Other Types of RNA Synthesis

In addition to the cellular enzyme(s) that catalyzes DNA-directed RNA synthesis, cellular enzymes are involved in polyribonucleotide synthesis that do not use a template. Some of the properties of these enzymes are summarized in table 28.5. We have already mentioned polynucleotide phosphorylase in this chapter, and in chapter 26 we discussed the importance of DNA primase to DNA synthesis. 

Two enzymes are known to add ribonucleotides post-transcriptionally to the 3 hydroxyl end of specific RNAs. One adds the CCA sequence found in all tRNAs at their 3 ends. The 3 terminal adenine in this sequence serves as the amino acid attachment site. The 3 -CCA is relatively unstable and is continually being rejuvenated by this enzyme, 

Comparative processing of major transcripts in prokaryotes and eukaryotes. 

In eukaryotes, 100-200 adenosine residues are added to the 3 ends of most mRNAs by a poly (A) polymerase. This addition occurs in the nucleus before the mRNA is fully processed and transported to the cytoplasm. 

Most RNAs are not made in their final functional forms as they peel off the DNA template (fig. 28.14). They must undergo backbone phosphodiester bond cleavages into smaller molecules (processing) and individual base changes 

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Many Viruses Encode Their Own RNA Polymerases RNA-Dependent RNA Polymerases of RNA Viruses Other Types of RNA Synthesis Posttranscriptional Alterations of Transcripts... 

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