …