## 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?

So, there are 2.19 × 10^{99} 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!