Discussion
If all the nucleotides have equal probability of occurrence in a 4 Mbp long DNA sequence, then how many times will the site of EcoRI, restriction endonuclease occur?
Answer: A
The EcoRI site is expected to occur about 976 times.
EcoRI recognises the six-base sequence GAATTC. With each base equally likely, the chance that a given position starts this sequence is \((1/4)^{6} = 1/4096\):
\[ \begin{aligned} \frac{4 \times 10^{6}}{4^{6}} &= \frac{4000000}{4096} \\ &\approx 976 \end{aligned} \]
\(4^{6}\) (4096) is the average spacing between sites, not the number of sites. \(6^{4}\) puts the base and exponent the wrong way round, and 1000 is a rough round number rather than the calculated value.
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