# DBT JRF 2020 — Question 14

Exam: DBT JRF 2020 · Section: A · Topic: Molecular Biology · Difficulty: Easy

## Question

What is the frequency with which a 4 bp cutter will cut the DNA, assuming random distribution of bases in the genome?

## Options

- **A.** 1/64
- **B.** 1/254
- **C.** 1/4096
- **D.** 1/256

## Answer

Option D

## Explanation

For a restriction enzyme that recognises a specific $n$ -base sequence, assuming equal and random distribution of all 4 bases (A, T, G, C), the probability of finding that exact sequence at any given position is $\left(\dfrac{1}{4}\right)^n$ . For a 4-base cutter: $\left(\dfrac{1}{4}\right)^4 = \dfrac{1}{256}$ . This means the enzyme cuts on average once every 256 bp. A 6-base cutter cuts every $4^6 = 4096$ bp, and an 8-base cutter every $4^8 = 65536$ bp. This is why rare-cutters (8-base) are used for pulsed-field gel electrophoresis of whole chromosomes.

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