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In eukaryotic cells, the synthesis of messenger RNA involves several complex steps, beginning with transcription and followed by multiple post-transcriptional modifications. Among the most important modifications that occur to the primary transcript, also known as pre-mRNA, are the addition of a 5’ cap and a 3’ poly-A tail. These modifications are crucial for the stability, processing, export, and translation of the mRNA. Without them, the mRNA would be highly unstable, degraded rapidly, and unable to participate efficiently in protein synthesis. Understanding the functions of the cap and poly-A tail is central to appreciating the regulation of gene expression in eukaryotes. The 5’ cap is added almost immediately after transcription begins. As the nascent RNA emerges from RNA polymerase II, a guanosine triphosphate (GTP) residue is added to the 5’ end of the transcript via a unique 5’-5’ triphosphate linkage. This guanosine is subsequently methylated at the N7 position, producing 7-methylguanosine, also called the m7G cap. In some transcripts, additional methylation occurs at the 2’-hydroxyl group of the ribose sugar of the first one or two nucleotides adjacent to the cap. The capping process is catalyzed by specific enzymes associated with RNA polymerase II, ensuring that the modification is tightly coupled with transcription. The primary function of the 5’ cap is to protect the mRNA from exonucleolytic degradation. In the absence of the cap, exonucleases could readily degrade the RNA from the 5’ end, preventing the transcript from surviving long enough to be processed and translated. The cap serves ______ _______ ______ ___ ___ ___ __________ ________.
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