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in the cell nucleus; once completely processed, it is transported to the
cytoplasm and
Translation (genetics) by the ribosome. At the end of its life, the mRNA is degraded.
Messenger Ribonucleic Acid (
mRNA) is a molecule of
RNA encoding a chemical "blueprint" for a protein product. mRNA is transcription (genetics) from a
DNA template, and carries coding information to the sites of
protein synthesis: the
ribosomes. Here, the nucleic acid polymer is translation (genetics) into a polymer of amino acids: a protein. In mRNA as in DNA, genetic information is encoded in the sequence of four nucleotides arranged into codons of three bases each. Each codon encodes for a specific amino acid, except the stop codon that terminate protein synthesis. This process requires two other types of RNA: Transfer RNA (
tRNA) mediates recognition of the codon and provides the corresponding amino acid, while Ribosomal RNA (
rRNA) is the central component of the ribosome's protein manufacturing machinery.
mRNA "life cycle"
The brief life of an mRNA molecule begins with transcription and ultimately ends in degradation. During its life, an mRNA molecule may also be processed, edited, and transported prior to translation.
eukaryotes mRNA molecules often require extensive processing and transport, while prokaryote molecules do not.
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.
Eukaryotic pre-mRNA processing
Processing of mRNA differs greatly between
eukaryotes,
bacteria and archea. Non-eukaryotic mRNA is essentially mature upon transcription and requires no processing, except in rare cases. Eukaryotic pre-mRNA, however, requires extensive processing.
5' cap addition
A
5' cap (also termed an RNA cap, an RNA 7-methylguanosine cap or an RNA m7G cap) is a modified guanine nucleotide that has been added to the "front" or
5' end of a eukaryotic messenger RNA shortly after the start of transcription. The 5' cap consists of a terminal 7-methylguanosine residue which is linked through a 5'-5'-triphosphate bond to the first transcribed nucleotide. Its presence is critical for recognition by the ribosome and protection from
RNases.
Cap addition is coupled to transcription, and occurs co-transcriptionally, such that each influences the other. Shortly after the start of transcription, the 5' end of the mRNA being synthesized is bound by a cap-synthesizing complex associated with
RNA polymerase. This enzyme complex
catalyzes the chemical reactions that are required for mRNA capping. Synthesis proceeds as a multi-step biochemistry reaction.
Splicing
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).
Editing
In some instances, an mRNA will be
RNA editing, 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.
Polyadenylation
Polyadenylation is the covalent linkage of a polyadenylyl moiety to a messenger RNA molecule. In eukaryotic organisms, most messenger RNA (mRNA) molecules are polyadenylated at the 3' end. The
messenger RNA#3' poly(A) tail and the protein bound to it aid in protecting mRNA from degradation by exonucleases. Polyadenylation is also important for transcription termination, export of the mRNA from the nucleus, and translation. mRNA can also be polyadenylated in prokaryotic organisms, where poly(A) tails act to facilitate, rather than impede, exonucleolytic degradation.
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.
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 cell nucleus to the cytoplasm. Mature mRNAs are recognized by their processed modifications and then exported through the
nuclear pore.
Translation
Because prokaryotic mRNA does not need to be processed or transported,
translation by the
ribosome can begin immediately after the end 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.
Degradation
After a certain amount of time, the message is degraded by
RNases into its component nucleotides. The limited longevity of mRNA enables a cell to alter protein synthesis rapidly in response to its changing needs.
Different mRNAs within the same cell have distinct lifetimes. In bacterial cells, individual mRNAs can survive from seconds to more than an hour; in mammalian cells, mRNA lifetimes range from several minutes to days. The greater the stability of an mRNA, the more protein may be produced from that transcript. The presence of AU-rich motifs in some mammalian mRNAs tends to destabilize those transcripts through the action of cellular proteins that bind these motifs. Rapid mRNA degradation via AU-rich motifs is a critical mechanism for preventing the overproduction of potent cytokines such as tumor necrosis factor (TNF) and granulocyte-macrophage colony stimulating factor (GM-CSF).Shaw, G. and Kamen, R. "A conserved AU sequence from the 3' untranslated region of GM-CSF mRNA mediates selective mRNA degradation." Cell. 1986 Aug 29;46(5):659-67. PMID: 3488815 CELL Base pairing with a small interfering RNA (siRNA) or microRNA (miRNA) can also accelerate mRNA degradation.
mRNA structure
, 5' UTR, coding region,
3' UTR, and poly(A) tail.
5' cap
The
5' cap is a modified guanine nucleotide added to the "front" (5' end) of the pre-mRNA using a 5',5-Triphosphate linkage. This modification is critical for recognition and proper attachment of mRNA to the ribosome, as well as protection from 5' exonucleases. It may also be important for other essential processes, such as splicing and transport.
