Structure and Synthesis of RNA Transcription

Transcription is the term used to describe the synthesis of RNA from a DNA template. Translation is the process by which information in RNA is used to synthesise a polypeptide chain. In a little more detail, the genetic information encoded in DNAis first transcribed into acomplementary copy of RNA (a primary RNA transcript) which is then processed to form messenger RNA (mRNA). This leaves the nucleus and is translated into a polypeptide in the cytosol. This then folds into a three-dimensional structure and may be further biochemically modified (post-transla-tional modification) to produce a protein (Figure 20.18). 

Boffa, L.C., Walker, J., Chen, T.A., Sterner, R., Mariani, M.R., and Allfrey, V.G. (1990) Factors effecting nucleosome structure in transcriptionally active chromatin. Histone acetylation, nascent RNA and inhibitors of RNA synthesis. Eur. J. Biochem. 194, 811-823. 

The sequence of the bases contains coded information for the synthesis of proteins. These sequences are transcribed into an RNA copy of the sequence messenger RNA (mRNA). The mRNA is translated in the cytoplasm. The DNA also encodes structural RNAs, with functions in transcription of the DNA, processing of the transcripts and translation of the transcripts. The genetic code shown in Table 8.2.1 is simple, but efficient. At each nucleotide position, there are only four possibilities A,... 

Chapter 17, which focuses on the structure of the nucleic acids, begins with a description of DNA structure and the investigations that led to its discovery. This is followed by a discussion of current knowledge of genome and chromosome structure, as well as the structure and roles of the several forms of RNA. Chapter 17 ends with the description of viruses, macromolecular complexes composed of nucleic acid and proteins that are cellular parasites. In the following chapter (Chapter 18), several aspects of nucleic acid synthesis and function (i.e., DNA replication and transcription) are discussed. Protein synthesis (translation) is described in Chapter 19. 

The promoter is necessary for expression of the lac operon and is thought to be the site for the initiation of RNA transcription. The site of the promoter cannot lie in the operator or structural genes, since mutations localized to the operator region do not lower or abolish the rate of synthesis of the enzymes, and deletion of the early portions of the z gene does not necessarily affect the rate of synthesis of the permease or transacetylase. The location of the promoter determined by deletion analysis is adjacent to the operator on the side away from the structural genes [22]. 

The information which specifies the amino-acid sequence of a protein is stored in the nucleotide sequence of the double helix of deoxyribonucleic acid (DNA). The transcription of sections of this information into ribonucleic acid (RNA) is catalysed by RNA polymerases. These enzymes not only control the synthesis of RNA but also recognize stop and start signals on the DNA. The start signals are complex and may be blocked by repressor molecules which inhibit the transcription process. Once synthesized, the (messenger) RNA is processed and exported to ribosomes where its nucleotide sequence is translated into protein. Triplets of three nucleotides (codons) in the messenger RNA each specify (encode) one amino acid. The linear sequence of nucleotides in the messenger RNA thus specifies the sequence of amino acids in the protein whose primary structure will therefore correspond directly to the sequence of nucleotides in the DNA. 

The structure of the DNA molecule is intimately related to its two primary roles replication (gene duplication by synthesis of more DNA) and transcription (gene expression by synthesis of RNA) (see Fig. 4.1). 

---