44. Which one of the following CANNOT be a recognition sequence for a Type II restriction enzyme?
(a) GAATTC (b) AGCT
(c) GCGGCCGC (d) ATGCCT

Type II restriction enzymes recognize specific palindromic DNA sequences typically 4-8 base pairs long. The correct answer is (d) ATGCCT, as it lacks the palindromic symmetry required for most Type II enzymes. Orthodox Type II enzymes function as homodimers that bind symmetric sites.​

Option Analysis

GAATTC (a): This is the EcoRI recognition site (5′-GAATTC-3′), a classic 6-bp palindrome where the sequence reads identical on the complementary strand (3′-CTTAAG-5′). EcoRI exemplifies orthodox Type II enzyme activity.​

AGCT (b): Equivalent to 5′-AGCT-3′, this 4-bp palindrome matches its reverse complement (3′-TCGA-5′). TaqI recognizes this sequence, confirming suitability for Type II enzymes.​

GCGGCCGC (c): This 8-bp palindrome (5′-GCGGCCGC-3′) reads the same forward and backward on complementary strands (3′-CGCCGGCG-5′). NotI recognizes it, fitting Type II criteria despite longer length.​

ATGCCT (d): This 6-bp sequence (5′-ATGCCT-3′) is asymmetric; its complement is 3′-TAC GGA-5′, which does not mirror the original. Type II enzymes overwhelmingly require palindromic sites for homodimer binding.​

Introduction to Type II Restriction Enzymes

Type II restriction enzyme recognition sequence determines precise DNA cleavage in molecular cloning and biotech applications. These enzymes target short, mostly palindromic DNA sequences of 4-8 bp, enabling homodimer binding without ATP. Understanding palindromic requirements explains why certain sequences fail recognition.​

Characteristics of Valid Recognition Sequences

Type II enzymes prefer symmetric palindromes for efficient binding:

• Length: 4-8 bp typically.​

• Symmetry: Sequence identical when read 5’→3′ on both strands.

• Examples:

  • GAATTC (EcoRI): Perfect 6-bp palindrome.​

  • AGCT (TaqI): 4-bp palindrome common in shorter sites.

  • GCGGCCGC (NotI): 8-bp palindrome, still valid.​

Palindromic structure allows two identical subunits to recognize half-sites simultaneously.​

Why ATGCCT Cannot Serve as Recognition Sequence

ATGCCT lacks palindrome symmetry:

• Forward: 5′-ATGCCT-3′

• Reverse complement: 5′-AGG CAT-3′ (not matching)

Non-palindromic sequences suit Type IIS enzymes (asymmetric, cut outside site), not orthodox Type II. This makes ATGCCT the invalid option.​

Applications in Molecular Biology

Recognition sequence specificity enables:

• Cloning: EcoRI cuts GAATTC for sticky ends.

• Genome editing: Palindromic sites ensure precision.

• Exam relevance: Tests palindromic DNA concepts in biotech curricula.