NEET 2020 Botany Question 21 Solution

_Eco_RI is a type II restriction endonuclease isolated from the bacterium _Escherichia coli_ strain RY 13: - It scans double-stranded DNA and specifically binds to a symmetrical 6-base pair palindromic sequence: $$\begin{aligned} 5'-\text{G } \downarrow \text{ A A T T C}-3' \\ 3'-\text{C T T A A } \uparrow \text{ G}-5' \end{aligned}$$ - The enzyme cleaves the phosphodiester bond specifically between G (guanine) and A (adenine) on each strand, equidistant from the axis of symmetry. - This staggered, asymmetrical cut leaves single-stranded $5'\text{-AATT-}3'$ protruding termini termed sticky or cohesive ends that readily form complementary hydrogen bonds with fragments cut by the same enzyme.

- Option (2): $5'\text{-GGAACC-}3'$ is not palindromic. - Option (3): Inverts 5' and 3' polarity ($5'\text{-CTTAAG-}3'$ is incorrect polarity orientation). - Option (4): $5'\text{-GGATCC-}3'$ is the recognition palindrome of a different restriction enzyme, _Bam_HI, not _Eco_RI.