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).
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.
Comparison to DNA
Structurally, RNA is indistinguishable from DNA except for the critical presence of a hydroxyl group attached to the pentose ring in the 2' position (DNA has a hydrogen atom rather than a hydroxyl group). This hydroxyl 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.
RNA is made by an enzyme, RNA polymerase, using DNA as a template. The synthesis begins when the enzyme binds to special promoter regions on the DNA helix; these regions mark the beginning of a gene. The DNA double helix is unwound by the helicase activity of the enzyme. RNA is then synthesised so that it is complementary to one of strands in the DNA. The strand from which the RNA is encoded is called the template strand; the opposite strand is called the coding strand (so called because it logically has the equivalent nucleotide sequence of the new RNA strand). A small stretch of DNA-RNA hybrid is present at the active site of the enzyme. The synthesis continues until a termination sequence is reached. The resulting RNA molecule is the primary transcript. Modulation of the rate of initiation of RNA synthesis is one of the most important ways in which gene expression is regulated.
RNA world hypothesis
The RNA world hypothesis proposes that the 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:
- Messenger RNA (mRNA) is transcribed directly from a gene's DNA and is used to encode proteins.
- RNA genes are genes that encode functional RNA molecules; in contrast to mRNA, these RNA do not code for proteins. The best-known examples of RNA genes are transfer RNA (tRNA) and ribosomal RNA (rRNA). Both forms participate in the process of translation, but many others exist.
- RNA forms the genetic material (genomes) of some kinds of viruses.
- Double-stranded RNA (dsRNA) is used as the genetic material of some RNA viruses and is involved in some cellular processes, such as RNA interference.
Messenger RNA (mRNA)
Main article: Messenger RNA
Messenger RNA is RNA that carries information from DNA to the ribosome sites of protein synthesis in the cell. Once mRNA has been transcribed from DNA, it is exported from the nucleus into the cytoplasm (in eukaryotes mRNA is "processed" before being exported), 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.
Non-coding RNA or "RNA genes"
Main article: Non-coding RNA
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.