Coding regions
Coding regions are composed of
codons, which are decoded and translated into one (mostly eukaryotes) or several (mostly prokaryotes) proteins by the ribosome. Coding regions begin with the start codon and end with the one of three possible
stop codon . In addition to protein-coding, portions of coding regions may also serve as regulatory sequences in the
pre-mRNA as
exon splicing enhancer or
exonic splicing silencer .
Untranslated regions
Untranslated regions (UTRs) are sections of the RNA before the start codon and after the stop codon that are not translated, termed the five prime untranslated region (5' UTR) and three prime untranslated region (3' UTR), respectively. These regions are transcribed as part of the same transcript as the coding region. Several roles in gene expression have been attributed to the untranslated regions, including mRNA stability, mRNA localization, and translational efficiency. The ability of a UTR to perform these functions depends on the sequence of the UTR and can differ between mRNAs.
The stability of mRNAs may be controlled by the 5' UTR and/or 3' UTR due to varying affinity for RNA degrading enzymes called ribonucleases and for ancillary proteins that can promote or inhibit RNA degradation.
Translational efficiency, including sometimes the complete inhibition of translation, can be controlled by UTRs. Proteins that bind to either the 3' or 5' UTR may affect translation by influencing the ribosome's ability to bind to the mRNA.
microRNAs bound to the
3' UTR also may affect translational efficiency or mRNA stability.
Cytoplasmic localization of mRNA is thought to be a function of the 3' UTR. Proteins that are needed in a particular region of the cell can actually be translated there; in such a case, the 3' UTR may contain sequences that allow the transcript to be localized to this region for translation.
Some of the elements contained in untranslated regions form a characteristic secondary structure when transcribed into RNA. These structural mRNA elements are involved in regulating the mRNA. Some, such as the
SECIS element, are targets for proteins to bind. One class of mRNA element, the riboswitches, directly bind small molecules, changing their fold to modify levels of transcription or translation. In these cases, the mRNA regulates itself.
===3' poly(A) tail===The 3' poly(A) tail is a long sequence of adenine nucleotides (often several hundred) added to the "tail" or 3' end of the pre-mRNA through the action of an enzyme, polyadenylate polymerase. In higher eukaryotes, the poly(A) tail is added onto transcripts that contain a specific sequence, the AAUAAA signal. The importance of the AAUAAA signal is demonstrated by a mutation in the human alpha 2-globin gene that changes the original sequence AATAAA into AATAAG, which can lead to hemoglobin deficiencies.
-->
Monocistronic versus polycistronic mRNA
An mRNA molecule is said to be
monocistronic when it contains the genetic information to Translation (genetics) only a single protein. This is the case for most of the Eukaryote mRNAs{{ cite journal| author = Kozak, M.
| year = 1983
| month = March
| title = Comparison of initiation of protein synthesis in procaryotes, eucaryotes, and organelles.
| journal = Microbiological Reviews
| volume = 47
| issue = 1
| pages = 1-45
| doi =
| id = {{PMID|6343825-->
| url = http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=281560&blobtype=pdf
| format = PDF
| accessdate = 2006-08-12
-->
. On the other hand,
polycistronic mRNA carries the information of several proteins, which are translated into single proteins. Most of the mRNA found in bacteria and archea are polycistronic. Dicistronic is the term used to describe a mRNA that encodes only two proteins.
Anti-sense mRNA
During transcription, double stranded DNA produces mRNA from the
sense strand; the other, complementary, strand of DNA is termed
anti-sense. Anti-sense mRNA is an RNA complementary in sequence to one or more mRNAs. In some organisms, the presence of an anti-sense mRNA can inhibit gene expression by base-pairing with the specific mRNAs. In biochemistry research, this effect has been used to study gene function, by simply shutting down the studied gene by adding its anti-sense mRNA transcript. Such studies have been done on the worm
Caenorhabditis elegans and the
bacterium Escherichia coli. This plays a part in RNA interference and RNA transcription.
See also
References
External links
- Life of mRNA Flash animation
in the
cell nucleus; once completely processed, it is transported to the cytoplasm and
Translation (genetics) by the ribosome. At the end of its life, the mRNA is degraded.
Messenger Ribonucleic Acid (
mRNA) is a molecule of RNA encoding a chemical "blueprint" for a
protein product. mRNA is transcription (genetics) from a
DNA template, and carries coding information to the sites of
protein synthesis: the
ribosomes. Here, the nucleic acid polymer is
translation (genetics) into a polymer of
amino acids: a protein. In mRNA as in DNA, genetic information is encoded in the sequence of four nucleotides arranged into
codons of three bases each. Each codon encodes for a specific amino acid, except the stop codon that terminate protein synthesis. This process requires two other types of RNA:
Transfer RNA (
tRNA) mediates recognition of the codon and provides the corresponding amino acid, while
Ribosomal RNA (
rRNA) is the central component of the ribosome's protein manufacturing machinery.
mRNA "life cycle"
The brief life of an mRNA molecule begins with transcription and ultimately ends in degradation. During its life, an mRNA molecule may also be processed, edited, and transported prior to translation. eukaryotes mRNA molecules often require extensive processing and transport, while
prokaryote molecules do not.
