# DBT JRF 2020 — Question 39

Exam: DBT JRF 2020 · Section: A · Topic: Microbiology · Difficulty: Easy

## Question

Bacteria protect themselves from phages by producing the following enzymes which fragment the phage genome

## Options

- **A.** topoisomerases
- **B.** endonucleases
- **C.** methylases
- **D.** exonucleases

## Answer

Option B

## Explanation

Bacteria use the restriction-modification (R-M) system to defend against bacteriophage DNA. Restriction endonucleases recognise specific short palindromic sequences in foreign DNA and cleave both strands. The bacterium's own DNA at the same recognition sites is protected by methylation (by cognate methyltransferases) — methylated bases are not cut by the cognate restriction enzyme. This creates a molecular immune system that distinguishes 'self' from 'non-self' DNA. Common restriction enzymes (EcoRI, BamHI, HindIII) are derived from bacterial R-M systems and are essential tools in molecular biology.

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