# 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?

(To make this problem interactive, turn on javascript!)

• I need a hint ... : Convert 5 trillion to scientific notation.

• ...another hint ... : Find percent by dividing the number existing by the total possible, then multiply by 100.

• ...another hint ... : 5 × 1012 / 2.19 × 1099 = ?

• ...another hint ... : To divide scientific notation -- divide the numbers, subtract the zeros.

• ...another hint ... : Don't forget to convert to a percentage by removing 2 more decimal places.

#### 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:

0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000023%