RNA
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Ribonucleic acid (RNA) is a
Some RNA molecules play an active role within cells by catalyzing biological reactions, controlling gene expression, or sensing and communicating responses to cellular signals. One of these active processes is protein synthesis, a universal function in which RNA molecules direct the synthesis of proteins on ribosomes. This process uses transfer RNA (tRNA) molecules to deliver amino acids to the ribosome, where ribosomal RNA (rRNA) then links amino acids together to form coded proteins.
It has become widely accepted in science
Comparison with DNA
Like DNA, most biologically active RNAs, including
In this fashion, RNAs can achieve chemical catalysis (like enzymes).[7] For instance, determination of the structure of the ribosome—an RNA-protein complex that catalyzes peptide bond formation—revealed that its active site is composed entirely of RNA.[8]
Structure
Each nucleotide in RNA contains a ribose sugar, with carbons numbered 1' through 5'. A base is attached to the 1' position, in general, adenine (A), cytosine (C), guanine (G), or uracil (U). Adenine and guanine are purines, and cytosine and uracil are pyrimidines. A phosphate group is attached to the 3' position of one ribose and the 5' position of the next. The phosphate groups have a negative charge each, making RNA a charged molecule (polyanion). The bases form hydrogen bonds between cytosine and guanine, between adenine and uracil and between guanine and uracil.[9] However, other interactions are possible, such as a group of adenine bases binding to each other in a bulge,[10] or the GNRA tetraloop that has a guanine–adenine base-pair.[9]
An important structural component of RNA that distinguishes it from DNA is the presence of a
RNA is transcribed with only four bases (adenine, cytosine, guanine and uracil),
There are more than 100 other naturally occurring modified nucleosides.
The functional form of single-stranded RNA molecules, just like proteins, frequently requires a specific
The naturally occurring enantiomer of RNA is D-RNA composed of D-ribonucleotides. All chirality centers are located in the D-ribose. By the use of L-ribose or rather L-ribonucleotides, L-RNA can be synthesized. L-RNA is much more stable against degradation by RNase.[26]
Like other structured biopolymers such as proteins, one can define topology of a folded RNA molecule. This is often done based on arrangement of intra-chain contacts within a folded RNA, termed as circuit topology.
Synthesis
Synthesis of RNA is usually catalyzed by an enzyme—
There are also a number of RNA-dependent RNA polymerases that use RNA as their template for synthesis of a new strand of RNA. For instance, a number of RNA viruses (such as poliovirus) use this type of enzyme to replicate their genetic material.[28] Also, RNA-dependent RNA polymerase is part of the RNA interference pathway in many organisms.[29]
Types of RNA
Overview
Messenger RNA (mRNA) is the RNA that carries information from DNA to the ribosome, the sites of protein synthesis (translation) in the cell. The mRNA is a copy of DNA. The coding sequence of the mRNA determines the amino acid sequence in the protein that is produced.[30] However, many RNAs do not code for protein (about 97% of the transcriptional output is non-protein-coding in eukaryotes[31][32][33][34]).
These so-called
In length
According to the length of RNA chain, RNA includes
In translation
Ribosomal RNA (rRNA) is the catalytic component of the ribosomes. The rRNA is the component of the ribosome that hosts translation. Eukaryotic ribosomes contain four different rRNA molecules: 18S, 5.8S, 28S and 5S rRNA. Three of the rRNA molecules are synthesized in the nucleolus, and one is synthesized elsewhere. In the cytoplasm, ribosomal RNA and protein combine to form a nucleoprotein called a ribosome. The ribosome binds mRNA and carries out protein synthesis. Several ribosomes may be attached to a single mRNA at any time.[30] Nearly all the RNA found in a typical eukaryotic cell is rRNA.
