MathBench > Probability

BLAST and (Im)probability

Ready for some big numbers?

Our story so far: George claims to have found a new, unique sequence of DNA which codes for the sleuthing gene. It is 165 nucleotides long.

Bess has pooh-poohed her friend's claim, stating that all possible 165 nucleotide sequences have already been discovered.

In fact, about how many unique 165-nucleotide sequences are possible?

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

I think I have the answer: 4^165 = 2.19 × 1099.

A Google is "one" with a hundred zeros, right? So the answer to the problem above ("2" with 99 zeros) is one-fifth of a google. It is a seriously huge number. If it doesn't boggle your mind, you need to get your mind-boggler checked.

one google written out

Put another way, there are a fifth of a google of distinct nucleotide sequences that would code for a 55-amino-acid peptide. That's an awful lot of sequences. And it seems pretty unlikely that they've all been discovered already!