Final answer:
The 3′ ends of eukaryotic mRNAs are generated by cleavage of the pre-mRNA followed by the addition of a poly-A tail by poly(A) polymerase. This modification protects the mRNA from degradation, assists in nuclear export, and controls mRNA half-life in the cytoplasm. Additionally, mRNAs are capped at the 5' end during transcription to further protect the RNA and to facilitate translation initiation.
Step-by-step explanation:
The 3′ ends of mRNAs are generated by a process known as cleavage and polyadenylation. This process involves the cutting of pre-mRNA and the addition of a poly-A tail, which is essential for the stability and function of mRNA. First, once elongation of the pre-mRNA is complete, an endonuclease cleaves the pre-mRNA between a specific AAUAAA consensus sequence and a GU-rich sequence.
Alongside polyadenylation, eukaryotic pre-mRNAs are also modified by 5' capping, where a 7-methylguanosine cap is added to the 5' end of the growing transcript. This cap also protects the mRNA from degradation and is essential for the initiation of translation by ribosomes.