Final answer:
The process of adding a 7-methylguanosine to the 5' end of a mature mRNA is known as 5' capping, a crucial step in protecting the mRNA from degradation and aiding in translation.
Step-by-step explanation:
The addition of a 7-methylguanosine to the 5' end of a mature mRNA is a process known as 5' Capping. During this process, while the pre-mRNA is still being synthesized, a 7-methylguanosine cap is added to the growing transcript by a phosphate linkage. This important modification not only protects the nascent mRNA from degradation but also plays a crucial role in the initiation of translation by ribosomes because factors involved in protein synthesis recognize the cap.