Ribonucleic acid (Ri | bo | nu | kle | in | säu | re; short RNS ; English RNA for ribonucleic acid ) (Latin-French-Greek artificial word ) is a nucleic acid that is a polynucleotide made up of a chain of many nucleotides composed. In certain virus types ( RNA viruses , retroviruses ) and the hypothetical primeval ribocytes, the biomolecule is the carrier of genetic information , i.e. the material basis of the genes . The word is made up of ribose and nucleic acid .
An essential function of the RNA in the biological cell is the conversion of genetic information into proteins (see protein biosynthesis , transcription and translation ), in the form of the mRNA it acts as a carrier of information. In addition, special types of RNA perform other tasks; in the case of RNA viruses , it even makes up the genome itself. Furthermore, parts of the cell components responsible for the conversion of this information also consist of RNA: snRNA and snoRNA are involved in the maturation of the mRNA , the catalytic components of the ribosomes form the rRNA , and the tRNA transports the building blocks for the proteins. Special RNAs are also involved in gene regulation .
Structure and difference to DNA
In terms of structure, RNA is similar to DNA . In contrast to double-stranded DNA, RNA molecules are usually single-stranded, but can develop characteristic refoldings in short stretches with complementary base sequences (AU, GC), which give the intramolecular impression of a double-stranded helix. Both are polynucleotides in which the nucleobases on sugars are linked to one another via phosphoric acid diesters. Single-strandedness increases the number of possibilities for three-dimensional structures in RNA and allows it to undergo chemical reactions that DNA cannot. In RNA, every nucleotide consists of a ribose (i.e. a pentose : a sugar with five carbon atoms), a phosphate residue and an organic base . The ribose of the RNA is identical to that of the DNA, except for a hydroxyl group instead of a hydrogen atom at the 2 'position in the pentose ring (hence also deoxy ribonucleic acid, D NA). This difference makes the RNA less stable compared to the DNA, since it enables hydrolysis by bases: the OH group at the 2 'position of the sugar is deprived of its proton by a negatively charged hydroxide ion and the oxygen that remains is deprived of its proton Ring bond with the phosphorus, whereby the bond to the next nucleotide is broken. The RNA is broken down into its nucleotides.
The following organic bases occur in RNA : adenine , guanine , cytosine and uracil . The first three bases are also found in DNA. Uracil, on the other hand, replaces thymine as a complementary base to adenine. RNA presumably uses uracil because it is less energetic to manufacture (no methyl substitution).
Hairpin, stemloop and loop structures are known as secondary structures in RNA, but a helix conformation is also possible, with hairpin and stemloop structures having both single-stranded and double-stranded areas. The loop structures denote single-stranded loop structures within a molecule.
Like DNA, RNA can also be present as a double-stranded molecule. It has the typical features of a Watson-Crick helix: anti-parallel arrangement of the RNA strands and right-hand helix. It takes the form of an A or A 'helix (see DNA ). The A-RNA is also referred to as RNA-11, homologous to the A'-RNA, which is referred to as RNA-12. The number after the dash indicates the number of base pairs per helix turn. A´-RNA often occurs at high salt concentrations (over 20%).
A-RNA: 11 base pairs per helix turn, pitch 2.7 nm to 2.8 nm, angle of inclination to the helix axis approx. 14 °
A'-RNA: 12 base pairs per helix turn, pitch 3 nm, angle of inclination to the helix axis 16 ° to 19 °
The enantiomer of RNA found in living things is D- RNA. It is made up of D -ribonucleotides. The centers of chirality are in the D- ribose. By using L ribose or L -Ribonukleotiden can be L RNA synthesis. This is comparatively more stable compared to the enzymatic degradation by RNases .
