Final answer:
The last step of 5' cap phosphorylation involves the methylation of the guanosine residue at the mRNA's 5' end, which protects the mRNA and aids in translation initiation.
Step-by-step explanation:
The last step of the 5' cap formation in mRNA processing is the enzymatic addition of a methyl group to the guanosine residue that has been added to the 5' end. The enzyme responsible for this methylation is known as a methyltransferase. This 5' capping is crucial as it protects the mRNA from degradation in the cytoplasm and facilitates its recognition by the ribosome for translation into protein. The phosphate linkage that connects the 7-methylguanosine cap to the mRNA is essential for mRNA stability and is a unique feature that distinguishes eukaryotic mRNAs from other RNA molecules.