Final answer:
The statement that a 7-methylguanosine cap is added to the 5' end of mRNA in eukaryotes during transcriptional elongation through a 5'-5' bond is true. This cap plays a crucial role in mRNA stability, translation, and nuclear export.
Step-by-step explanation:
The statement is True: The "CAP" of an mRNA in eukaryotes, which is a methylated guanine, is indeed added to the 5' end of the RNA through a 5'-5' bond during transcriptional elongation. While the pre-mRNA is still being synthesized, a 7-methylguanosine cap is added to the 5' end of the growing transcript by a phosphate linkage. This 5' cap not only protects the nascent mRNA from degradation, but also serves as a recognition site for protein synthesis machinery. It helps in the attachment of mRNA to ribosomes for translation and is significant for the exit of mRNA from the nucleus into the cytoplasm. The capping enzyme attaches this cap by forming an unusual 5',5'- triphosphate linkage to the first nucleotide of the transcript, which remains present in mature mRNA.