Metabolism and Energy Transformations in Body Cells - Renata Armenakovna Petrosova 2004

Template synthesis reactions. Transcription
Transcription

The process of RNA Biosynthesis on a DNA template is called transcription. This process takes place in The Nucleus. All types of RNA—messenger, transfer, and ribosomal—are synthesized on the DNA template and subsequently participate in Protein Synthesis. During transcription, METABOLISM/28.html">The Genetic Code is copied from DNA onto Messenger RNA. The reaction is based on THE PRINCIPLE OF complementarity.

RNA Synthesis has several distinct features. An RNA molecule is significantly shorter and represents a copy of only a small region of DNA. Therefore, the template is strictly a specific segment of DNA that contains the information for the given nucleic acid. The newly synthesized RNA never remains bound to the original DNA template, but is released upon completion of the reaction. The transcription process occurs in three stages.

The First stage is initiation, the onset of the process. The synthesis of RNA copies begins at a specific region on the DNA called the promoter. This region contains a specific set of NUCLEOTIDES that act as start signals. The process is catalyzed by Enzymes called RNA polymerases. The RNA polymerase enzyme binds to the promoter, unwinds The Double Helix, and breaks the Hydrogen Bonds between the two DNA strands, though only one of them serves as the template for RNA synthesis.

The Second Stage is elongation. This phase constitutes the main process. Nucleotides align along one of the DNA strands According to the principle of complementarity, acting as a template (Fig. 19). Moving step-by-step along the DNA chain, the RNA polymerase enzyme links the nucleotides together while continuously unwinding the DNA double helix further ahead. As a result of this movement, an RNA copy is synthesized.

The Third Stage is termination. This is the final phase. RNA synthesis continues until a stop signal is reached—a specific nucleotide sequence that halts the enzyme's movement and stops RNA synthesis. The polymerase detaches from both the DNA and the synthesized RNA copy. Simultaneously, the RNA molecule is released from the template, and the DNA restores its double helix. Synthesis is complete. Depending on the DNA region, ribosomal, transfer, and messenger RNAs are synthesized in this manner.

Only one of the DNA strands serves as the template for the transcription of an RNA molecule. However, the templates for two adjacent genes can be different DNA strands. Which of the two strands is used for synthesis is determined by the promoter, which directs the RNA polymerase enzyme in one direction or the other.

Following transcription, the messenger RNA molecule in Eukaryotic Cells undergoes Processing. Nucleotide sequences that do not carry information about the specific protein are excised. This process is called splicing. Depending on The Cell type and developmental stage, different Regions of the RNA molecule may be removed. Consequently, various RNAs carrying information for different Proteins can be synthesized from a single DNA segment. This ensures the transmission of substantial Genetic information from a single Gene and also facilitates genetic recombination.

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Fig. 19. Synthesis of messenger RNA. 1 — DNA strand; 2 — synthesized RNA