Expert: Dana Krempels, Ph.D. - 12/11/2007

Question
2.   The base sequence of the gene coding for a short polypeptide is 3’-CTACGCTAGGCGATTGACT-5’. What would be the base sequence of the mRNA transcribed from this gene? Using the genetic code in your text book give the amino acid sequence of the polypeptide translated from this mRNA.

What i was able to get, which i am still confused on is 5'-GAUGCGAUCCGCUAACUGA-3' The amino sequence for the polypeptide translated from the mRNA is 5'-G|AUG|CGA|UCC|GCU|AAC|UGA|-3'. Is this correct? i'm still confused. :(

Answer
Dear Lorraine,

The sequence you transcribed from the original DNA strand is correct.  But you need a little more information to answer the question.

You need to know whether the question wants you to determine the amino acid sequence of that particular strand *as it stands*, or whether you're supposed to look for a start codon.  

You've found an AUG one nucleotide downstream from the 5' end, but I'm not sure the question is asking for that.  If it is, then go ahead and start with the AUG (methionine), and work from there with the Crick code:

www.bio.miami.edu/dana/pix/gencode.jpg

to decipher the remaining amino acids.

However, if the question is asking for the amino acid sequence shown as if it were just part of the middle of a coding sequence, then the first amino acid would be encoded by GAU, and be aspartate, the next would be alanine (GCG), etc.

Hope that helps.

Dana