Final answer:
The 5' guanosine cap, which consists of a 7-methylguanosine, is uniquely linked to mRNA and protects it from degradation, aids in nuclear export, and facilitates translation initiation.
Step-by-step explanation:
The guanosine in the 5' guanosine cap differs from the guanosine used in the body of the mRNA in that it is methylated at the 7th nitrogen. This specially modified nucleotide, known as 7-methylguanosine, is added to the 5' end of the nascent mRNA transcript with a unique 5',5'-triphosphate linkage. This modification creates a cap that serves multiple important functions: the cap protects the mRNA from degradation by exonucleases, it assists in the mRNA transport from the nucleus to the cytoplasm, and it is recognized by protein synthesis factors to help initiate translation by ribosomes.