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.
Eukaryotic pre-mRNA processing
Processing of mRNA differs greatly between
eukaryotes,
bacteria and archea. Non-eukaryotic mRNA is essentially mature upon transcription and requires no processing, except in rare cases. Eukaryotic pre-mRNA, however, requires extensive processing.
5' cap addition
A
5' cap (also termed an RNA cap, an RNA 7-methylguanosine cap or an RNA m7G cap) is a modified guanine nucleotide that has been added to the "front" or
5' end of a eukaryotic messenger RNA shortly after the start of transcription. The 5' cap consists of a terminal 7-methylguanosine residue which is linked through a 5'-5'-triphosphate bond to the first transcribed nucleotide. Its presence is critical for recognition by the
ribosome and protection from
RNases.
Cap addition is coupled to transcription, and occurs co-transcriptionally, such that each influences the other. Shortly after the start of transcription, the 5' end of the mRNA being synthesized is bound by a cap-synthesizing complex associated with RNA polymerase. This enzyme complex catalyzes the chemical reactions that are required for mRNA capping. Synthesis proceeds as a multi-step biochemistry reaction.
Splicing
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).
Editing
In some instances, an mRNA will be RNA editing, 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.
Polyadenylation
Polyadenylation is the covalent linkage of a polyadenylyl moiety to a messenger RNA molecule. In eukaryotic organisms, most messenger RNA (mRNA) molecules are polyadenylated at the 3' end. The messenger RNA#3' poly(A) tail and the protein bound to it aid in protecting mRNA from degradation by exonucleases. Polyadenylation is also important for transcription termination, export of the mRNA from the nucleus, and translation. mRNA can also be polyadenylated in prokaryotic organisms, where poly(A) tails act to facilitate, rather than impede, exonucleolytic degradation.
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.
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
cell nucleus to the cytoplasm. Mature mRNAs are recognized by their processed modifications and then exported through the
nuclear pore.
Translation
Because prokaryotic mRNA does not need to be processed or transported,
translation by the
ribosome can begin immediately after the end 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.
Degradation
After a certain amount of time, the message is degraded by
RNases into its component nucleotides. The limited longevity of mRNA enables a cell to alter protein synthesis rapidly in response to its changing needs.
Different mRNAs within the same cell have distinct lifetimes. In bacterial cells, individual mRNAs can survive from seconds to more than an hour; in mammalian cells, mRNA lifetimes range from several minutes to days. The greater the stability of an mRNA, the more protein may be produced from that transcript. The presence of AU-rich motifs in some mammalian mRNAs tends to destabilize those transcripts through the action of cellular proteins that bind these motifs. Rapid mRNA degradation via AU-rich motifs is a critical mechanism for preventing the overproduction of potent cytokines such as tumor necrosis factor (TNF) and granulocyte-macrophage colony stimulating factor (GM-CSF).Shaw, G. and Kamen, R. "A conserved AU sequence from the 3' untranslated region of GM-CSF mRNA mediates selective mRNA degradation." Cell. 1986 Aug 29;46(5):659-67. PMID: 3488815 CELL Base pairing with a small interfering RNA (siRNA) or microRNA (miRNA) can also accelerate mRNA degradation.
mRNA structure
, 5' UTR,
coding region, 3' UTR, and poly(A) tail.
5' cap
The
5' cap is a modified guanine nucleotide added to the "front" (
5' end) of the pre-mRNA using a 5',5-Triphosphate linkage. This modification is critical for recognition and proper attachment of mRNA to the ribosome, as well as protection from 5' exonucleases. It may also be important for other essential processes, such as splicing and transport.
Coding regions
Coding regions are composed of
codons, which are decoded and translated into one (mostly eukaryotes) or several (mostly prokaryotes) proteins by the ribosome. Coding regions begin with the
start codon and end with the one of three possible
stop codon . In addition to protein-coding, portions of coding regions may also serve as regulatory sequences in the
pre-mRNA as exon splicing enhancer or
exonic splicing silencer .
Untranslated regions
Untranslated regions (UTRs) are sections of the RNA before the start codon and after the stop codon that are not translated, termed the five prime untranslated region (5' UTR) and three prime untranslated region (3' UTR), respectively. These regions are transcribed as part of the same transcript as the coding region. Several roles in gene expression have been attributed to the untranslated regions, including mRNA stability, mRNA localization, and
translational efficiency. The ability of a UTR to perform these functions depends on the sequence of the UTR and can differ between mRNAs.
