BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

Part IV INFORMATION

CHAPTER 25. RNA INFORMATION AND TRANSCRIPTION

Summary

The flow of Genetic information in normal Cells proceeds in the direction: DNA → RNA → protein. The synthesis of RNA on a DNA template is called METABOLISM/31.html">Transcription, and the synthesis of protein on an RNA template is called Translation. There are Three types of RNA molecules: Messenger RNA (mRNA), Transfer RNA (tRNA), and ribosomal RNA (rRNA). All these RNA molecules are single-stranded. Transfer RNA and ribosomal RNA contain extensive double-helical regions formed by the folding of the chain into hairpin loops. The smallest RNA molecules are tRNAs, containing about 75 NUCLEOTIDES. The longest RNA molecules are found among mRNAs, which can contain over 5,000 nucleotides. The nucleotide sequence in long RNA chains can be determined by cleaving them and separating the fragments using gel Electrophoresis.

All cellular RNAs are synthesized by RNA polymerase in accordance with the DNA template sequence. Ribonucleoside triphosphates serve as activated intermediates. RNA Synthesis, much like DNA Synthesis, proceeds in the 5' → 3' direction. RNA polymerase differs from DNA polymerase in that it does not require a primer and lacks nuclease activities. Another difference is that the DNA template is entirely conserved during RNA synthesis, whereas it is conserved semi-conservatively during DNA Replication. E. coli RNA polymerase has an α₂ββ'σ subunit Structure and a molecular mass reaching 500 kDa. The core enzyme has the composition α₂ββ'. The sigma subunit increases both the rate and Specificity of transcription by enabling RNA polymerase to recognize Transcription initiation signals in the template DNA. The binding of RNA polymerase to such promoter regions causes local unwinding of the template, creating the conditions necessary for The formation of the first phosphodiester bond. RNA chains begin with either pppG or pppA. Following the initiation of the new chain synthesis, the sigma subunit dissociates from the holoenzyme. Transcription termination is regulated just as precisely as initiation. The Template DNA contains transcription stop signals. Some of these signals are recognized by the ρ protein. Certain specific inhibitors can block transcription. For instance, the antibiotic rifampicin inhibits the initiation of RNA synthesis, whereas actinomycin D blocks RNA elongation. Many RNA molecules undergo Cleavage and chemical modification (such as methylation) post-transcriptionally. In prokaryotes, tRNA and rRNA molecules undergo extensive Processing, whereas mRNA molecules undergo almost no modifications. In eukaryotes, all transcripts are extensively modified.