How does a eukaryotic ribosome initially bind to the mRNA during translation initiation?

• A. By attaching to the poly(A) tail at the 3' end of the mRNA
• B. By recognizing a Shine-Dalgarno sequence upstream of the start codon
• C. By directly binding to the start codon (AUG) without any additional factors
• D. By recognizing the 5' cap structure of the mRNA with the help of initiation factors

Answer: (D)

Verified step by step guidance

1. Understand the context of translation initiation in eukaryotic cells: Translation is the process by which ribosomes synthesize proteins using mRNA as a template. In eukaryotes, this process begins with the ribosome binding to the mRNA.
2. Recognize the structural features of eukaryotic mRNA: Eukaryotic mRNA has a 5' cap structure, which is a modified guanine nucleotide added to the 5' end of the mRNA during transcription. This cap plays a crucial role in translation initiation.
3. Learn the role of initiation factors: In eukaryotic cells, initiation factors (eIFs) are proteins that assist the ribosome in recognizing the 5' cap structure of the mRNA. These factors help guide the small ribosomal subunit to the mRNA and stabilize the interaction.
4. Clarify why other options are incorrect: The poly(A) tail at the 3' end of the mRNA is involved in mRNA stability and translation efficiency but does not directly mediate ribosome binding. The Shine-Dalgarno sequence is specific to prokaryotic translation and is not present in eukaryotic mRNA. The ribosome does not directly bind to the start codon (AUG) without the assistance of initiation factors.
5. Conclude the correct mechanism: The eukaryotic ribosome initially binds to the mRNA by recognizing the 5' cap structure with the help of initiation factors, which position the ribosome to scan for the start codon (AUG) downstream.