Synthesis of RNA
The enzyme RNA polymerase catalyzes the DNA through the process of transcription from nucleoside triphosphate (NTP) the RNA. To do this, the RNA polymerase attaches itself to a nucleotide sequence of the DNA called a promoter ( transcription initiation ). Then it separates the DNA double helix into two single strands of DNA by breaking the hydrogen bonds in a short area. On codogenic strand of DNA are deposited by base pairing complementary ribonucleotides on. With elimination of a pyrophosphate, they are linked to one another by an ester-like bond between phosphoric acid and ribose . The reading direction of the DNA runs from the 3 'end to the 5' end, the synthesis of the complementary RNA accordingly 5 '→ 3'. The opening of the DNA double helix takes place only in a short area, so that the part of the RNA that has already been synthesized hangs out of this opening, with the 5 'end of the RNA first. The synthesis of the RNA is terminated at a DNA segment called a terminator. The RNA transcript is then released and the RNA polymerase separates from the DNA.
RNA can be artificially produced by phosphoramidite synthesis .
RNA molecules can perform different functions. The RNA can transmit genetic information. Other RNA molecules help translate this information into proteins and regulate genes . In addition, RNA can also have catalytic functions similar to an enzyme . RNA is therefore given different names, depending on its function. Preceding lower case letters indicate the different RNA types:
- The mRNA , messenger RNA (engl. Messenger RNA ) copies the information in a gene lying on the DNA and transmits it to the ribosome , where with the help of this information, protein synthesis can take place. In each case three nucleotides lying next to one another in the reading frame of the polynucleotide strand form a codon with the help of which a specific amino acid that is to be incorporated into a protein can be clearly determined. This connection was found in 1961 by Heinrich Matthaei and Marshall Warren Nirenberg . The decoding of the genetic code marks a new beginning in almost all bio-sciences.
The following classes of RNA are commonly referred to as non-coding ribonucleic acids .
- The asRNA , antisense RNA , is used to regulate gene expression .
- The circRNA , circular RNA, is involved in regulation by binding to miRNA.
- The hnRNA , heterogeneous nuclear RNA, occurs in the cell nucleus of eukaryotes and is a precursor of the mature mRNA, which is why it is often referred to as pre-mRNA (or pre-mRNA for precursor mRNA) .
- The miRNAs , microRNAs are closely related to the siRNAs and are used to regulate cellular processes such. B. Proliferation and Cell Death.
- The riboswitches are used for gene regulation . They can be either activating or repressing.
- The ribozymes are catalytically active RNA molecules. Like enzymes, they catalyze chemical reactions.
- The rRNA , ribosomal RNA , like the tRNA, does not carry any genetic information, but is involved in the construction of the ribosome and is also catalytically active in the formation of the peptide bond .
- The saRNA , self-ammunition RNA , is used in RNA vaccines to extend the duration of action.
- The siRNA , small interfering RNA , arises from a signal path of the cell, which is summarized as RNAi (RNA Interference). Here, dsRNA (double-stranded RNA) is broken up into many smaller fragments of approx. 22 nucleotides in length by the enzyme Dicer (the siRNAs ) and incorporated into the enzyme complex RISC (RNA-induced silencing complex). With the help of the incorporated RNA fragments, RISC binds complementarily to DNA, e.g. B. gene areas or mRNA and can "switch off" them. siRNAs are currently (2006) intensively researched for their involvement in various cell processes and diseases.
- The shRNA is used for RNAi.
- The snoRNA , small nucleolar RNA , is found in the nucleolus , and the closely related scaRNAs in the Cajal bodies .
- The snRNA , small nuclear RNA , in the cell nucleus of eukaryotes is responsible for splicing the hnRNA at the spliceosome .
- The lncRNA , long non-coding RNA , are longer than 200 nucleotides and thus differ from small regulatory RNAs such as miRNAs and siRNAs.
- The piRNA , Piwi-interacting RNA, are 26–31 nucleotides long and thus differ from the somewhat smaller miRNAs and siRNAs. They form complexes with PIWI proteins that are involved in epigenetic and post-transcriptional silencing in germ cells.
- The tRNA , transfer RNA , does not encode any genetic information, but serves as an auxiliary molecule in protein synthesis by taking up a single amino acid from the cytoplasm and transporting it to the ribosome. The tRNA is encoded by a specific RNA gene .
- The tracrRNA , which plays an important role in the CRISPR / Cas9 system.
