text: rna acid ( rna ) is a polymeric molecule implicated in various biological roles in coding , decoding , regulation , and expression of genes . rna and dna are nucleic acids , and , along with proteins and carbohydrates , constitute the three major macromolecules essential for all known forms of life . like dna , rna is assembled as a chain of nucleotides , but unlike dna it is more often found in nature as a single - strand folded onto itself , rather than a paired double - strand . cellular organisms use messenger rna ( mrna ) to convey genetic information ( using the letters g , u , a , and c to denote the nitrogenous bases guanine , uracil , adenine , and cytosine ) that directs synthesis of specific proteins . many viruses encode their genetic information using an rna genome . 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 wherein mrna molecules direct the assembly 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 proteins . comparison with dna like dna , most biologically active rnas , including mrna , trna , rrna , snrnas , and other non - coding rnas , contain self - complementary sequences that allow parts of the rna to fold and pair with itself to form double helices . analysis of these rnas has revealed that they are highly structured . unlike dna , their structures do not consist of long double helices , but rather collections of short helices packed together into structures akin to proteins . in this fashion , rnas can achieve chemical catalysis , like enzymes . for instance , determination of the structure of the ribosome — an enzyme that catalyzes peptide bond formation — revealed that its active site is composed entirely of rna . 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 , 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 ( ) . the bases form hydrogen bonds between cytosine and guanine , between adenine and uracil and between guanine and uracil . however , other interactions are possible , such as a group of adenine bases binding to each other in a bulge , or the that has a guanine – adenine base - pair . an important structural feature of rna that distinguishes it from dna is the presence of a hydroxyl group at the 2 ' position of the ribose sugar . the presence of this functional group causes the helix to mostly adopt the a - form geometry , although in single strand dinucleotide contexts , rna can rarely also adopt the b - form most commonly observed in dna . the a - form geometry results in a very deep and narrow major groove and a shallow and wide minor groove . a second consequence of the presence of the 2 ' -hydroxyl group is that in conformationally flexible regions of an rna molecule ( that is , not involved in formation of a double helix ) , it can chemically attack the adjacent phosphodiester bond to cleave the backbone . rna is transcribed with only four bases ( adenine , cytosine , guanine and uracil ) , but these bases and attached sugars can be modified in numerous ways as the rnas mature . ( ψ ) , in which the linkage between uracil and ribose is changed from a c – n bond to a c – c bond , and ( t ) are found in various places ( the most notable ones being in the loop of trna ) . another notable modified base is hypoxanthine , a deaminated adenine base whose nucleoside is called inosine ( i ) . plays a key role in the wobble hypothesis of the genetic code . there are more than 100 other naturally occurring modified nucleosides , the greatest structural diversity of modifications can be found in trna , while and nucleosides with 2 ' -o - often present in rrna are the most common . the specific roles of many of these modifications in rna are not fully understood . however , it is notable that , in ribosomal rna , many of the post - transcriptional modifications occur in highly functional regions , such as the peptidyl transferase center and the subunit interface , implying that they are important for normal function . the functional form of single - stranded rna molecules , just like proteins , frequently requires a specific tertiary structure . the scaffold for this structure is provided by secondary structural elements that are hydrogen bonds within the molecule . this leads to several recognizable " domains " of secondary structure like hairpin loops , bulges , and internal loops . since rna is charged , metal ions such as mg2 + are needed to stabilise many secondary and tertiary structures . 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 . 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 — rna polymerase — using dna as a template , a process known as transcription . initiation of transcription begins with the binding of the enzyme to a promoter sequence in the dna ( usually found " upstream " of a gene ) . the dna double helix is unwound by the helicase activity of the enzyme . the enzyme then progresses along the template strand in the 3 ’ to 5 ’ direction , synthesizing a complementary rna molecule with elongation occurring in the 5 ’ to 3 ’ direction . the dna sequence also dictates where termination of rna synthesis will occur . primary transcript rnas are often modified by enzymes after transcription . for example , a poly ( a ) tail and a 5 ' cap are added to eukaryotic pre - mrna and introns are removed by the spliceosome . 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 . also , rna - dependent rna polymerase is part of the rna interference pathway in many organisms . 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 coding sequence of the mrna determines the amino acid sequence in the protein that is produced . however , many rnas do not code for protein ( about 97 % of the transcriptional output is non - protein - coding in eukaryotes ) . these so - called non - coding rnas ( " " ) can be encoded by their own genes ( rna genes ) , but can also derive from mrna introns . the most prominent examples of non - coding rnas are transfer rna ( trna ) and ribosomal rna ( rrna ) , both of which are involved in the process of translation . there are also non - coding rnas involved in gene regulation , rna processing and other roles . certain rnas are able to catalyse chemical reactions such as cutting and ligating other rna molecules , and the catalysis of peptide bond formation in the ribosome ; these are known as ribozymes . in length according to the length of rna chain , rna includes small rna and long rna . usually , small rnas are 200 nt . long rnas , also called large rnas , mainly include long non - coding rna ( ) and mrna . small rnas mainly include 5.8s ribosomal rna ( rrna ) , 5s rrna , transfer rna ( trna ) , microrna ( mirna ) , small interfering rna ( sirna ) , small nucleolar rna ( snornas ) , piwi - interacting rna ( pirna ) , trna - derived small rna ( ) and small rdna - derived rna ( ) . in translation messenger rna ( mrna ) carries information about a protein sequence to the ribosomes , the protein synthesis factories in the cell . it is coded so that every three nucleotides ( a codon ) correspond to one amino acid . in eukaryotic cells , once precursor mrna ( pre - mrna ) has been transcribed from dna , it is processed to mature mrna . this removes its introns — non - coding sections of the pre - mrna . the mrna is then exported from the nucleus to the cytoplasm , where it is bound to ribosomes and translated into its corresponding protein form with the help of trna . in prokaryotic cells , which do not have nucleus and cytoplasm compartments , mrna can bind to ribosomes while it is being transcribed from dna . after a certain amount of time the message degrades into its component nucleotides with the assistance of ribonucleases . transfer rna ( trna ) is a small rna chain of about 80 nucleotides that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation . it has sites for amino acid attachment and an anticodon region for codon recognition that binds to a specific sequence on the messenger rna chain through hydrogen bonding . rna ( rrna ) is the catalytic component of the ribosomes . 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 . nearly all the rna found in a typical eukaryotic cell is rrna . transfer - messenger rna ( ) is found in many bacteria and plastids . it tags proteins encoded by mrnas that lack stop codons for degradation and prevents the ribosome from stalling . regulatory rnas several types of rna can downregulate gene expression by being complementary to a part of an mrna or a gene 's dna . micrornas ( mirna ; 21 - 22 nt ) are found in eukaryotes and act through rna interference ( rnai ) , where an effector complex of mirna and enzymes can cleave complementary mrna , block the mrna from being translated , or accelerate its degradation . while small interfering rnas ( sirna ; 20 - 25 nt ) are often produced by breakdown of viral rna , there are also endogenous sources of sirnas act through rna interference in a fashion similar to mirnas . some mirnas and sirnas can cause genes they target to be methylated , thereby decreasing or increasing transcription of those genes . animals have piwi - interacting rnas ( pirna ; 29 - 30 nt ) that are active in germline cells and are thought to be a defense against transposons and play a role in gametogenesis . many prokaryotes have crispr rnas , a regulatory system similar to rna interference . antisense rnas are widespread ; most downregulate a gene , but a few are activators of transcription . one way antisense rna can act is by binding to an mrna , forming double - stranded rna that is enzymatically degraded . there are many long noncoding rnas that regulate genes in eukaryotes , one such rna is xist , which coats one x chromosome in female mammals and inactivates it . an mrna may contain regulatory elements itself , such as riboswitches , in the 5 ' untranslated region or 3 ' untranslated region ; these cis - regulatory elements regulate the activity of that mrna . the untranslated regions can also contain elements that regulate other genes . in rna processing many rnas are involved in modifying other rnas . introns are spliced out of pre - mrna by spliceosomes , which contain several small nuclear rnas ( snrna ) , or the introns can be ribozymes that are spliced by themselves . rna can also be altered by having its nucleotides modified to other nucleotides than a , c , g and u. in eukaryotes , modifications of rna nucleotides are in general directed by small nucleolar rnas ( snorna ; 60 - 300 nt ) , found in the nucleolus and snornas associate with enzymes and guide them to a spot on an rna by basepairing to that rna . these enzymes then perform the nucleotide rrnas and trnas are extensively modified , but snrnas and mrnas can also be the target of base modification . rna can also be methylated . 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 . in reverse transcription reverse transcribing viruses replicate their genomes by reverse transcribing dna copies from their rna ; these dna copies are then transcribed to new rna . also spread by copying dna and rna from one another , and telomerase contains an rna that is used as template for building the ends of eukaryotic chromosomes . double - stranded rna double - stranded rna ( dsrna ) is rna with two complementary strands , similar to the dna found in all dsrna forms the genetic material of some viruses ( double - stranded rna viruses ) . double - stranded rna such as viral rna or sirna can trigger rna interference in eukaryotes , as well as interferon response in vertebrates . circular rna recently , 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 ) . circrnas are thought to arise via a " back - splice " reaction where the spliceosome joins a downstream donor to an upstream acceptor 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 nobel prizes . nucleic acids were discovered in 1868 by friedrich miescher , who called the material ' nuclein ' since it was found in the nucleus . it was later discovered that prokaryotic cells , which do not have a nucleus , also contain nucleic acids . the role of rna in protein synthesis was suspected already in 1939 . severo ochoa won the 1959 nobel prize in medicine ( shared with arthur kornberg ) after he discovered an enzyme that can synthesize rna in the laboratory . however , the enzyme discovered by ochoa ( polynucleotide phosphorylase ) was later shown to be responsible for rna degradation , not rna synthesis . in 1956 alex rich and david davies hybridized two separate strands of rna to form the first crystal of rna whose structure could be determined by x - ray crystallography . the sequence of the 77 nucleotides of a yeast trna was found by robert w. holley in 1965 , winning holley the 1968 nobel prize in medicine ( shared with har gobind khorana and marshall nirenberg ) . in 1967 , carl woese hypothesized that rna might be catalytic and suggested that the earliest forms of life ( self - replicating molecules ) could have relied on rna both to carry genetic information and to catalyze biochemical reactions — an rna world . during the early 1970s , retroviruses and reverse transcriptase were discovered , showing for the first time that enzymes could copy rna into dna ( the opposite of the usual route for transmission of genetic information ) . for this work , david baltimore , renato and howard temin were awarded a nobel prize in 1975 . in 1976 , walter fiers and his team determined the first complete nucleotide sequence of an rna virus genome , that of bacteriophage ms2 . in 1977 , introns and rna splicing were discovered in both mammalian viruses and in cellular genes , resulting in a 1993 nobel to philip sharp and richard roberts . catalytic rna molecules ( ribozymes ) were discovered in the early 1980s , leading to a 1989 nobel award to thomas cech and sidney altman . in 1990 , it was found in petunia that introduced genes can silence similar genes of the plant 's own , now known to be a result of rna interference . at about the same time , 22 nt long rnas , now called micrornas , were found to have a role in the development of c. elegans . studies on rna interference gleaned a nobel prize for andrew fire and craig mello in 2006 , and another nobel was awarded for studies on the transcription of rna to roger kornberg in the same year . the discovery of gene regulatory rnas has led to attempts to develop drugs made of rna , such as sirna , to silence genes . evolution in march 2015 , complex dna and rna organic compounds of life , including uracil , cytosine and thymine , were reportedly formed in the laboratory under outer space conditions , using starting chemicals , such as pyrimidine , found in meteorites . pyrimidine , like polycyclic aromatic hydrocarbons ( pahs ) , the most carbon - rich chemical found in the universe , may have been formed in red giants or in interstellar dust and gas clouds , according to the scientists .