Some Restriction Enzymes Cut at Specific and Unambiguous Sequences (E

Multiple Choice

Some restriction enzymes cut at specific and unambiguous sequences (e.g., NotI at GCGGCCGC) . As such, the distance between two such sites in a large random DNA sequence is 4⁸, assuming a 50% G+C DNA content. However, the sequence recognized by XhoII is said to be ambiguous (Pu-GATC-Py, where Pu = any purine and Py any pyrimidine) . What, therefore, is the expected distance between two XhoII sites in a large random sequence?

A) 4 × 4 × 4 × 4
B) 1 × 4 × 4 × 4 × 4 × 1
C) 2 × 4 × 4 × 4 × 4 × 2
D) 4⁶
E) 4² × 2²