Small nucleolar RNAs snoRNAs

FIGURE 26-22 Processing of pre-rRNA transcripts in vertebrates. In step (T), the 45S precursor is methylated at more than 100 of its 14,000 nucleotides, mostly on the 2 -OH groups of ribose units retained in the final products. (5) A series of enzymatic cleavages produces the 18S, 5.8S, and 28S rRNAs. The cleavage reactions require RNAs found in the nucleolus, called small nucleolar RNAs (snoRNAs), within protein complexes reminiscent of spliceosomes. The 5S rRNA is produced separately. 

Polymerase II Transcription of most genes to give precursors to mRNA and most small nuclear RNAs (snRNAs and small nucleolar RNAs (snoRNAs)... 

Small nucleolar RNAs (snoRNAs) contain between 60 and 300 nucleotides. They bind to specific target sequences by hydrogen-bond base pairing and then specialized proteins bind to the complex. The complex catalyzes modifications of the target RNA molecule, thus snoRNAs are particularly important in processing rRNA in mammals. 

Noncoding RNA is RNA that is generated by transcription from DNA but that is not translated into protein. It inclndes the major strnctnral RNAs rRNA and tRNA as well as small nuclear RNA (snRNA), small nucleolar RNA (snoRNA), microRNA (miRNA), and short interfering RNA (siRNA). 

Transport of RNA between the nucleus and cytoplasm is an essential step in the expression of messenger RNA (mRNA), and the function of ribosomal RNA (rRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA). Most nuclear RNA transcripts are covalently modified, complexed with proteins into ribo-nucleoprotein particles (RNPs), and then exported through the nuclear pore complex to the cytoplasm. After export, most RNAs remain in the cytoplasm where they function in protein synthesis. However, many snRNAs are imported back to the nucleus as mature snRNP particles, where they function in pre-mRNA splicing. Small nucleolar RNAs (snoRNAs), which function in ribosome biogenesis, are not exported to the cytoplasm, but their intranuclear targeting to the nucleolus is an area of active research. 

Noncoding RNAs are summarized as transcripts that are not translated (synonym nonprotein-coding RNA). They can be divided into small (small nucleolar RNAs [snoRNAs], Piwi interacting RNAs [piRNAs], etc.) and medium/large size molecules, and thanks to their popularity, miRNAs are often classified as an independent group (review in Costa [39]). Another... 

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