Editing Protein biosynthesis (section)

===Post-transcriptional modifications===
[[File:Post-transcriptional modification of pre-mRNA.png|upright=1.2|thumb|alt=three strands of RNA at different stages of maturation, the first strand contains introns and exons only, the second strand has gained a 5' cap and 3' tail and contains still introns and exons, the third strand has the cap and tail but the introns have been removed| Outlines the process of post-transcriptionally modifying pre-mRNA through capping, polyadenylation and splicing to produce a mature mRNA molecule ready for export from the nucleus.]]

Once transcription is complete, the pre-mRNA molecule undergoes post-transcriptional modifications to produce a mature mRNA molecule.{{cn|date=December 2024}}

There are 3 key steps within post-transcriptional modifications:{{cn|date=April 2023}}
# Addition of a [[Five-prime cap|5' cap]] to the 5' end of the pre-mRNA molecule
# Addition of a 3' [[polyadenylation|poly(A) tail]] is added to the 3' end pre-mRNA molecule
# Removal of [[intron]]s via [[RNA splicing]]
The 5' cap is added to the 5' end of the pre-mRNA molecule and is composed of a guanine nucleotide modified through [[Protein methylation|methylation]]. The purpose of the 5' cap is to prevent break down of mature mRNA molecules before translation, the cap also aids binding of the ribosome to the mRNA to start translation<ref name="Khan2020">{{Cite web |title=Eukaryotic pre-mRNA processing |url=https://www.khanacademy.org/science/biology/gene-expression-central-dogma/transcription-of-dna-into-rna/a/eukaryotic-pre-mrna-processing |access-date=9 March 2020 |website=Khan Academy}}</ref> and enables mRNA to be differentiated from other RNAs in the cell.<ref name="Alberts 2015" /> In contrast, the 3' Poly(A) tail is added to the 3' end of the mRNA molecule and is composed of 100-200 adenine bases.<ref name="Khan2020" /> These distinct mRNA modifications enable the cell to detect that the full mRNA message is intact if both the 5' cap and 3' tail are present.<ref name="Alberts 2015" />

This modified pre-mRNA molecule then undergoes the process of RNA splicing. Genes are composed of a series of introns and [[exon]]s, introns are nucleotide sequences which do not encode a protein while, exons are nucleotide sequences that directly encode a protein. Introns and exons are present in both the underlying DNA sequence and the pre-mRNA molecule, therefore, to produce a mature mRNA molecule encoding a protein, splicing must occur.<ref name="Toole2015" /> During splicing, the intervening introns are removed from the pre-mRNA molecule by a multi-protein complex known as a [[spliceosome]] (composed of over 150 proteins and RNA).<ref name="Jo2015">{{Cite journal |vauthors=Jo BS, Choi SS |date=December 2015 |title=Introns: The Functional Benefits of Introns in Genomes |journal=Genomics & Informatics |volume=13 |issue=4 |pages=112–118 |doi=10.5808/GI.2015.13.4.112 |pmc=4742320 |pmid=26865841}}</ref> This mature mRNA molecule is then exported into the cytoplasm through nuclear pores in the envelope of the nucleus.{{cn|date=December 2024}}