Covers RNA synthesis from a DNA template, starting with its definition and general features such as strand complementarity and the 5' to 3' direction. It then describes the DNA-dependent RNA polymerase, the nucleoside triphosphate energy supply and pyrophosphate removal, and the initiation, elongation and termination stages. The note also outlines the transcription unit, prokaryotic polymerase subunits, rho-independent terminators, the toxin alpha-amanitin, and which polymerase transcribes which RNA class in eukaryotes.
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Topic 6 of 64 in the MU Sofia syllabus for Biology (Dental Medicine, Year 1). The free AceMed note covers it in 1 section, about 429 words (3 min of reading).
In this note
Introduction
Introduction
Transcription:
Definition: The process of RNA synthesis using a specific DNA segment as a template.
General Characteristics:
DNA primarily serves as a store for genetic information.
Genes are expressed through transcription, converting DNA into RNA.
Specific beginning and end sequences define the copied DNA segment.
Each DNA strand acts as a template for certain genes and is complementary to others.
DNA and synthesized RNA are antiparallel and complementary (A-U, G-C).
RNA synthesis occurs in the 5' to 3' direction.
The Enzyme:
DNA-dependent RNA Polymerase Functions:
Separates the two DNA strands within a short region.
Utilizes one of the DNA strands as a template.
Catalyzes the formation of covalent bonds between nucleotides.
Participates in the regulation of gene activity.
RNA Polymerase:
Complex protein with a quaternary structure.
Energy in Transcription:
Energy is provided by ribonucleotides, which are nucleoside triphosphates (ATP, UTP, GTP, and CTP).
Only one phosphate group remains in the RNA product, with the RNA polymerase breaking the bond between it and the next phosphate to utilize the energy.
Pyrophosphate, the other product, is hydrolyzed by pyrophosphatase, shifting the reaction in the necessary direction.
The breakage between phosphorus provides energy to the process.
Stages of Transcription:
Initiation
Elongation
Termination
Initiation:
Transcription factors recognize the TATA box (promoter) and bind to it.
RNA polymerase (holoenzyme) follows the transcription factors, forming the Transcription initiation complex.
DNA strands open up the DNA helix, forming phosphodiester bonds between nucleotides.
Elongation:
RNA moves along the template strand.
The core enzyme transcribes the DNA.
Complementary RNA nucleotides are added to the 3' end.
RNA peels away, and the DNA double helix reforms.
Topoisomerases relax DNA supercoiling before and after RNA polymerase action.
Termination:
The enzyme recognizes a specific nucleotide sequence called a terminator, signaling the end of transcription.
Transcription Unit:
The DNA segment transcribed into an RNA molecule.
Includes three regions: promoter, RNA coding region, terminator.
May contain one or more genes.
Prokaryotic RNA Polymerase:
Comprises five subunits: two identical α subunits, β, β', and σ subunits (forming the holoenzyme).
Binds to DNA segments and initiates transcription at promoters.
During elongation, only the core enzyme functions.
Terminators:
Included in genes, containing palindromes or symmetric sequences.
Rho-independent Termination:
Sequence motifs cause transcription to stop, consisting of a short, often GC-rich hairpin followed by a sequence enriched in thymine residues.
Death Cap Toxicity:
Contains α-amanitin, a peptide toxin inhibiting RNA Polymerase II.
Transcription in Eukaryotes:
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