Eukaryotic transcripts processing/modification

No. In the vast majority of cases, the mRNA, which is always monocistronic in eukaryotes, is formed by the processing (modification and splicing) of primary transcripts. 

Messenger RNA processing/modification. Most primary mRNA transcripts in prokaryotes function in translation without further modification. Eukaryotic mRNA transcripts, however, undergo extensive posttranscriptional modifications. 

Most eukaryotic tRNAs are formed from monomeric precursors, each gene acting as a transcriptional unit. Processing is similar to that in bacteria (Section A, 7). Eukaryotic RNase P usually cleaves the 5 end,555-558c and another enzyme cuts at the 3 end.542 556 The 3 CCA sequence of the mature tRNA is usually not present in the primary transcript but is added.559/559a b As in bacteria (p. 1620) extensive modification of bases also occurs in the tRNA precursors of eukaryotes.235 560-562 Many tRNA genes contain introns, which must be removed by splicing (Section 5). 

In eukaryotic organisms, transcription regulation is a complex process that demands coordinated interaction of several genetic elements. The efficiency of this process mainly depends on the promoter/enhancer sequences, the copy number of the gene, and the structure and elements present at the insertion site in the host s chromatin. On the other hand, the co-transcriptional modifications (capping, splicing, polyadenylation, and transport to cytoplasm) on the primary transcript determine the stability, turnover rate, and translational capacity of the future mRNA. 

All classes of RNA transcripts must be processed into mature species. The reactions include several types Nucleolytic cleavage, as in the separation of the mature rRNA species from the primary transcript of RNA polymerase I action Chain extension (non-template-directed), as in the synthesis or regeneration of the common CCA sequence at the 3 end of transfer RNAs or of PolyA at the 3 end of mRNAs and Nucleotide modification, for example, the synthesis of methylated nucleotides in tRNA or rRNA. These reactions are a feature of both prokaryotic and eukaryotic gene expression, and the biological consequences are diverse. For example, modified nucleotides can affect the way in which a tRNA recognizes different codons. 

Prokaryotic ribosomes attach to the nascent mRNA while it is still being transcribed. Because transcription and translation are coupled, prokaryotic mRNAs undergo little modification and processing before being used as templates for protein synthesis. Prokaryotic tRNA and rRNA are transcribed in units larger than those ultimately used and must be processed to generate the functional molecules. The processing of these and the eukaryotic primary transcripts, almost all of which require modification, is discussed in a later section. 

Nascently transcribed eukaryotic iRNAs are among the most processed of all RNA polymerase III transcripts. Like those of prokaryotic tRNAs, the 5 leader is cleaved by RNase P, the 3 trailer is removed, and CCA is added by the CCA-adding enzyme (Figure 29.25). Eukaryotic tRNAs are also heavily modified on base and ribose moieties these modifications are important for function. In contrast with prokaryotic tRNAs, many eukaryotic pre-tRNAs are also spliced by an endonuclease and a ligase to remove an intron. 

In prokaryotic cells, which have no nuclei, translation of an mRNA into protein can begin from the 5 end of the mRNA even while the 3 end is still being synthesized by RNA polymerase. In other words, transcription and translation can occur concurrently in prokaryotes. In eukaryotic cells, however, not only is the nucleus separated from the cytoplasm where translation occurs, but also the primary transcripts of protein-coding genes are precursor mRNAs (pre-mRNAs) that must undergo several modifications, collectively termed RNA processing, to yield a functional mRNA (see Figure... 

The three principal kinds of RNA—tRNA, rRNA, and niRNA—are all modihed enzymatically after transcription to give rise to the functional form of the RNA in question. The type of processing in prokaryotes can differ greatly from that in eukaryotes, especially in the case of mRNA. The initial size of the RNA transcripts is greater than the hnal size because of leader sequences at the 5 end and trailer sequences at the 3 end. The leader and trailer sequences must be removed, and other forms of trimming are also possible. Terminal sequences can be added after transcription, and base modification is frequently observed, especially in tRNA. 

Describe the splicing of eukaryotic mRNA, give the consensus sequences at the splice site Junctions, and designate the other nucleotide sequence elements involved in the process. List the functions of this post-transcriptional modification, and explain the importance of alternative splicing in gene expression. 

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