Ribonucleic acid ( RNA) is a nucleic acid, a polynucleotide composed of a chain of many nucleotides. In scientific parlance ribonucleic acid is named with the acronym RNA ( ribonucleic acid ), often also in German.
An essential function of the RNA in the biological cell, the implementation of the genetic information in the protein (see protein, transcription and translation ), in the form of the mRNA, it acts in this case as an information carrier. In addition, special types of RNA perform other tasks; in RNA viruses it makes even the genome itself. Furthermore, there are also parts of the body responsible for the implementation of this information cell constituents from RNA: With the maturation of mRNA snRNA and snoRNA involved, the catalytic components of the ribosome is rRNA and tRNA transports the building blocks for proteins. Further specific RNAs are involved in gene regulation.
Structure and contrast to DNA
Structurally, the RNA of the DNA is similar. RNA molecules are - in contrast to the double-stranded DNA - usually single-stranded, but can form characteristic foldover that bring intramolecularly the impression of a double-stranded helix in short distances with complementary base sequences ( AU, GC). Both are polynucleotides in which the nucleobases are connected via phosphoric sugars linked together. The Einzelsträngigkeit increases the number of possibilities for three-dimensional structures of RNA and allowed their chemical reactions, which are the DNA is not possible. Each nucleotide consists of a ribose in RNA (i.e., a pentose: a sugar with five carbon atoms ), a phosphate group and an organic base. The ribose of the RNA is identical with that of the DNA, up to a hydroxyl group (instead of a hydrogen atom, lat Hydrogenium ) at the 2'- position in the pentose ring (hence deoxyribonucleic acid, DNA). This difference makes RNA less stable than DNA because it enables hydrolysis by bases: The OH group at the 2 ' position of the sugar is robbed by a negatively charged hydroxide ion of a base of their proton and the then remaining oxygen is a ring bond with the a phosphor, whereby the bond to the next nucleotide is released. The RNA is then separated into its nucleotides.
In RNA the following organic bases can be found: adenine, guanine, cytosine and uracil. The first three bases are also present in the DNA. Uracil replaces thymine on the other hand as complementary base to adenine. Probably uses RNA uracil, since this is energetically less expensive to produce ( no methyl substitution ).
As secondary structures especially hairpin, Stemloop and loop structures are known in the RNA, a helical conformation but is also possible, wherein said hairpin and Stemloop structures both single-stranded and double- stranded regions. The loop structures called single- loop structures within a molecule.
RNA, as DNA is also present as a double-stranded molecule. It thereby has the typical characteristics of a Watson- Crick helix: antiparallel arrangement of the RNA strands and right-handed helix. It assumes the form of an A- or A'- helix at (cf. DNA). The A RNA is referred to as an RNA -11, homologous to the a ' RNA which is referred to as an RNA -12. Here are the number after the indent the number of base pairs per helical turn again. A'- RNA often occurs at high salt concentrations (above 20 %).
A- RNA: 11 base pairs per helical turn, pitch of 2.7 nm to 2.8 nm, angle of inclination to the helical axis about 14 ° A'- RNA: 12 base pairs per helical turn, pitch of 3 nm, angle of inclination to the helical axis 16 ° to 19 °
Synthesis of RNA
The enzyme RNA polymerase catalyses the DNA by the process of transcription of nucleoside triphosphate (NTP) RNA. In return, the RNA polymerase recognizes a promoter called nucleotide sequence of DNA ( transcription initiation ). Then it separates the DNA double helix by releasing the hydrogen bonds in a short range into two single strands of DNA on. On codogenic strand of DNA deposited by base pairing complementary ribonucleotides to. You are eliminating a pyrophosphate linked by an ester type bond between phosphoric acid and ribose. The reading direction of the DNA extending from the 3'- end to the 5'-end, the synthesis of the complementary RNA accordingly 5 '→ 3'. The opening of the DNA double helix is only in a short region, such that the portion of the RNA already synthesized hanging out of this opening and that with the above 5'- end of the RNA. The synthesis of RNA is terminated at a terminator said DNA section. Then the RNA transcript is released, and the RNA polymerase detaches from the DNA.
RNA molecules can exert different functions. The RNA can transfer genetic information. Other RNA molecules that contribute to the translation information into proteins, and in the regulation of genes. In addition, RNA and catalytic functions similar to hold an enzyme. RNA is therefore - also named differently - depending on their function. Leading lowercase letters indicate the different types of RNA:
- The mRNA, messenger RNA (English messenger RNA) copies the information lying in a gene on the DNA and carries them to the ribosome, where with the help of this information, the protein can take place. Each three adjacent in the reading frame of the polynucleotide strand of nucleotides form a codon, with the help of which a specific amino acid to be incorporated into a protein can be determined unambiguously. This relationship was found in 1961 by Heinrich Matthaei and Marshall Warren Nirenberg. The deciphering of the genetic code marks a new beginning in almost all bio - sciences.
