MathBench > Probability

BLAST and (Im)probability

To be or not to be ... in blast


Back to Bess -- who still believes that all the possible 165-nucleotide sequences have already been discovered and cataloged. In fact, she's read that you can access the NCBI human genomic database through BLAST, and it contains over 5 trillion sequences of that length! ... which does seem like an awful lot.

Recall, however, that we found that 2.19 × 1099 165-nucleotide sequences are theoretically possible. What percentage of the theoretically possible sequences are actually contained in BLAST?

Assume there are 5 trillion 165-nucleotide sequences in BLAST. What percentage of 165-nucleotide sequences is this?

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I think I have the answer: 2.3 × 10-85.

In other words, if you wanted to write this as a percentage, you would need more than 80 zeros after the decimal point, like this: