Final answer:
All proteins in eukaryotes begin with methionine during translation initiation because the initiator tRNA is charged with methionine. However, not all mature proteins necessarily start with methionine as it may be removed. The eukaryotic methionine-charged initiator tRNA is recognized by the 40S subunit, which then binds with the mRNA at the 5' cap before forming the complete initiation complex.Option 3 is the correct answer.
Step-by-step explanation:
In eukaryotic organisms, it's a common misperception that all proteins begin with methionine (MET) because the initiator tRNA carries it. Here's the reality: during translation initiation, the initiator tRNA in eukaryotes is charged with methionine and is referred to as Met-tRNAi. This differs from regular Met-tRNAs as it can bind with initiation factors (IFs). Unlike in prokaryotes where the start codon codes for N-formyl-methionine (fMet), in eukaryotes it codes for methionine. The misconception might arise because, although methionine is indeed the first amino acid incorporated into nascent proteins, not all mature proteins begin with methionine since it can be post-translationally removed or processed.
The initiation complex in eukaryotes involves the 40S small ribosomal subunit recognizing the 5' cap of the mRNA, and scanning to locate the start codon AUG. Upon recognition, the 60S large ribosomal subunit binds to form the complete initiation complex, ready for protein synthesis to begin. This process is ATP-dependent and uses other nucleoside triphosphates like GTP for energy. Notably, the eukaryotic 5' cap structure plays a critical role in this process, contrasting with the prokaryotic Shine-Dalgarno sequence which is absent in eukaryotes.