[kevviiinn]

Iirc there's a protein that adds 12-15 slowly then another protein comes in and adds another 200+ when it detects the start of the tail. Not sure about the details about how or why it stops tho. At least when considering mRNA getting prepped for nuclear export

[subroutine]

You are probably right, I'm not an expert on this process. But there's likely a difference between the innate eukaryotic cell polyadenylation process vs. how coronavirus accomplishes polyadenylation because coronavirus rna never enters the nucleus.