Messenger RNA (mRNA) is RNA that carries information from DNA to the ribosome sites of protein synthesis in the cell. mRNA runs through several steps during its usually brief existence: During transcription, an enzyme called RNA polymerase makes a copy of a gene from the DNA to mRNA as needed. In prokaryotes, no further processing of mRNA occurs (except in rare cases), and often translation of the mRNA into protein occurs even while transcription is going on. In eukaryotes, transcription and translation occur in different parts of the cell (transcription in the nucleus, where DNA is kept, and translation in the cytoplasm, where ribosomes reside). Also in eukaryotes, mRNA undergoes several processing steps before it is ready to be translated:
The addition of a 5' cap. A modified guanine nucleotide is added to the "front" of the message. This is critical for recognition and proper attachment of the ribosome.
splicing - The pre-mRNA (unprocessed or partially-processed messenger RNA is called "pre-mRNA" or "hnRNA" for heterogeneous nuclear RNA) 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 one pre-mRNA message may be spliced in several different ways, allowing a single gene to encode multiple proteins. This process is called alternative splicing. Most RNA splicing is performed by enzymes, but some RNA molecules are also capable of catalyzing their own splicing (see ribozymes).
polyadenylation - A sequence (often several hundred) of adenine nucleotides is added to the 3' end of the pre-mRNA through the action of an enzyme, polyA polymerase (this modification does not occur in prokaryotic mRNA). The polyadenylated tail is added on to the transcripts that contain a specific sequence, the AAUAAA signal. The importance of the AAUAAA signal is demonstrated by a mutation in the coding DNA sequence (AATAAA) which can lead to some hemoglobin deficiencies (see Higgins et al. Nature. 1983 Nov 24-30;306(5941):398-400)
Polyadenylation helps increase the half-life of the transcript, so that the transcript lasts longer in the cell and consequently is translated more and produces more protein.
After the mRNA has been processed, it is exported from the nucleus into the cytoplasm, where it is bound to ribosomes and translated into protein. After a certain amount of time the message degrades into its component nucleotides, usually with the assistance of RNases.
Messenger RNA that has been processed and is ready for translation is called a "mature transcript" or "mature mRNA" or sometimes simply "mRNA".