start and stop codon help?

[go4kil]

i have this homework and im just not sure if i got the right answer
"the protein coding portion of a section of mRNA is 450 bases long. assuming that the start codon does get translated into an amino acid but the stop codon does not, how many amino acids will be in the corresponding polypeptide?"

i was thinking that it would finish where ever the stop codon is so there is no definit number of amino acids, on the other hand, i was thinking that it would be 450/3=150 polypeptides minus 9 stop codons. can you guys/gals help me out. thank you!

[blcr11]

Why 9 stop codons? There should only be one 3-base stop codon. (There can be more than one stop codon, but the way the gene is described I assume you are to consider it a single, cleanly read gene--no small orfs or anything like that, and no splicing, no non-translated upstream or downstream sequence beyond the one stop codon, etc.) So 450 bases, including start and stop, will have 150 (including the starting amino acid) - 1 (non-translated stop codon) = 149 amino acids.

[go4kil]

i thought there was 3 stop codons so i multiplied 3 bases x 3 codons and subtracted it to the 150. so the stop codon would be in the end assuming it is a single read gene and not anywhere in between the mrna strand right?

[MrMistery]

yes. You don't generally want to make an mRNA that will only be read half. Not efficient...

[canalon]

i thought there was 3 stop codons so i multiplied 3 bases x 3 codons and subtracted it to the 150. so the stop codon would be in the end assuming it is a single read gene and not anywhere in between the mrna strand right?

Yes there are 3 possible stop codons, but only one is used at the same time. So you need to substract only one codon as blcr11 was explaining.