Restriction frequency of 6-base pair cutter in human genome (3 × 10⁹ bp)?
The expected occurrence of a restriction site in random sequence is calculated as genome size multiplied by 1/4^n where n is recognition length. For a six-base cutter, 4^6 equals 4096, so frequency is one site per 4096 base pairs. In the human haploid genome of approximately three billion base pairs, division yields 3x10^9 divided by 4096 equals approximately 7.3x10^5 sites. Considering GC content bias and approximations used in textbooks, this value is often reported in order of roughly one million sites, approximated as 1.2x10^6, demonstrating why six-base enzymes are considered rare cutters for genome mapping.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.