Final answer:
The start codon is a sequence on mRNA, specified as AUG, to which the ribosome attaches, marking the beginning of the translation process. In prokaryotes, the Shine-Dalgarno sequence helps locate the AUG codon, while eukaryotes use a cap structure and Kozak's rules. This start codon is critical for establishing the correct reading frame for protein synthesis.
Step-by-step explanation:
The ribosome attaches to the mRNA at a precise site, called the start codon, which is specified as AUG. At the initiation of protein synthesis, the translation process begins when the ribosome binds to the mRNA in the cytoplasm. The small ribosomal subunit, aided by initiation factors, identifies the AUG start codon, and the initiator tRNA with the anticodon UAC base-pairs with this codon.
In prokaryotes such as E. coli, the Shine-Dalgarno sequence situated upstream of the AUG codon assists in the proper positioning of the 30S ribosomal subunit on the mRNA. Conversely, in eukaryotic cells, a 7-methylguanosine cap at the 5' end of the mRNA and certain consensus sequences, as outlined by Kozak's rules, guide the ribosome in locating the start codon.
Once the AUG start codon is recognized, the large ribosomal subunit joins to form an intact ribosome, establishing the correct reading frame for the synthesis of the polypeptide. The initiator methionine-tRNA is then situated in the P-site, and the elongation of the polypeptide chain commences as the ribosome moves and new aminoacyl-tRNAs enter the A-site.