From Genes to Proteins: Start to Finish

We just discussed that the journey from DNA to mRNA to protein requires that a cell identify where a gene begins and ends. This must be done both during the transcription and the translation process.

Transcription Transcription, the synthesis of an RNA copy from a sequence of DNA, is carried out by an enzyme called RNA polymerase. This molecule has the job of recognizing the DNA sequence where transcription is initiated, called the promoter site. In general, there are two "promoter" sequences upstream from the beginning of every gene. The location and base sequence of each promoter site vary for prokaryotes (bacteria) and eukaryotes (higher organisms), but they are both recognized by RNA polymerase, which can then grab hold of the sequence and drive the production of an mRNA. Eukaryotic cells have three different RNA polymerases, each recognizing three classes of genes. RNA polymerase II is responsible for synthesis of mRNAs from protein-coding genes. This polymerase requires a sequence resembling TATAA, commonly referred to as the TATA box, which is found 25-30 nucleotides upstream of the beginning of the gene, referred to as the initiator sequence. Transcription terminates when the polymerase stumbles upon a termination, or stop signal. In eukaryotes, this process is not fully understood. Prokaryotes, however, tend to have a short region composed of G's and C's that is able to fold in on itself and form complementary base pairs, creating a stem in the new mRNA. This stem then causes the polymerase to trip and release the nascent, or newly formed, mRNA.

Translation The beginning of translation, the process in which the genetic code carried by mRNA directs the synthesis of proteins from amino acids, differs slightly for prokaryotes and eukaryotes, although both processes always initiate at a codon for methionine. For prokaryotes, the ribosome recognizes and attaches at the sequence AGGAGGU on the mRNA, called the Shine-Delgarno sequence, that appears just upstream from the methionine (AUG) codon. Curiously, eukaryotes lack this recognition sequence and simply initiate translation at the amino acid methionine, usually coded for by the bases AUG, but sometimes GUG. Translation is terminated for both prokaryotes and eukaryotes when the ribosome reaches one of the three stop codons.