Final answer:
The fourth step in the initiation phase of mRNA translation is the binding of the 60S subunit to the pre-assembled complex of Met-tRNA1, mRNA, and the 40S subunit, forming an 80S ribosome ready for elongation.
Step-by-step explanation:
Step 4 in the initiation of polypeptide chain synthesis involves the completion of the initiation complex in eukaryotic translation. After the initiation factors have identified the appropriate start codon AUG on the mRNA, they, along with other proteins and the cap-binding complex (CBP), dissociate. This allows for the 60S ribosomal subunit to bind to the already assembled complex of Met-tRNA1, mRNA, and the 40S subunit.
The binding of the 60S subunit creates a functional 80S ribosome that is ready to commence the elongation phase of protein synthesis. The initiator tRNA is positioned in the P site of the ribosome with methionine, the first amino acid of the chain. This juncture marks the completion of the assembly of the initiation complex and the transition towards the elongation phase where amino acids will be successively added to the growing polypeptide chain.