A DNA-binding protein recognizes a specific eight-nucelotide sequence in DNA. Assuming that its binding is perfectly specific, how many binding sites are expected to exist for it in human genomic DNA, which is composed of about 6 × 10⁹ nucleotide pairs? What about if the protein forms a homodimer? Note that the target sequence can be oriented either way in the double-stranded DNA.
A) About 200,000; about 30,000
B) About 2000; about 300
C) About 200; about 30
D) About 200,000; about 300
E) About 200,000; about 3

Question

Quizplus · Accepted Answer

The expected number of target 8-mer sequences is calculated as:
(6 × 10⁹ 8-mer/genome) × (2 orientations per double strand) / (4⁸ 8-mers/target sequence)
= ~200,000 target sequences/genome.
Similarly, the expected number of target 16-mer sequences is calculated as:
(6 × 10⁹ 16-mer/genome) × (2 orientations per double strand) / (4¹⁶ 16-mers/target sequence)
= ~3 target sequences/genome.

A DNA-Binding Protein Recognizes a Specific Eight-Nucelotide Sequence in DNA