Regulatory RNA
The earliest known regulators of
RNA interference by miRNAs
Post-transcriptional expression levels of many genes can be controlled by
Long non-coding RNAs
Next to be linked to regulation were
Enhancer RNAs
The third major group of regulatory RNAs is called
Regulatory RNA in prokaryotes
At first, regulatory RNA was thought to be a eukaryotic phenomenon, a part of the explanation for why so much more transcription in higher organisms was seen than had been predicted. But as soon as researchers began to look for possible RNA regulators in bacteria, they turned up there as well, termed as small RNA (sRNA).
Archaea also have systems of regulatory RNA.[58] The CRISPR system, recently being used to edit DNA in situ, acts via regulatory RNAs in archaea and bacteria to provide protection against virus invaders.[46][59]
In RNA processing
Many RNAs are involved in modifying other RNAs.
RNA genomes
Like DNA, RNA can carry genetic information. RNA viruses have genomes composed of RNA that encodes a number of proteins. The viral genome is replicated by some of those proteins, while other proteins protect the genome as the virus particle moves to a new host cell. Viroids are another group of pathogens, but they consist only of RNA, do not encode any protein and are replicated by a host plant cell's polymerase.[65]
In reverse transcription
Reverse transcribing viruses replicate their genomes by
Double-stranded RNA
Double-stranded RNA (dsRNA) is RNA with two complementary strands, similar to the DNA found in all cells, but with the replacement of thymine by uracil and the adding of one oxygen atom. dsRNA forms the genetic material of some
Circular RNA
In the late 1970s, it was shown that there is a single stranded covalently closed, i.e. circular form of RNA expressed throughout the animal and plant kingdom (see circRNA).[73] circRNAs are thought to arise via a "back-splice" reaction where the spliceosome joins a upstream 3' acceptor to a downstream 5' donor splice site. So far the function of circRNAs is largely unknown, although for few examples a microRNA sponging activity has been demonstrated.
Key discoveries in RNA biology
Research on RNA has led to many important biological discoveries and numerous
The sequence of the 77 nucleotides of a yeast tRNA was found by
In the early 1970s,
In 1977,
At about the same time, 22 nt long RNAs, now called microRNAs, were found to have a role in the development of C. elegans.[82] Studies on RNA interference gleaned a Nobel Prize for
Relevance for prebiotic chemistry and abiogenesis
In 1968,
In March 2015,
Medical applications
RNA, initially deemed unsuitable for therapeutic use due to its short half-life, has been proven to possess numerous therapeutic properties through advancements in stabilization chemistry. RNA molecules have potential therapeutic applications due to their ability to fold into complex conformations and binding proteins, nucleic acids, small molecules, and form catalytic centers.[91] RNA-based vaccines are thought to be a quicker way to obtain immunological resistance than the traditional approach of vaccines that rely on a killed or altered version of the pathogen, because it can take months or even years to grow and study a pathogen in order to determine which molecular parts to extract, inactivate, and use in a vaccine. Small molecules with conventional therapeutic properties can target RNA and DNA structures, thereby treating novel diseases. However, research on small molecules targeting RNA and approved drugs for human illness therapy is scarce. Ribavirin, branaplam, and ataluren are currently available medications that stabilize double-stranded RNA structures and control splicing in a variety of disorders.[92][93]
Protein-coding mRNAs have emerged as new therapeutic candidates, with RNA replacement being particularly beneficial for brief but torrent-like protein expression.[94] In vitro transcribed mRNAs (IVT-mRNA) have been used to deliver proteins for bone regeneration, pluripotency, and heart function in animal models.[95][96][97][98][99] SiRNAs, short RNA molecules, play a crucial role in innate defense against viruses and chromatin structure. They can be artificially introduced to silence specific genes, making them valuable for gene function studies, therapeutic target validation, and drug development.[94]
See also
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External links
- RNA World website Link collection (structures, sequences, tools, journals)
- Nucleic Acid Database Images of DNA, RNA, and complexes.
- Anna Marie Pyle's Seminar: RNA Structure, Function, and Recognition