The first step in protein synthesis is the transcription of mRNA from a DNA gene in the nucleus. At some other prior time, the various other types of RNA have been synthesized using the appropriate DNA.
Step 1: Transcription
The RNAs migrate from the nucleus into the cytoplasm. Prior to the beginning of the protein synthesis, all of the component parts are assembled in the ribosome which is the brown/tan structure in the left graphic.
- 30S Subunit Ribosome - graphic
See Link for an overall diagram.
Step 2: Initiation
In the cytoplasm, protein synthesis is actually initiated by the AUG codon on mRNA. The AUG codon signals both the interaction of the ribosome with m-RNA and also the tRNA with the anticodons (UAC). The tRNA which initiates the protein synthesis has N-formyl-methionine attached. The formyl group is really formic acid converted to an amide using the -NH2 group on methionine (left most graphic).
The next step is for a second tRNA to approach the mRNA (codon - CCG). This is the code for proline. The anticodon of the proline tRNA which reads this is GGC. The final process is to start growing peptide chain by having amine of proline to bond to the carboxyl acid group of methinone (met) in order to elongate the peptide. The other graphic below demonstrates this process at the molecular level.
Click for larger image
Step 3: Elongation
Elongation of the peptide begins as various tRNA's read the next codon. In the example on the left the next tRNA to read the mRNA is tyrosine. When the correct match with the anticodons of a tRNA has been found, the tyrosine forms a peptide bond with the growing peptide chain. The proline is now hydrolyzed from the tRNA. The proline tRNA now moves away from the ribosome and back into the cytoplasm to reattach another proline amino acid.
Step 4: Elongation and Termination
When the stop signal on mRNA is reached, the protein synthesis is terminated. The last amino acid is hydrolyzed from its t-RNA. The peptide chain leaves the ribosome. The N-formyl-methionine that was used to initiate the protein synthesis is also hydrolyzed from the completed peptide at this time. The ribosome is now ready to repeat the synthesis several more times.
- Charles Ophardt, Professor Emeritus, Elmhurst College; Virtual Chembook