Final answer:
The mammalian cleavage/polyadenylation machinery is crucial for pre-mRNA processing and includes endonucleases, poly-A polymerase, and additional factors that add a poly-A tail and 5' cap, ensuring mRNA stability and facilitating nuclear export.
Step-by-step explanation:
The mammalian cleavage/polyadenylation machinery is responsible for processing the pre-mRNA and involves multiple components. Key elements include an endonuclease, which cleaves the pre-mRNA between an AAUAAA consensus sequence and a GU-rich sequence. Following this, poly-A polymerase adds approximately 200 adenine residues, forming the poly-A tail, which is crucial for mRNA stability, nuclear export, and translation initiation.
After the 5' methylguanosine cap is added to the growing transcript, the pre-mRNA is then processed through splicing, where introns are excised, and exons are joined to form a contiguous coding sequence. These modifications, including the poly-A tail and 5' cap, protect the mRNA from degradation and assist in its export through the nuclear pores into the cytoplasm, where it can be used as a template for protein synthesis.
The entire process is essential for the accurate expression and regulation of genetic information in eukaryotic cells, demonstrating the complex and highly coordinated nature of gene expression at the molecular level.