BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 10. METABOLISM OF NUCLEIC ACIDS

10.7.Biosynthesis of RNA (Transcription)

General Principles. The expression of Genetic information stored in DNA is mediated by RNA molecules (mRNA, tRNA, rRNA). First, the genetic information from a Gene is rewritten (transcribed) into an mRNA molecule, which delivers this information to the site of the corresponding polypeptide synthesis—the ribosome. Therefore, the Selection/27.html">Realization of Genetic information as protein molecules is impossible without prior RNA Synthesis (METABOLISM/31.html">Transcription).

A double-stranded DNA molecule serves as the physiological template for the synthesis of all cellular RNAs. Even when The Genome is represented by single-stranded DNA, as in some Viruses, it must convert into a double-stranded, replicative form prior to transcription. Either of the two strands of genomic DNA can be transcribed, but only one of them typically serves as the template during the transcription of a specific gene. Accordingly, a distinction is made between the coding DNA strand—from which the genetic information is read—and the non-coding strand. From this perspective, transcription is an asymmetric process. Furthermore, upon completion of RNA synthesis, the DNA molecule returns to its original state, making transcription a conservative process. The conservative nature of RNA synthesis and the need to replicate only a single template strand rather than both in certain instances have driven the evolution not only of specialized mechanisms for recognizing the start site of synthesis, but also of systems for strand selection in asymmetric synthesis, as well as process termination.

The nucleotide precursors for RNA synthesis are four ribonucleoside-5'-triphosphates: ATP, GTP, CTP, and UTP. As in DNA Synthesis, upon incorporation into the growing chain, they lose pyrophosphate residues. This supplies energy to the process, meaning no additional Energy Sources are required for this synthesis.

RNA synthesis begins at specific DNA sequences (sites) called promoters and terminates at termination regions (termination sites). The DNA segment bounded by a promoter and a termination site constitutes a unit of transcription—the transcription unit (transcripton). The resulting RNA molecules are referred to as primary transcripts. In prokaryotes, a primary transcript often contains RNA copies of several genes simultaneously, whereas in eukaryotes, it typically contains only a single gene.

The Synthesis of the RNA chain proceeds in the 5'→3' direction. This means that successive NUCLEOTIDES are added to the 3'-end of the growing chain. Consequently, the template DNA strand is always antiparallel to the newly synthesized strand. Thus, the coding DNA strand is transcribed in the 3'→5' direction.

Transcription is not coupled to the Phases of the Cell Cycle; it can be accelerated or slowed down depending on The Cell's or Organism's demand for a specific protein.