Final answer:
The initiation stage of protein synthesis is when the translation process begins with the formation of an initiation complex, involving the assembly of various components including the small ribosomal unit, mRNA, initiation factors, GTP, and a special initiator tRNA.
Step-by-step explanation:
Initiation Stage of Protein Synthesis
The initiation stage of protein synthesis is where the translation of mRNA into a polypeptide chain begins. This process entails the assembly of an initiation complex which includes the small ribosomal subunit, the mRNA template, initiation factors, guanosine triphosphate (GTP), and a special initiator tRNA. In E. coli, the initiator tRNA carries a formylated methionine (fMet-tRNAMet) recognizing the start codon AUG on the mRNA. The formation of this complex is essential for the ribosome to begin synthesising the protein.
In prokaryotes like E. coli, the small 30S ribosome, along with initiation factors (IF-1, IF-2, and IF-3), forms part of this complex. Additionally, in prokaryotic cells, the presence of the Shine-Dalgarno sequence within the mRNA facilitates the ribosomal binding. In contrast, eukaryotic translation begins when the initiator tRNA carrying methionine binds to the AUG start codon, and the large ribosomal subunit binds to the initiation complex. In eukaryotes, ribosomes typically bind to the 5' cap structure of the mRNA rather than a Shine-Dalgarno sequence.