MathBench > Probability

BLAST and (Im)probability

How many distinct 55-amino-acid sequences are there?

Recall that there are 64 different 3-letter combinations of nucleotides, but only 20 amino acids. Furthermore, you know that the first codon in the sequence has to be a start codon, and the last one has to be a stop codon.

So, how many distinct amino acid sequences are there?

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I think I have the answer: 20^53 = 9.00 × 1068.

So, there are 2.19 × 1099 distinct 165-nucleotide sequences, but only 9.00 × 10 68 distinct 53-amino-acid peptides. That's a big difference, and suggests a lot of repetition in DNA.

On average, how many ways can each 53-peptide sequence be coded?



If you don't want to do problems like the above, you will need to know how to divide numbers in scientific notation!