Restriction endonucleases cleave foreign viral DNA via specific recognition sequences, while bacterial DNA containing those sequences is protected through methylation modification, not absence.
Correct Answer
(A) is correct but (R) is not correct.
Assertion Analysis
Assertion (A) states that restriction endonucleases cleave foreign viral DNA inside bacterial cells, but bacterial DNA is protected from attack. This is correct. Bacteria use restriction-modification (R-M) systems where endonucleases recognize palindromic sequences (4-8 bp) in foreign DNA like bacteriophages and cleave it, while their own DNA is shielded.
Reason Analysis
Reason (R) claims bacterial DNA lacks recognition sequences of the restriction endonuclease. This is incorrect. Bacterial DNA contains the same recognition sequences but is protected by methyltransferases (modification enzymes) that add methyl groups (—CH₃) to adenine/cytosine within those sequences, preventing cleavage. Foreign DNA lacks this methylation and is degraded.
Option Evaluation
| Option |
Evaluation |
| Both (A) and (R) correct; (R) explains (A) |
Incorrect; (R) wrongly states no recognition sequences exist in bacterial DNA . |
| Both correct but (R) not explanation |
Incorrect; (R) is factually wrong . |
| (A) correct, (R) incorrect |
Correct. Protection occurs via methylation of recognition sites, not their absence . |
| (A) incorrect, (R) correct |
Incorrect; both (A) and bacterial DNA protection are true . |
Introduction to Restriction Endonuclease Action
Restriction endonuclease cleave foreign viral DNA inside bacterial cells through specific recognition sequences, while bacterial DNA protection occurs via methylation modification. This R-M system is key for NEET molecular biology.
Mechanism of Foreign DNA Cleavage
Restriction endonucleases (e.g., EcoRI) recognize palindromic sequences like GAATTC and hydrolyze phosphodiester bonds, fragmenting unmethylated viral DNA. This prevents bacteriophage replication in bacteria.
Bacterial DNA Protection Strategy
Bacterial DNA contains identical recognition sequences but methylases add CH₃ groups (e.g., to adenine), blocking endonuclease binding. Foreign DNA enters unmethylated and gets cleaved.
Exam Relevance for Assertion-Reason
This question tests R-M system details: Assertion correct (cleavage vs. protection), Reason wrong (sequences present but modified). Common NEET trap assuming “no sequences”.
