What molecule is added to the 5' end of eukaryotic RNA transcripts?

In eukaryotic cells, the process of transcription results in the production of immature RNA transcripts that undergo several modifications before they are translated into proteins. One of the key modifications is the addition of a 7-methylguanylate cap to the 5' end of the RNA transcript. This cap is formed from guanosine triphosphate (GTP).

The addition of GTP is crucial for several reasons. First, it helps protect the RNA transcript from degradation by exonucleases, which are enzymes that degrade RNA from the ends. Second, the 5' cap is involved in the initiation of translation by facilitating the binding of the ribosome to the mRNA. The cap structure is also important for splicing, as it assists in the recognition of the mRNA during the splicing process which removes introns and joins exons together.

In contrast, the other nucleotides—ATP, UTP, and CTP—are primarily involved in RNA synthesis during transcription but do not play a role in the capping process. Thus, the correct answer emphasizes the unique function of GTP in establishing the protective and functional cap on eukaryotic RNA transcripts.