5' cap structure
Capping of the pre-mRNA involves the addition of 7-methylguanosine (m7G) to the 5' end. The cap protects the 5' end of the primary RNA transcript from attack by ribonucleases and is recognized by eukaryotic initiation factors involved in assembling the ribosome on the mature mRNA prior to initiating translation.
Source
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"An Introduction to Molecular Biology/Transcription of RNA and its modification."
http://en.wikibooks.org/wiki/An_Introduction_to_Molecular_Biology/Transcription_of_RNA_and_its_modification%23Splicing_of_RNA
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