Main steps in translation of eukaryotic mRNA.

(A) mRNAs display two structural features that are important for translation initiation: the methylated cap at the 5′ end (yellow pentagon) and the AUG initiation codon at the start of the coding region (blue box). The distance between the 5′-cap and the initiator AUG may range from about 40 to more than 1000 bases, although it usually is less than 100 bases. (B) A preinitiation complex consisting of the ribosomal 40S subunit (green) and several initiation factors (IFs) is assembled at the the methylated 5′-cap. This initiation complex then migrates along the 5′-UTR in search of an AUG initiation codon (scanning process). (C) Recognition of an AUG leads to assembly of the 60S ribosomal subunit and initiation of protein synthesis.