The stability of mRNAs may be controlled by the 5' UTR and/or 3' UTR due to varying affinity for RNA degrading enzymes called
ribonucleases and for ancillary proteins that can promote or inhibit RNA degradation.
Translational efficiency, including sometimes the complete inhibition of translation, can be controlled by UTRs. Proteins that bind to either the 3' or 5' UTR may affect translation by influencing the ribosome's ability to bind to the mRNA. microRNAs bound to the
3' UTR also may affect translational efficiency or mRNA stability.
Cytoplasmic localization of mRNA is thought to be a function of the 3' UTR. Proteins that are needed in a particular region of the cell can actually be translated there; in such a case, the 3' UTR may contain sequences that allow the transcript to be localized to this region for translation.
Some of the elements contained in untranslated regions form a characteristic
secondary structure when transcribed into RNA. These structural mRNA elements are involved in regulating the mRNA. Some, such as the SECIS element, are targets for proteins to bind. One class of mRNA element, the
riboswitches, directly bind small molecules, changing their fold to modify levels of transcription or translation. In these cases, the mRNA regulates itself.
===3' poly(A) tail===The 3' poly(A) tail is a long sequence of
adenine nucleotides (often several hundred) added to the "tail" or 3' end of the pre-mRNA through the action of an enzyme, polyadenylate polymerase. In higher eukaryotes, the poly(A) tail is added onto transcripts that contain a specific sequence, the AAUAAA signal. The importance of the AAUAAA signal is demonstrated by a mutation in the human alpha 2-globin gene that changes the original sequence AATAAA into AATAAG, which can lead to hemoglobin deficiencies.
-->
Monocistronic versus polycistronic mRNA
An mRNA molecule is said to be
monocistronic when it contains the genetic information to
Translation (genetics) only a single protein. This is the case for most of the
Eukaryote mRNAs{{ cite journal| author = Kozak, M.
| year = 1983
| month = March
| title = Comparison of initiation of protein synthesis in procaryotes, eucaryotes, and organelles.
| journal = Microbiological Reviews
| volume = 47
| issue = 1
| pages = 1-45
| doi =
| id = {{PMID|6343825-->
| url = http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=281560&blobtype=pdf
| format = PDF
| accessdate = 2006-08-12
-->
. On the other hand, polycistronic mRNA carries the information of several proteins, which are translated into single proteins. Most of the mRNA found in
bacteria and
archea are polycistronic. Dicistronic is the term used to describe a mRNA that encodes only two proteins.
Anti-sense mRNA
During transcription, double stranded DNA produces mRNA from the
sense strand; the other, complementary, strand of DNA is termed
anti-sense. Anti-sense mRNA is an RNA complementary in sequence to one or more mRNAs. In some organisms, the presence of an anti-sense mRNA can inhibit
gene expression by base-pairing with the specific mRNAs. In biochemistry research, this effect has been used to study gene function, by simply shutting down the studied gene by adding its anti-sense mRNA transcript. Such studies have been done on the
worm Caenorhabditis elegans and the bacterium
Escherichia coli. This plays a part in
RNA interference and RNA transcription.
See also
References
External links
- Life of mRNA Flash animation
Messenger RNA - Wikipedia, the free encyclopedia
Messenger ribonucleic acid (mRNA) is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding ...
Definition: mRNA from Online Medical Dictionary
messenger RNA -->
Definition: messenger RNA from Online Medical Dictionary
The Online Medical Dictionary is a searchable dictionary of definitions from medicine, science and technology.
genome.gov | Talking Glossary: "messenger rna (mrna)"
Easy-to-use, talking glossary of genetic terms, produced by the National Human Genome Research Institute.
Messenger RNA - Glossary - Pharmaceutical International
Messenger RNA (or mRNA) is RNA that copies information from DNA in a cell nucleus and transfers it into the cytoplasm that surrounds the nucleus.
messenger RNA - definition of messenger RNA in the Medical dictionary ...
mes·sen·ger RNA (m s n-j r)
Transcription
The upper image is an electron micrograph of a mRNA-DNA hybrid molecule formed by mixing the messenger RNA (mRNA) from a clone of antibody-secreting cells with single-stranded DNA ...
1600218148 : 9781600218149:Messenger RNA Research Perspectives
Gazelle Book Services Limited. White Cross Mills, Hightown, LANCASTER LA1 4XS, United Kingdom. Telephone: +44(0)1524 68765 Fax: +44(0)1524 63232
glossary
Messenger RNA (m-RNA) m-RNA carries the genetic code for a protein from the DNA to the ribosomes where the code is read and the protein manufactured.
Messenger RNA
Messenger RNA Author: brador Text: How does messenger RNA work? Response #: 1 of 1 Author: moodywj Text: mRNA attaches to an organelle in the ...
