NEET 2021 Zoology Question 185 Solution

Restriction endonucleases (Type II restriction enzymes) serve as precision "molecular scissors" in genetic engineering: - The enzyme scans duplex DNA until it locates its specific recognition sequence (typically 4 to 8 base pairs in length). - These recognition sequences are palindromes with rotational dyad symmetry: the sequence of base pairs reads identical when reading both antiparallel strands in the same orientation (either $5' \to 3'$ or $3' \to 5'$). - For example, the restriction endonuclease *Eco*RI recognizes the 6-base pair palindrome: $$\begin{aligned} 5'-\text{G A A T T C}-3' \\ 3'-\text{C T T A A G}-5' \end{aligned}$$ - *Eco*RI cuts both strands between the G and A bases, generating overhanging, complementary single-stranded ends termed sticky ends.

- Degenerate primer sequence: Oligonucleotide primer mixtures used in PCR when target sequences exhibit codon redundancy, not restriction sites. - Okazaki fragments / sequences: Short lagging-strand DNA segments synthesized discontinuously during semi-conservative DNA replication. - Poly(A) tail sequences: A stretch of 200–300 adenylate residues added post-transcriptionally to the $3'$ end of eukaryotic pre-mRNA for stability and nuclear export.