Messenger RNA is a type of single-stranded ribonucleic acid that is transcribed from a strand of DNA as a template and carries genetic information to guide protein synthesis. Using genes in cells as templates, mRNA is transcribed according to the principle of complementary base pairing, and mRNA contains base sequences corresponding to certain functional fragments in the DNA molecule, serving as a direct template for protein biosynthesis. Although mRNA only accounts for 2% to 5% of total cellular RNA, it has the most types and is very active in metabolism. It is the RNA with the shortest half-life. It is broken down within minutes to hours after synthesis.
Figure 1. The "life cycle" of an mRNA in a eukaryotic cell. RNA is transcribed in the nucleus; after processing, it is transported to the cytoplasm and translated by the ribosome. Finally, the mRNA is degraded.
Messenger RNA is a direct template that directs protein biosynthesis. mRNA accounts for 2% to 5% of the total intracellular RNA. There are many types of mRNAs, and their molecular sizes vary greatly. Messenger RNA (mRNA) is a large class of RNA molecules that transfer genetic information from DNA to the ribosome, where it serves as a template for protein synthesis and determines the amino acid sequence of the peptide chain of the gene expression protein product. RNA polymerase transcribes the primary transcript mRNA (called pre-mRNA) into processed mature mRNA, which is translated into protein. As in DNA, mRNA genetic information is also stored in a nucleotide sequence, which is arranged as a codon consisting of each three base pairs. Each codon encodes a particular amino acid, with the exception of the stop codon, which stops protein synthesis. The process of translating codons into amino acids requires two other types of RNA: transfer RNA (tRNA) and ribosomal RNA (rRNA). tRNA mediates the recognition of codons and provides the corresponding amino acids. rRNA is a core component of ribosomal protein manufacturing machinery. The existence of mRNA was first proposed by Jacques Monod and François Jacob, and subsequently discovered by Jacob, Sydney Brenner and Matthew Meselson at the California Institute of Technology in 1961.
Prokaryotic and eukaryotic mRNAs have different characteristics
Transcription refers to the process of synthesizing RNA from DNA. During transcription, RNA polymerase copies the DNA of a gene into mRNA as needed, a process that is similar in eukaryotes and prokaryotes.
Unlike prokaryotes, eukaryotic RNA polymerase binds to mRNA processing enzymes during transcription. Therefore, eukaryotic mRNA processing can be performed quickly after transcription begins. Short-lived unprocessed or partially processed transcription products are called precursor mRNAs or pre-mRNAs; once processed, they are called mature mRNAs.
Another difference between eukaryotes and prokaryotes is the transport of mRNA. Since eukaryotic transcription and translation are performed in different organelles, eukaryotic mRNA must be exported from the nucleus to the cytoplasm. This process may be regulated by different signaling pathways. Mature mRNAs are recognized by their processed modifications and are exported to the cytoplasm through the nuclear pores after binding to the cap-binding proteins CBP20 and CBP80 and the transcription / export complex (TREX).
Translation of mRNA
Because prokaryotic mRNAs do not require processing or transport, translation of prokaryotic mRNAs in the ribosome can begin immediately after transcription is complete. Therefore, it can be said that prokaryotic mRNA translation and transcription coupling occur. Translation of eukaryotic mRNAs (ie mature mRNAs) that have been processed and transported to the cytoplasm in the ribosome occurs in the free-floating ribosomes in the cytoplasm, or is directed to the endoplasmic reticulum by signal recognition particles. Therefore, unlike prokaryotes, eukaryotic mRNA translation is not directly coupled to transcription. In some case it may even happen that a decrease in mRNA levels is accompanied by an increase in protein levels.
1. Quaresma AJ.; et al. Regulation of mRNA export by the PI3 kinase/AKT signal transduction pathway. Molecular Biology of the Cell. 2013, 24 (8): 1208–21.