Final answer:
The Internal Ribosomal Entry Site (IRES) in eukaryotic mRNA allows the ribosome to directly bind and initiate translation without the need to scan from the 5' cap, providing an alternative mechanism for protein synthesis, especially under stressful conditions that inhibit cap-dependent translation.
Step-by-step explanation:
The Internal Ribosomal Entry Site (IRES) is a nucleotide sequence found in some eukaryotic mRNA molecules and plays a critical role in the initiation of translation. Unlike the typical mechanism where the small ribosomal subunit binds to the 5' cap of the mRNA and scans for the start codon, the IRES allows the initiation complex to directly bind to the mRNA, bypassing the need for the 5' cap structure and the scanning process.
This is particularly important under circumstances where cap-dependent translation is inhibited, such as during stress or viral infections that compromise the host's cap-dependent translation machinery. The IRES functions as an alternate means for ribosomes to be recruited to the mRNA, ensuring that protein synthesis can occur under a variety of cellular conditions.