RNA
Ribonucleic acid (RNA) is a nucleic acid consisting of a string of covalently-bound nucleotides. It is biochemically distinguished from DNA by the presence of an additional hydroxyl group, attached to each pentose ring; as well as by the use of uracil, instead of thymine. RNA transmits genetic information from DNA (via transcription) into proteins (by translation).
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2 Comparison to DNA 3 RNA world hypothesis 4 Biological role 5 Messenger RNA 6 RNA genes 7 Double-stranded RNA 8 See also |
RNA has four different bases: adenine, guanine, cytosine, and uracil. The first three are the same as those found in DNA, but uracil replaces thymine as the base complementary to adenine. This may be because uracil is energetically less expensive to produce, although it easily degenerates into cytosine. Thus, uracil is appropriate for RNA, where quantity is important but lifespan is not, whereas thymine is appropriate for DNA.
Structurally, RNA is indistinguishable from DNA except for the critical presence (noted above) of an additional hydroxyl group attached to the pentose ring in the 2' position. This additional group gives the molecule far greater catalytic versatility and allows it to perform reactions that DNA is incapable of performing; but at the same time, it makes RNA sensitive to alkaline hydrolysis, to which DNA is not.
The other major difference between RNA and DNA is that RNA is almost exclusively found in the single-stranded form (an exception being the genetic material of some kinds of viruses). RNA molecules often fold into more complex structures by making use of complementary internal sequences; that is, one part of a single RNA molecule is the nucleic acid complement of another part of the same molecule (for example, 5'-ACUCGA-3' and 5'-UCGAGU-3'), so that the two strands bind together. This allows the formation of hairpin loops, coils, etc., which then direct the formation of higher-order structures.
The RNA world hypothesis proposes that the a universal ancestor to all life relied on RNA both to carry genetic information like DNA and to catalyze biochemical reactions like an enzyme. In effect, RNA was, before the emergence of the first cell, the dominant, and probably the only, form of life. This hypothesis is inspired by the fact that retroviruses use RNA as their sole genetic material, while peptide bond formation in the ribosome is carried out by an RNA-derived ribozyme. From this perspective, retroviruses and ribozymes are remnants, or molecular fossils, left over from that RNA world. Assuming that DNA is better suited for storage of genetic information and proteins are better suited for the catalytic needs of cells, one would expect reduced use of RNA in cells, and greater use of DNA and proteins.
RNA plays several roles in biology:
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:
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 heteronucleic RNA).
There are sections of the RNA before and after its start and stop sequences that are not translated. These come from the template DNA strand that the RNA was transcribed from. These regions, known as the 5'UTR and 3'UTR (five-prime and three-prime untranslated regions, respectively, due to the fact that DNA and RNA run from 5' to 3' and this region is on the end of the RNA sequence) code for no protein sequences. However, their importance lies in the belief that the sequence of the 5' UTR and 3' UTR may, by their varying affinity for certain RNase enzymes, promote or inhibit the relative stability of the RNA molecule. Certain UTRs may allow the RNA to survive longer in the cytoplasm before being degraded, thus allowing them to produce more protein, while others may be degraded sooner, thus lasting a shorter time and producing a smaller relative amount of protein.
Also, there is evidence that certain complexes within the UTRs may not only affect the stability of the molecule, but that they may promote translational efficiency or even cause inhibition of translation altogether, depending on the sequences in the UTRs.
Some functional 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.
Anti-sense mRNA can inhibit gene translation in many eukaryotes, when
the anti-sense RNA's sequence is complementary to that of the mRNA of the gene. This means a gene is not expressed as protein if a matching anti-sense mRNA is present in the cell. This may be a defense mechanism against retroposons (transposons that use dsRNA as an intermediate state) or viruses, because both can use double-stranded mRNA as an intermediate. In biochemical research, this effect has been used to study gene function, simply shutting down the
studied gene by adding its anti-sense mRNA transcript. Such studies
have been done on the worm C. elegans.
Compare RNA interference.
RNA genes (sometimes referred to as non-coding RNA or small RNA) are genes that encode RNA that is not translated into a protein.
The most prominent examples of RNA genes are transfer RNA (tRNA) and ribosomal RNA (rRNA), both of which are involved in the process of translation. However, since the late 1990s, many new RNA genes have been found, and thus RNA genes may play a much more significant role than previously thought.
Double-stranded RNA (or dsRNA) is RNA with two complementary strands, similar to the DNA found in all "higher" cells. dsRNA forms the genetic material of some viruses. In eukaryotes, it may play a role in the process of RNA interference and in microRNAs.Chemical structure
Comparison to DNA
RNA world hypothesis
Biological role
Messenger RNA
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.Untranslated regions
Anti-sense mRNA
RNA genes
Main article: RNA geneDouble-stranded RNA