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I'm studying for a test and I am confused by these problems/statements.

  1. How many amino acids will 18 bases code for? Answer: 6. I got this right.
  2. If a certain complete protein has 33 amino acids that compose it, then exactly how many bases will it take to code the entire thing?

The answer to 2. is 105, but I'm confused on how we arrived at this answer. If possible or if necessary, please explain the related concepts to me!

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  • $\begingroup$ Think I figured it out. $\endgroup$
    – biologyflair
    Commented Jan 29, 2016 at 0:17

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I don't think that's right. I think 102 would make sense, because you want 99 nucleotides to code for 33 amino acids and more more triplet to code for stop. Maybe the question writer didn't quite realize that the starting methionine counts as one of the 33 amino acids.

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  • $\begingroup$ I realized my own question after a lot more mulling, but yeah, you need to account for the Start and Stop codon. According to my teacher, the Start codon = Start codon when it's the first in the sequence and it codes for Methionine if it's not the first. $\endgroup$
    – biologyflair
    Commented Jan 29, 2016 at 0:17
  • $\begingroup$ No. It codes for methionine when it's the first. The first amino acid of virtually every protein is methionine, at least before any post-translational modification. $\endgroup$
    – swbarnes2
    Commented Jan 29, 2016 at 0:23
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    $\begingroup$ @biologyflair your teacher is incorrect. Met is START, as well as being the first AA in many proteins. Take this Entrez Protein search for the ribosomal S6 protein, which is conserved across pretty much all species. Pick entries at random and look at the FASTA sequence - the first letter of the second line (beginning of capitalized sequence) is always M. Or, try here for all the proteins in the E. coli chromosome. $\endgroup$
    – MattDMo
    Commented Jan 29, 2016 at 0:29
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    $\begingroup$ Then the question is, should I listen to her and pass the test or listen to this for long term benefit? $\endgroup$
    – biologyflair
    Commented Jan 29, 2016 at 0:36
  • $\begingroup$ Do you have a textbook? I'd be surprised if the textbook was wrong. You could try and point out that your teacher is wrong, if you don't think that will work, give them the answer they wnat, but remember the real answer for the future. That link to all the e.coli proteins is proof. $\endgroup$
    – swbarnes2
    Commented Jan 29, 2016 at 0:39

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