Final answer:
The third step in the creation of a 5' cap involves the enzymatic addition of CHâ‚‚-guanosine to the 5' end of the transcript by a capping enzyme. The 5' cap protects the mRNA from degradation and helps initiate translation.
Step-by-step explanation:
The third step in the creation of the 5' cap on eukaryotic mRNA involves the addition of a methyl group to the guanine nucleotide at the initial transcribed nucleotide. This process is known as "methylation" and occurs at the 7th position of the guanine base.
The enzyme responsible for this methylation is called guanine-7-methyltransferase or mRNA (guanine-N7-)-methyltransferase. This modification plays a crucial role in mRNA stability, processing, and recognition by the translation machinery during protein synthesis.
The 5' cap structure is essential for various cellular processes, including mRNA export, translation initiation, and protection from degradation by exonucleases.