Tiêu đề 24. TRANSCRIPTION AND TRANSCRIPTION FACTORS.pdf ✅

PHARMD GURU Page 1 1. INTRODUCTION: Transcription is the process by which genetic information encoded in DNA is copied into RNA. This is a critical step in gene expression and is regulated by a variety of factors to ensure precise control of cellular function. While the fundamental process of transcription is conserved across prokaryotic and eukaryotic organisms, the complexity and regulation differ significantly. 2. BASIC PRINCIPLES OF TRANSCRIPTION IN PROKARYOTES: A. OVERVIEW: Transcription in prokaryotes is relatively simple due to the absence of a defined nucleus and the streamlined nature of their genetic material. The process involves the synthesis of RNA from a DNA template, mediated by RNA polymerase. B. STAGES OF TRANSCRIPTION IN PROKARYOTES: a) INITIATION:  Promoter Recognition: RNA polymerase binds to a specific DNA sequence called the promoter, which contains conserved sequences at the -10 (TATAAT, Pribnow box) and -35 (TTGACA) regions relative to the transcription start site.  Role of Sigma Factor (σ): The core RNA polymerase enzyme requires the sigma factor to recognize and bind to promoter sequences. The sigma factor dissociates after initiation.  Open Complex Formation: Once bound, the DNA strands unwind, forming an open complex, exposing the template strand for RNA synthesis. b) ELONGATION:  RNA Synthesis: RNA polymerase adds ribonucleotides complementary to the DNA template strand in the 5’ to 3’ direction.  Unwinding and Rewinding: DNA unwinds ahead of RNA polymerase and rewinds behind it. TRANSCRIPTION AND TRANSCRIPTION FACTORS
PHARMD GURU Page 2 c) TERMINATION:  Rho-Dependent Termination: Involves the Rho protein, which binds to a specific RNA sequence and moves toward RNA polymerase, causing transcription termination.  Rho-Independent Termination: The RNA transcript forms a hairpin loop followed by a poly-U tail, destabilizing the RNA-DNA hybrid and leading to polymerase dissociation. C. TRANSCRIPTION EFFICIENCY AND REGULATION:  Coupling with Translation: In prokaryotes, transcription and translation occur simultaneously.  Regulatory Mechanisms:  Sigma Factors: Allow RNA polymerase to recognize different promoters.  Operons: Clusters of functionally related genes controlled by a single promoter (e.g., lac operon). D. SIGNIFICANCE:  Rapid Adaptation: Enables quick responses to environmental changes.  Efficient Gene Regulation: Ensures coordinated expression of genes involved in related functions. 3. BASIC PRINCIPLES OF TRANSCRIPTION IN EUKARYOTES: A. OVERVIEW: Transcription in eukaryotes is highly regulated due to the presence of a nucleus and the complexity of chromatin structure. It involves multiple RNA polymerases, general transcription factors, and additional regulatory elements.
PHARMD GURU Page 3 Figure 1: Diagram illustrating the structure of RNA polymerase and its interaction B. STAGES OF TRANSCRIPTION IN EUKARYOTES: a) INITIATION:  RNA Polymerases:  RNA Polymerase I: Transcribes rRNA (except 5S rRNA).  RNA Polymerase II: Transcribes mRNA and small nuclear RNAs (snRNAs).  RNA Polymerase III: Transcribes tRNA, 5S rRNA, and other small RNAs.  Promoter Elements:  TATA Box: Located ~25-30 bp upstream of the transcription start site, recruits TATA-binding protein (TBP).  Initiator (Inr) Sequence: Surrounds the transcription start site, essential for transcription initiation.  Proximal Promoter Elements (CAAT box, GC box): Help regulate transcription efficiency.  Pre-Initiation Complex (PIC): A multi-protein complex involving TFIID, TFIIA, TFIIB, TFIIE, TFIIF, and TFIIH, which stabilizes RNA polymerase II at the promoter.
PHARMD GURU Page 4 b) ELONGATION:  RNA Polymerase II moves along the DNA, synthesizing RNA in the 5’ to 3’ direction.  Capping: As the RNA is synthesized, a 7-methylguanosine cap is added to the 5’ end to protect against degradation and aid in translation. c) TERMINATION:  Polyadenylation Signal (AAUAAA): Cleavage occurs downstream, followed by the addition of a poly-A tail.  Release of RNA Polymerase: Facilitated by termination factors. C. POST-TRANSCRIPTIONAL MODIFICATIONS:  Splicing: Removal of introns and joining of exons by the spliceosome.  Alternative Splicing: Generates different protein isoforms from a single gene.  5’ Capping & 3’ Polyadenylation: Essential for mRNA stability and translation efficiency. D. REGULATION OF TRANSCRIPTION:  Enhancers & Silencers: Regulate transcription efficiency from distant sites.  Epigenetic Modifications:  Histone Acetylation: Loosens chromatin, increasing transcription.  DNA Methylation: Silences gene expression by compacting chromatin.  Transcription Factors: Activate or repress gene expression by interacting with RNA polymerase and chromatin modifiers. 4. TRANSCRIPTION FACTORS THAT REGULATE TRANSCRIPTION: Figure 2: Roles of transcription factors and their binding to specific DNA