Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

RNA, RNA Processing
mRNA Processing

During the maturation of eukaryotic mRNA, a 5'-cap is added, introns are removed, and a poly(A) tail is synthesized at the 3'-end. Unlike eukaryotes, prokaryotic mRNA undergoes Processing only in rare cases.

Eukaryotes

mRNA maturation takes place in The Nucleus of Eukaryotic Cells. As a rule, this process begins already during METABOLISM/31.html">Transcription. A cap is attached to the 5'-end of the mRNA. Upon completion of transcription, a poly(A) tail is formed at the 3'-end of the mRNA; the signal for polyadenylation is the AAUAAA sequence located 11–30 NUCLEOTIDES upstream of the polyadenylation site. Eukaryotic mRNA also undergoes splicing. The mature mRNA is transported from the nucleus into the Cell Cytoplasm, where it serves as a template for Protein Synthesis (Fig. 5.18).

Class="center">

Fig. 5.18. Processing of eukaryotic mRNA

It should be noted that some eukaryotic mRNAs lack poly(A) tails, and some mRNA precursors do not contain introns. Naturally, such molecules do not undergo splicing.

Prokaryotic mRNA processing

Prokaryotic mRNA generally does not undergo processing because transcription and Translation are coupled in these organisms. Even before transcription is complete, Ribosomes interact with the mRNA being synthesized by RNA polymerase and initiate the synthesis of polypeptide chains (Fig. 5.19).

Fig. 5.19. Transcription and translation are coupled in prokaryotes.

Certain polycistronic mRNAs in prokaryotes can be cleaved to generate individual mRNAs. In some cases, such Cleavage is essential for successful translation, whereas in others it is optional.

In certain instances, the 3'-end of prokaryotic mRNA undergoes post-transcriptional polyadenylation, yielding poly(A) tails of 14–60 nucleotides in length. Some prokaryotic pre-mRNAs also contain introns.