The following RNA classes commonly referred to as non-coding RNAs.
- The asRNA, antisense RNA, regulation of gene expression serves.
- The circRNA, circular RNA is involved by binding to miRNA in the regulation.
- The hnRNA, heterogeneous nuclear RNA ( engl. heterogeneous nuclear RNA) is found in the nucleus of eukaryotic cells and is a precursor of the mature mRNA, often it is therefore also known as pre-mRNA ( or English. Pre-mRNA for precursor mRNA) designated.
- The miRNAs, microRNAs are closely related to the siRNAs and serves the regulation of cellular processes such as cell proliferation and cell death.
- The riboswitches are used for gene regulation. They can act either activating or repressing.
- The Ribozymes are catalytically active RNA molecules. They catalyze chemical reactions such as enzymes.
- The rRNA, ribosomal RNA, similarly to the tRNA, no genetic information, but is involved in the structure of the ribosome and is also catalytically active in the formation of the peptide bond.
- The siRNA, small interfering RNA, is formed in a pathway of the cell, which is summarized as RNAi (RNA interference). It is dsRNA ( double-stranded RNA; double-stranded RNA English ) by the enzyme Dicer into many smaller fragments of about 22 nucleotides in length divided ( siRNAs ) and incorporated into the enzyme complex RISC (RNA - induced silencing complex ). Using the incorporated RNA fragments binds RISC to complementary DNA, such as gene regions or mRNA, and this can "turn off" it. siRNAs are currently (2006) researched intensively on their involvement in various cellular processes and diseases.
- The snoRNA, small nucleolar RNA can be found in the nucleolus, and the closely related scaRNAs in the Cajal bodies.
- SnRNA, small nuclear RNA in the nucleus of eukaryotic cells and is responsible for the splicing of the spliceosome hnRNA.
- The tRNA, transfer RNA does not encode genetic information, but also serves as an auxiliary molecule in protein biosynthesis by accommodating a single amino acid from the cytoplasm and transported to the ribosome. TRNA is encoded by a specific RNA gene.
In the majority of living organisms, the RNA plays as an information carrier of a DNA minor role: The DNA is here the permanent storage medium for genetic information, RNA serves as a buffer. Only RNA viruses ( the majority of all viruses) use RNA instead of DNA as a permanent storage medium. For the taxonomy of viruses are divided into the following types of RNA:
- DsRNA: double-stranded RNA;
- Ss ( ) RNA: single-stranded RNA is used as a mRNA;
- Ss (-) RNA: single-stranded RNA, which serves as a template for mRNA production.
In addition, some viruses use RNA as Replikationsintermediat (eg, retroviruses and hepadnaviruses ).
Degradation of RNA
Because new RNA is formed and as needed at different times different transcripts ( differential gene expression ), the RNA also a reduction in the cell may not be too stable, but must be subject. This is done with the aid of RNases, enzymes, separating the compounds from the sugar - backbone of the RNA, and thus, the monomers ( or oligomers ) form which can be re-used to form new RNA. When an RNA is to be reduced, is primarily ( but not exclusively) determined by the length of the poly - A tail, which is successively shortened with increasing residence time of the RNA in the cytoplasm. Decreases the length of this tail below a critical value, the RNA is degraded rapidly. In addition, the RNAs may contain stabilizing or destabilizing elements that enable a further regulation.
At least with the mRNA of eukaryotes RNA degradation is not somewhere in the cytoplasm takes place, but in the so-called "P- bodies " (processing bodies ) involved in the RNases and others in RNA turnover ( degradation ) involved very rich enzymes are. Along with "stress granules " these bodies continue to serve the short-term storage of mRNA and demonstrate that, in turn, the close linkage of RNA metabolism (in this translation, and RNA degradation ).
The RNA world hypothesis
The RNA world hypothesis states that RNA molecules were the forerunners of the organisms. The hypothesis can be derived from the ability of the RNA to the storage, transmission and reproduction of genetic information, as well as their ability to catalyze reactions as ribozymes. In an evolutionary environment those RNA molecules would frequently occur, the self-spawning preferred.
Several Nobel Prizes have already been awarded for research on RNA: