<P> 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 . </P> <P> 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 nucleotides other 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 cajal bodies . snoRNAs associate with enzymes and guide them to a spot on an RNA by basepairing to that RNA . These enzymes then perform the nucleotide modification . rRNAs and tRNAs are extensively modified, but snRNAs and mRNAs can also be the target of base modification . RNA can also be methylated . </P> <P> 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 . </P> <P> Reverse transcribing viruses replicate their genomes by reverse transcribing DNA copies from their RNA; these DNA copies are then transcribed to new RNA . Retrotransposons 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 . </P>

Rna consists of how many chains of subunits