Biosynthesis and Transcription of RNA

The ability to mutate in response to DNA damage confers an evolutionary advantage to the organism. 

Translesion synthesis occurring under SOS mutagenesis attempts to extract the available base-pairing information from the lesion and, in the case of certain lesions, can 

It may be a primary importance for resisting particular classes of environmental agents, which introduce lesions that are not subject to excision repair or other accurate repair systems. 

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The corticosteroids exert their pharmacological effect by modifying RNA and protein synthesis in target tissue. After binding to the specific cytoplasmic protein receptors, the steroid-receptor complex is transported to the nucleus, where it binds to the nuclear chromatin, thereby altering the transcription of RNA. Since the steroids are not stored, the steroid concentration in the plasma is maintained by regulation of steroid biosynthesis. 

Cellular protein biosynthesis involves the following steps. One strand of double-stranded DNA serves as a template strand for the synthesis of a complementary single-stranded messenger ribonucleic acid (mRNA) in a process called transcription. This mRNA in turn serves as a template to direct the synthesis of the protein in a process called translation. The codons of the mRNA are read sequentially by transfer RNA (tRNA) molecules, which bind specifically to the mRNA via triplets of nucleotides that are complementary to the particular codon, called an anticodon. Protein synthesis occurs on a ribosome, a complex consisting of more than 50 different proteins and several stmctural RNA molecules, which moves along the mRNA and mediates the binding of the tRNA molecules and the formation of the nascent peptide chain. The tRNA molecule carries an activated form of the specific amino acid to the ribosome where it is added to the end of the growing peptide chain. There is at least one tRNA for each amino acid. 

Thomas That is not absolutely true. In bacteria, if you change the energy charge you see a direct effect on translation and ribosome biosynthesis. As soon as ATP drops, ribosomal RNA synthesis decreases but not transcription of other genes. 

These three compounds exert many similar effects in nucleotide metabolism of chicks and rats [167]. They cause an increase of the liver RNA content and of the nucleotide content of the acid-soluble fraction in chicks [168], as well as an increase in rate of turnover of these polynucleotide structures [169,170]. Further experiments in chicks indicate that orotic acid, vitamin B12 and methionine exert a certain action on the activity of liver deoxyribonuclease, but have no effect on ribonuclease. Their effect is believed to be on the biosynthetic process rather than on catabolism [171]. Both orotic acid and vitamin Bu increase the levels of dihydrofolate reductase (EC 1.5.1.4), formyltetrahydrofolate synthetase and serine hydroxymethyl transferase in the chicken liver when added in diet. It is believed that orotic acid may act directly on the enzymes involved in the synthesis and interconversion of one-carbon folic acid derivatives [172]. The protein incorporation of serine, but not of leucine or methionine, is increased in the presence of either orotic acid or vitamin B12 [173]. In addition, these two compounds also exert a similar effect on the increased formate incorporation into the RNA of liver cell fractions in chicks [174—176]. It is therefore postulated that there may be a common role of orotic acid and vitamin Bj2 at the level of the transcription process in m-RNA biosynthesis [174—176]. 

RNA synthesis is the transcription of the base sequence of DNA to that of RNA. All RNAs are synthesized on a DNA template. The enzyme that catalyzes the process is DNA-dependent RNA polymerase. All four ribonucleoside triphosphates—ATP, GTP, GTP, and UTP—are required, as is Mg. There is no need for a primer in RNA synthesis. As is the case with DNA biosynthesis, the RNA chain grows from the 5 to the 3 end. The enzyme uses one strand of the DNA (the anti-sense strand, or template strand) as the template for RNA synthesis. The RNA product has a sequence that matches the other strand of DNA, the coding strand. 

CioHijNjO, Mr 251.24, needles, mp. 225 -226°C, [oId -47° (H2O), well soluble in water. Metabolite from culture broths of Cordyceps, Isaria, Emericella, and Aspergillus species. C. exhibits strong cytotoxic activity by inhibition of RNA biosynthesis. Because of this property C. is used for investigations on messen-ger-RNA transcription. C. was the first nucleoside antibiotic to be described (1951) and was isolated from Cordyceps militaris. C. selectively inhibits the poly-adenylation of hnRNA in HeLa cells at 50 pg/mL. Furthermore not only 45S-rRNA biosynthesis but also adenylate cyclase are inhibited. 

Figure 3.13. The hydrogen bonds exhibited by the A-T and G-C base pairs as they occur in the structure of DNA. This is the type of base pairing responsible for accurate replication of DNA, for transcription of DNA to form RNA, and for translation of RNA to form protein, as discussed in Chapter 4 in relation to the energetics of protein biosynthesis. (Reproduced with permission from S. Arnott, S.D. Dover, and A.J. Wonacott, Acta Cryst. B25, 2196, 1969. Copyright 1969 lUCr journals.)...

Biosynthesis of protein can be seen in three steps replication, the production of the DNA for a daughter cell transcription, the conversion of the DNA into the equivalent sequence of RNA and translation, the conversion of the ribonucleic acid sequence into the specified protein sequence, that is,... 

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