<P> 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 . </P> <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>

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