Final answer:
The mRNA molecule can bind to the small ribosomal subunit when the subunits are dissociated, which is the initial step of protein synthesis before the large subunit attaches to form the complete ribosome.
Step-by-step explanation:
An mRNA molecule can bind to the small ribosome subunit only when the subunits (small and large) are separated or dissociated. This occurs during the initial stage of protein synthesis, also known as translation. Specifically, the 30S ribosomal subunit in bacteria (equivalent to the small subunit in eukaryotes) binds to the mRNA template. This association is facilitated by the presence of initiation factor proteins that help in recognizing the start codon of the mRNA. Once the small subunit is properly bound to the mRNA, the large subunit, known as the 50S ribosomal subunit in bacteria, attaches to form a complete initiation complex, thereby starting the process of translation where amino acids are assembled into proteins.