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Gene Expression

Posttranscriptional Modifications

Following transcription, modifications to the RNA may occur, including capping, poly-A tail addition, and RNA splicing, in which short segments of RNA are joined together to make a single mRNA message.

For prokaryotes, once mRNA is produced, it is ready for immediate translation. After immature messenger RNA (mRNA) has been synthesized during transcription in eukaryotes, however, it may be modified before exiting the nucleus. At this point, it is considered pre-mRNA. In eukaryotes, the first modification is capping, in which the 5′ end of the RNA is "capped" with a methylated guanosine triphosphate (GTP), where a methyl functional group is added to the guanosine molecule. The capping modification to the 5′ end of mRNA is necessary for stability in the cytoplasm, efficient export through nuclear pores, and efficient gene expression. Capping occurs as the RNA is being transcribed, after about 20–30 nucleotides have been added to the strand. This cap protects the RNA from degrading and assists in the synthesis of proteins in later steps.

The second modification in eukaryotes that mRNA undergoes is polyadenylation. Polyadenylation is the addition of a poly-A tail, or multiple adenosine monophosphates, to an mRNA strand. In this modification, a tail of repeating adenine (A) nucleotides is added to the 3′ end of the transcript. This poly-A tail is approximately 250 nucleotides long. The poly-A tail serves many functions, but its major role is the protection of the mRNA molecule against enzymatic degradation.

In eukaryotes, RNA splicing is the process by which introns are removed from mRNA and exons are joined together following transcription. This allows for the same gene to encode various messages depending on posttranscriptional modification. An exon is a DNA sequence within a gene sequence that codes for a polypeptide sequence. Exons must be combined during mRNA processing. An intron is a DNA sequence within a gene sequence that does not code for a specific protein. Introns must be removed from the mRNA during processing. After initial transcription, enzymes cut the pre-RNA, the primary RNA transcript, at intron-exon borders. Enzymes then attach exons together, leaving the introns unbound. Some exons may also be excluded from the final mRNA. Most genes contain many exons, so a variety of possible final mRNA transcripts can be synthesized from a single gene via RNA splicing, a process known as alternative splicing. However, for any specific cellular tissue, the products produced from RNA splicing are usually consistently the same each time. Each mRNA modification occurs before the message exits the nucleus for translation.

RNA Splicing

RNA splicing is a posttranscriptional modification. DNA contains exons, regions that code for proteins, and introns, regions that do not code for proteins. In splicing, introns and some exons are cut from the strand, resulting in different mRNA strands from a single gene.