In eukaryotes, transcription produces a pre-mRNA molecule that be processed into mature mRNA by the addition of a 5′ cap, a 3′ poly-A tail, and through RNA splicing.
At the end of eukaryotic transcription, a pre-mRNA molecule is formed. Before the pre-mRNA can be used as a template for protein synthesis in the process of translation, it needs to first be processed. The additional steps involved in eukaryotic mRNA maturation create a molecule with a much longer half-life than a prokaryotic mRNA. Eukaryotic mRNAs last for several hours, whereas the typical E. coli mRNA lasts no more than five seconds.
While the pre-mRNA is still being transcribed, a modified guanine nucleotide (called 7-methylguanosine) cap is added to the 5′ end of pre-mRNA strand. This protects the mRNA from being degraded and also helps in the attachment of ribosomes to the mRNA later during translation.
Once the full coding region of a gene is transcribed, an enzyme called poly-A polymerase (PAP) cuts the pre-mRNA at the 3′ end and adds a string of around 200 adenine (A) nucleotides, called the poly-A tail. The poly-A tail also protects the mRNA from degradation, helps the mRNA exist the nucleus into the cytoplasm, and aid in protein binding during the initiation of translation. Take a look at the diagram below to visualize where the 5′ cap and poly-A tail are located on an mRNA strand.
RNA splicing is also an important part of mRNA processing. Eukaryotic genes are composed of exons, which correspond to protein-coding sequences (ex-on signifies that they are expressed), and intervening sequences called introns (int-ron denotes their intervening role), which may be involved in gene regulation, but are removed from the pre-mRNA during processing. Intron sequences in mRNA do not encode functional proteins. Introns are removed and degraded while the pre-mRNA is still in the nucleus, a process called splicing. The splicing of pre-mRNAs is conducted by complexes of proteins and RNA molecules called spliceosomes. Each spliceosome is composed of five subunits called snRNPs (for small nuclear ribonucleoparticles, and pronounced “snurps”.) Each snRNP is itself a complex of proteins and a special type of RNA found only in the nucleus called snRNAs (small nuclear RNAs). Most introns contain marker sequences at both of their ends, which are recognized by the small RNAs and direct the spliceosome to remove the intron. Once the intron has been cut out, the spliceosome will “glue” (ligate) the flanking exons together. Splicing needs to precise and consistent, or the reading frame of the mRNA will be incorrect.
Another form of splicing, called alternative splicing, allows more than one mRNA to be made from the same gene. Through alternative splicing, eukaryotes actually encode many more different proteins than we have genes in our DNA.
Key Points
• A 5′ cap is added to the end of pre-mRNA while elongation is still in progress. The 5′ cap protects the mRNA from degradation and assists in ribosome binding during translation.
• A roughly 200 nucleotide-long poly-A tail is also added to the 3′ end of the pre-mRNA once elongation is complete. It also protects the mRNA from degradation, aids in the export of the mature mRNA to the cytoplasm, and is involved in binding proteins involved in initiating translation.
• Introns are intervening sequences within a pre-mRNA molecule that do not code for proteins and are removed during RNA processing by the spliceosome before the mRNA is exported to the cytoplasm.
• Exons are the coding sequences within a pre-mRNA molecule that are spliced together once the introns are removed to form mature mRNA molecules that are translated into proteins.
• Alternative splicing can produce more than one mRNA from a single gene, leading to more than one protein being encoded by that gene.
Key Terms
5′ cap: A modified guanine nucleotide (called 7-methylguanosine) that is added to a pre-mRNA strand to protect it from degradation and aid in ribosome binding during translation.
3′ poly-A tail: A string of around 200 adenine nucleotides that is added to the 3′ end of pre-mRNA to protect it from degradation, export it to the cytoplasm, and aid in translation initiation.
RNA splicing: When introns are removed from a pre-mRNA strand, leaving only protein-coding exons.
Exon: A region of a transcribed gene present in the final functional mRNA molecule.
Intron: Portions of a gene that are included in pre-mRNA transcripts but are removed during RNA processing and rapidly degraded.
Spliceosome: A dynamic complex of RNA and protein subunits that removes introns from pre-mRNA.
snRNPs: RNA-protein complexes (called small nuclear ribonucleoparticles) that make up the spliceosome.
Ligate: The chemical joining of two strands of DNA.
Alternative splicing: Another splicing mechanism that allows multiple mRNA sequences to be produced from a single gene, encoding separate proteins.
