MRNA post-transcriptional modification

In bacteria, which are prokaryotes, termination releases a mature mRNA for translation. In fact, because prokaryotes have no nuclear membrane separating the DNA from the cytoplasm, translation begins long before the mRNA is completed. In eukaryotes, transcription produces a primary transcript that must undergo extensive post-transcriptional modification before it is exported out of the nucleus for translation in the cytoplasm. 

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. 

Polyadanyllc acid. Poly A a Homopolymer (see) consisting entirely of residues of adenylic acid. Poly A sequences of varying lengths are found at the 3 end of many eukaryotic mRNA molecules (see Post-transcriptional modification of RNA, Poly A polymerase). The length of the poly A sequence depends on the source of the RNA and the physiological state of the cell, and is inversely related to the half-life of the mRNA. 

The most important difference between RNA synthesis in eukaryotic and prokaryotic cells is, however, at the level of the primary transcript, particularly in the case of hnRNA which must undergo three types of post-transcriptional modification (Figure 21.2) before it is transported to the cytoplasm as a fully mature mRNA molecule. These processes are as follows, (i) Modification of the 5 end of the RNA to form a cap structure. This entails the enzyme addition of a guanosine residue via a 5 -5 triphosphate linkage to the 5 end, as well as varying degrees of methylation of the first two or three nucleotides, (ii) In most eukaryotic mRNAs the addition of... 

Once a gene is cloned it is necessary to convert the information contained in it into a functional protein. There are a number of steps in gene expression (i) transcription of DNA into mRNA (ii) translation of the mRNA into a protein sequence and (iii) in some instances, post-translational modification of the protein. In discussing these steps in more detail, expression of a cloned insulin gene will be used as an example. 

FIGURE 6.1 The different stages of protein synthesis. Transcription and processing of RNA messages occur within the nucleus. The mRNA is then transported into the cytoplasm for translation and post-translational modifications. 

The transport from nucleus to cytoplasm is accompanied by modification at the 5 - and 3 -end of the pre-RNA, as well as by processing (splicing) of the primary transcript. The 3 -end modifications and sphcing decide which information contained in the primary transcript is made available for protein biosynthesis. The information content of the processed mRNA can be specifically influenced by these processes. This has an important impact on the tissue- and cell-specific protein expression. 3 -modification and splicing are tightly coupled to extranuclear transport. Interventions in the transport process are another possibihty for a regulation at the post-transcriptional level. 

In prokaryotes, RNA transcribed from protein-coding genes (messenger RNA, mRNA), requires little or no modification prior to translation. In fact, many mRNA molecules begin to be translated even before RNA synthesis has finished. However, ribosomal RNA (rRNA) and transfer RNA (tRNA) are synthesized as precursor molecules that do require post-transcriptional processing (see Topics G9 and G10, respectively). 

The data of Figure 10 illustrate that post-synthetic modification, including hydrolysis, is not limited to proteins. In this example, the large pre-mRNA undergoes post-transcriptional cleavage, splicing, modification of the 3 and 5 ends, and finally methylation to give the specific active mRNA. 

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