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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:

1. 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.
2. splicing - The 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 exonIn some organisms exons are situated between introns which are spliced from the strand before it is exported from the nucleus and do not code for protein parts. Exons are the regions of a transcribed gene that are not spliced out and which are retained ins. Sometimes one pre-mRNA message may be spliced in several different ways, allowing a single gene to encode multiple proteins. Most RNA splicing is performed by enzymeAn enzyme is a protein, or protein complex, that catalyzes a chemical reaction. Like any catalyst, enzymes work by lowering the activation energy of a reaction, thus allowing the reaction to proceed to its steady state or completion much faster than it ots, but some RNA molecules are also capable of catalyzing their own splicing (see ribozymeA ribozyme or RNA enzyme is an RNA molecule that can catalyze a chemical reaction. Many natural ribozymes catalyze either their own cleavage or the cleavage of other RNAs, but they have also been found to catalyze the aminotransferase activity of the ribos).
3. polyadenylation - A sequence (often several hundred) of adenineAdenine is one of the two purine bases used in forming nucleotides of the nucleic acids DNA and RNA. In DNA, adenine (A) binds to thymine (T) to assist in stabilizing the nucleic acid structures. In RNA, adenine binds to uracil (U). Adenine forms adenosin nucleotides is added to the 3' end of the pre-mRNA. This helps increase the half-life of the message, 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 RNase s.

Messenger RNA that has been processed and is ready for transcription is called a "mature transcript" or "mature mRNA" or sometimes simply "mRNA". Unprocessed or partially-processed messenger RNA is called "pre-mRNA" or "hnRNA" (for heterogeneous nuclear RNA).