Messenger RNA: Encyclopedia II - Messenger RNA - mRNA life cycle

Messenger RNA - mRNA life cycle

The brief life of an mRNAs begins with transcription and ultimately ends in degradation. During their life, mRNAs may also be processed, edited, and transported prior to translation. Eukaryotic mRNAs often require extensive processing and transport, while prokaryotic mRNAs do not.

Messenger RNA - Transcription

During transcription, RNA polymerase makes a copy of a gene from the DNA to mRNA as needed. This process is similar in eukaryotes and prokaryotes. One notable difference, however, is that eukaryotic RNA polymerase associates with mRNA processing enzymes during transcription so that processing can proceed quickly after the start of transcription. The short-lived, unprocessed or partially processed, product is termed pre-mRNA; once completely processed, it is termed mature mRNA.

Messenger RNA - Eukaryotic pre-mRNA processing

Processing of mRNA differs greatly between eukaryotes and prokaryotes. Prokaryotic mRNA is essentially mature upon transcription and requires no processing (except in rare cases). Eukaryotic pre-mRNA, however, requires extensive processing.

The 5' cap is modified guanine nucleotide is added to the "front" (5' end) of the pre-mRNA. This modification is critical for recognition and proper attachment of mRNA to the ribosome. It may also be important for other essential processes, such as splicing and transport.

Splicing is the process by which pre-mRNA is modified to remove certain stretches of non-coding sequences called introns; the stretches that remain include protein-coding sequences and are called exons. Sometimes pre-mRNA messages may be spliced in several different ways, allowing a single gene to encode multiple proteins. This process is called alternative splicing. Splicing is usually performed by an RNA-protein complex called the spliceosome, but some RNA molecules are also capable of catalyzing their own splicing (see ribozymes).

Polyadenylation is the covalent linkage of a polyadenylyl moiety to a messenger RNA molecule. In eukaryotic organisms, polyadenylation is the mechanism by which most messenger RNA (mRNA) molecules are terminated at their 3' ends. The poly(A) tail aids in mRNA stability by protecting it from exonucleases. Polyadenylation is also important for transcription termination, export of the mRNA from the nucleus, and translation. Some prokaryotic mRNAs also are polyadenylated, although the poly(A) tail's function is different from that in eukaryotes.

Polyadenylation occurs during and immediately after transcription of DNA into RNA. After transcription has been terminated, the mRNA chain is cleaved through the action of an endonuclease complex associated with RNA polymerase. The cleavage site is characterized by the presence of the base sequence AAUAAA near the cleavage site. After the mRNA has been cleaved, 80 to 250 adenosine residues are added to the free 3' end at the cleavage site. This reaction is catalyzed by polyadenylate polymerase.

Messenger RNA - Editing

In some instances, an mRNA will be edited, changing the nucleotide composition of that mRNA. An example in humans is the apolipoprotein B mRNA, which is edited in some tissues, but not others. The editing creates an early stop codon, which upon translation, produces a shorter protein.

Messenger RNA - Transport

Another difference between eukaryotes and prokaryotes is mRNA transport. Because eukaryotic transcription and translation is compartmentally separated, eukaryotic mRNAs must be exported from the nucleus to the cytoplasm. Mature mRNAs are recognized by their processed modifications and then exported through the nuclear pore.

Messenger RNA - Translation

Because prokaryotic mRNA does not need to be processed or transported, translation by the ribosome can begin immediately after the start of transcription. Therefore, it can be said that prokaryotic translation is coupled to transcription and occurs co-transcriptionally.

Eukaryotic mRNA that has been processed and transported to the cytoplasm (i.e. mature mRNA) can then be translated by the ribosome. Translation may occur at ribosomes free-floating in the cytoplasm, or directed to the endoplasmic reticulum by the signal recognition particle. Therefore, unlike prokaryotes, eukaryotic translation is not directly coupled to transcription.

Messenger RNA - Degradation

After a certain amount of time the message degrades into its component nucleotides, usually with the assistance of RNases. Due to mRNA processing, eukaryotic mRNAs are generally more stable than prokaryotic mRNAs.

Adapted from the Wikipedia article "mRNA life cycle", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki