Small nuclear RNA snRNA

In contrast, RNA occurs in multiple copies and various forms (Table 11.2). Cells contain up to eight times as much RNA as DNA. RNA has a number of important biological functions, and on this basis, RNA molecules are categorized into several major types messenger RNA, ribosomal RNA, and transfer RNA. Eukaryotic cells contain an additional type, small nuclear RNA (snRNA). With these basic definitions in mind, let s now briefly consider the chemical and structural nature of DNA and the various RNAs. Chapter 12 elaborates on methods to determine the primary structure of nucleic acids by sequencing methods and discusses the secondary and tertiary structures of DNA and RNA. Part rV, Information Transfer, includes a detailed treatment of the dynamic role of nucleic acids in the molecular biology of the cell. 

In all eukaryotic cells there are small nuclear RNA (snRNA) species that are not directly involved in protein synthesis but play pivotal roles in RNA processing. These relatively small molecules vary in size from 90 to about 300 nucleotides (Table 35-1). 

Small nuclear RNAs (snRNAs), a subset of these RNAs, are significantly involved in mRNA processing and gene regulation. Of the several snRNAs, Ul, U2, U4, U5, and U6 are involved in intron removal and the processing of hnRNA into mRNA (Chapter 37). The U7 snRNA may be involved in production of the correct 3 ends of histone mRNA—which lacks a poly(A) tail. The U4 and U6 snRNAs may also be required for poly(A) processing. 

All eukaryotic cells have four major classes of RNA ri-bosomal RNA (rRNA), messenger RNA (mRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA). The first three are involved in protein synthesis, and snRNA is involved in mRNA splicing. As shown in Table 37-1, these various classes of RNA are different in their diversity, stability, and abundance in cells. 

Spliceosome The macromolecular complex responsible for precursor mRNA splicing. The spliceosome consists of at least five small nuclear RNAs (snRNA Ul, U2, U4, U5, and U6) and many proteins. 

Small nuclear RNA (snRNA), vrfiich is also only found in the nucleus of eukaryotes. One of its major functions is to participate in splicing (removal of introns) mRNA. 

The removal of introns from pre-messenger RNAs in eukaryotes is catalyzed by the spliceosome, which is a large ribonucleoprotein consisting of at least 70 proteins and five small nuclear RNAs (snRNA) [144]. This splicing pathway involves two phosphotransfer reactions. In the first step, the 5 splice site is attacked by a 2 hydroxy group of an adenosine nucleotide within the intron [indicated by A in Fig. 12] that corresponds to the branch point in the lariat intermediate (Fig. 12,middle). In the second step, the 3 -OH group of the free 5 exon attacks the phosphodiester bond between the intron and... 

Small nuclear RNAs (snRNAs) are involved in the splicing of mRNA precursors (see p.246). They associate with numerous proteins to form spliceosomes. ... 

E. Small nuclear RNA (snRNA) molecules are components of splicesomes, which... 

The RNA product may encode transfer RNAs (tRNAs), ribosomal RNAs (rRNAs), or small nuclear RNAs (snRNAs) that have end point functions in the cell. 

RNA polymerase II This enzyme synthesizes the precursors of messenger RNAs that are subsequently translated to produce proteins. Polymerase II also synthesizes certain small nuclear RNAs (snRNA, see p. 424), and is used by some viruses to produce viral RNA. 

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

To assist in the modification and processing of mRNAs, eukaryotic cells contain in their nuclei small nuclear RNAs (snRNAs), which are complexed with specific proteins to form small nuclear ribonucleoprotein particles (snRNPs). These RNAs have been named U1, U2, U3,.. . , U13 and range in size from 100 to 220 bases. One, U3, is... 

RNA polymerase II (RNA Pol II) is located in the nucleoplasm and transcribes protein-coding genes, to yield pre-mRNA, and also the genes encoding small nuclear RNAs (snRNAs) involved in mRNA processing (see Topic G8), except for U6 snRNA. 

One final class of eukaryotic RNA that deserves mention are the small nuclear RNAs (snRNAs). As their name suggests, this class of RNA is found in the nucleus and participates in the splicing of in-trons from certain types of eukaryotic mRNAs. Five snRNAs (Ul, U2, U4, U5, and U6) take part in these splicing reactions when combined with a... 

RNA pol 11 synthesises mRNAs and some small nuclear RNAs (snRNAs) involved in RNA sphcing. 

Ribosomal RNA is the most abundant of the three types of RNA. Transfer RNA comes next, followed by messenger RNA, which constitutes only 5% of the total RNA. Eukaryotic cells contain additional small RNA molecules. Small nuclear RNA (snRNA) molecules, for example, participate in the splicing of RNA exons. A small RNA molecule in the cytosol plays a role in the targeting of newly synthesized proteins to intracellular compartments and extracellular destinations. 

Most striking is the splicing of mRNA precursors, which is catalyzed by spliceosomes consisting of small nuclear ribonucleoprotein particles (snRNPs). The small nuclear RNA (snRNA) molecules in these complexes play a key role in directing the alignment of splice sites and in mediating catalysis. Indeed, some RNA molecules can splice themselves in the absence of protein. This landmark discovery by Thomas Cech and Sidney Altman revealed that RNA molecules can serve as catalysts and greatly influenced our view of molecular evolution. 

In previous sections many of the eukaryotic proteins and DNA sequences that participate in transcription and its control have been introduced. In this section, we focus on assembly of transcription preinitiation complexes involving RNA polymerase II (Pol II). Recall that this eukaryotic RNA polymerase catalyzes synthesis of mRNAs and a few small nuclear RNAs (snRNAs). Mechanisms that control the assembly of Pol II transcription preinitiation complexes, and hence the rate of transcription of protein-coding genes, are considered in the next section. 

Small nuclear ribonucleoproteins contain small nuclear RNAs (snRNAs)known as Ul, U2, and so onwhich are complexed with proteins to form the snRNPs. The snRNPs recognize specific nucleotide sequences on the nascent transcript and bind to the transcript. Those snRNPs that are involved in RNA splicing form a complex called a spliceosome that removes introns from the nascent transcripts. The spliceosome is nearly as large as a ribosome. 

There are four major classes of RNA in cells messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA). The relative abundance, and complexity (number of distinct types), of these RNA molecules is quite different. About 80% of RNA in a cell consists of rRNA, the RNA component of ribosomes. There are only four types of eukaryotic rRNA (28S, 18S, 5.8S, and 5S) and, therefore, the sequence complexity of this class is actually quite low. In contrast, mRNA constitutes just 5% of total cellular RNA, yet it is the most diverse, with an estimated 104105 different species which correspond to the same number of protein coding genes. Both tRNA and snRNA (in eukaryotes) make up the remaining fraction of RNA in a cell (-15%), with -50 different types of tRNA and -10 different snRNAs. The total number of molecules per cell of each class of RNA is therefore based on the relationship between the total mass of each RNA class, the average length of RNA molecules in that class, and the sequence complexity. 

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