In the majority of living beings, RNA as an information carrier plays a subordinate role to DNA: Here, DNA is the permanent storage medium for genetic information, while RNA serves as a temporary storage facility. Only RNA viruses (the majority of all viruses) use RNA instead of DNA as a permanent storage medium. A distinction is made between the following RNA types for the taxonomy of viruses :
* dsRNA : double-stranded RNA;
* ss (+) RNA : single-stranded RNA used as mRNA;
* ss (-) RNA : single-stranded RNA that serves as a template for mRNA production.
Breakdown of RNA
Since new RNA is constantly being formed and since different transcripts are required at different times (differential gene expression), the RNA in the cell must not be too stable, but must also be subject to degradation. This is done with the help of RNases , enzymes that separate the connections of the sugar structure of the RNA and thus form the monomers (or oligomers), which can be used again to form new RNA. When an RNA is to be broken down is mainly (but not exclusively) determined by the length of the poly-A tail , which is successively shortened as the RNA remains in the cytoplasm. If the length of this tail falls below a critical value, the RNA is quickly degraded. In addition, the RNAs can contain stabilizing or destabilizing elements that enable further regulation.
At least in the case of the mRNA of eukaryotes , the RNA breakdown does not take place somewhere in the cytoplasm , but in the so-called “P-Bodies” ( processing bodies ), which are very rich in RNases and other involved in the RNA turnover (breakdown) Are enzymes. Together with Stress Granules , these bodies continue to serve for the short-term storage of mRNA and thus again demonstrate the close link between RNA metabolism (here translation and RNA breakdown).
The RNA World Hypothesis
The RNA world hypothesis states that RNA molecules were probably the precursors of organisms in chemical evolution . The hypothesis can be derived from the ability of RNA to store, transfer and replicate genetic information and from its ability to catalyze reactions as ribozymes . In an evolutionary environment, those RNA molecules would occur more frequently that preferentially reproduce themselves.
Several Nobel Prizes have already been awarded for research on RNA :
- In 1959, S. Ochoa and A. Kornberg received the Nobel Prize in Medicine for their studies on the synthesis of RNA by RNA polymerases .
- For their discovery of the catalytic activity of RNA molecules (cf. ribozyme ), S. Altman and T. Cech were honored with the 1989 Nobel Prize in Chemistry.
- In 1993 R. Roberts and P. Sharp received the Nobel Prize for Medicine for their studies on the processing of RNA in eukaryotes (see also splicing (biology) ).
- In 2006 Andrew Fire and Craig Mello received the Nobel Prize in Medicine for the discovery of RNA interference ; In the same year Roger Kornberg (son of Nobel Prize winner A. Kornberg) was honored with the Nobel Prize in Chemistry for his studies on RNA polymerase.
- In 2009, Ada Yonath was awarded the Nobel Prize in Chemistry “for studies on the structure and function of the ribosome” together with Venkatraman Ramakrishnan and Thomas A. Steitz .
RNA purification and detection
RNA can be purified by RNA purification , e.g. B. by RNA extraction , are separated from other biomolecules . The amount and purity of the isolated RNA is determined by photometric measurement at a wavelength of 260 and 280 nm. Further information on the quality of the RNA is obtained by agarose gel electrophoresis followed by staining with dyes such as SYBR Green II , methylene blue , Stains-All or by a silver coloring . The qualitative detection of RNA (whether a certain RNA is present) is usually done by RT-PCR , sometimes with subsequent DNA sequencing , or by Northern blot . The quantitative detection (how much of a certain RNA is present) is usually carried out by qRT-PCR ; in the case of purified samples with only one RNA sequence, the concentration can also be determined by photometry . The RNA can be stretched and aligned through molecular combining . Using in situ hybridization , specific RNAs can be detected in cells and tissues without prior isolation.
RNA is used for different purposes. In the case of ribozymes , the RNA has an enzymatic activity , while aptamers enter into a longer-term bond to a target structure. Short double-stranded RNA in the form of siRNA and shRNA is used to temporarily suppress gene expression by means of RNA interference . RNA vaccines are one of the genetic vaccines in which the antigen is made within the cells of the person being vaccinated.
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