GATE 2021 BT – Question 64

Recombinant DNA Technology and Other Tools in Biotechnology · Recombinant DNA technology: Restriction and modification enzymes, vectors, expression vectors, libraries, gene isolation and cloning, transposons, gene targeting · 2 marks · Numerical answer

The possible number of SalI restriction sites in 9 kb double-stranded DNA with equiprobable bases (nearest integer) is _______.

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Correct answer: 2

Explanation

**Recognition site.** SalI recognises a specific **6-base-pair** sequence (G^TCGAC).

**Probability of a given site at any position.** If the four bases are equally likely, the chance that a particular 6-mer occurs at a given position is
$$\left(\frac14\right)^6=\frac1{4096}.$$

**Expected number of sites** in 9 kb (9000 bp):
$$\frac{9000}{4096}=2.20\approx\mathbf{2}.$$