Thursday, December 23, 2010

Clinical Case: Number Of Nucleotides....

Clinical Vignette: Transcription  of a  prokaryotic  gene by  RNA  polymerase yields  an mRNA  corresponding  to a  single polypeptide  chain  containing  64  amino  acids.  The  mRNA  has  a  5'  untranslated  region  of  120 nucleotides  and a  3'  untranslated region  of  240 nucleotides.  Approximately  how many  nucleotides are in the coding region of the mRNA? 

       A. 64 
       B. 128 
       C. 192 
       D. 424 
       E. 552 


The  correct  answer is  C.  This vignette has distractors, so beware. Regardless  of  how long  the  untranslated  regions  are, the  number  of nucleotides  in  the coding  region  of  an mRNA  is  three  times the  number  of  amino acids,  since three  nucleotides  are required  to code  for each  amino acid, and  3 X  64 =  192. In reality, three nucleotides   code  for   the  first  amino  acid  (formyl-methionine  in  prokaryotes,  methionine  in eukaryotes),  which  may be  removed  in  posttranslational steps,  and  three  nucleotides at  the  3' 
end  (are  needed to  terminate the  process (i.e.,  a STOP codon),  so the  actual number would likely be slightly higher.