Which molecule serves as a template for transcription?
DNA serves as a template for the synthesis of RNA during transcription.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Human Genome Project and DNA Fingerprinting
#RNA synthesis
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Which of the following is the template strand of DNA in transcription?
The non-coding strand (also called the template strand) serves as the template for RNA synthesis in transcription.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
What is the function of the terminator sequence in transcription?
The terminator sequence signals RNA polymerase to stop transcription and release the RNA transcript.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
What is the role of a promoter in transcription?
The promoter is a specific DNA sequence where RNA polymerase binds to initiate transcription.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
Which of the following enzymes catalyzes the synthesis of RNA from a DNA template?
RNA polymerase catalyzes the transcription of RNA from a DNA template.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
Sensitivity of RNA polymerases to α-amanitin is
Sensitivity to alpha-amanitin toxin distinguishes three nuclear polymerases and is exploited in nuclear run-on assays. Pol II shows highest sensitivity, IC50 approximately 0.01 microgram per milliliter, blocked by binding near bridge helix. Pol III requires about ten micrograms per milliliter, intermediate sensitivity, while Pol I remains active even at hundreds of micrograms per milliliter, classified as resistant. Ranking therefore Pol II greater than Pol III greater than Pol I. Structural divergence in funnel domains and trigger loop residues that contact bicyclic peptide explains differential binding affinities and differential toxin susceptibility among polymerases.
Ref: Berg et al., Biochemistry, 9th ed., Chapter 28: Sensitivity II > III > I; Alberts et al., Molecular Biology of the Cell, Amanitin sensitivity ranking
α-amanitin strongly inhibits
Alpha-amanitin cyclic octapeptide from Amanita phalloides death cap mushroom penetrates hepatocytes and binds with extremely high affinity in funnel and trigger loop beneath RNA polymerase II active site, obstructing translocation and nucleotide incorporation, leading to slow elongation arrest and degradation of RPB1 subunit. RNA polymerase II is most sensitive with inhibition constant around ten nanomolar, blocked at one microgram per milliliter, causing fatal liver failure. Polymerase III requires tenfold higher concentration, while polymerase I and mitochondrial polymerase resist up to high doses due to structural differences.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: alpha-amanitin strongly inhibits Pol II; Berg et al., Biochemistry, Amanitin mechanism
Abortive initiation refers to synthesis of RNA of length
During initial transcription, RNA polymerase undergoes iterative cycles of synthesizing and releasing short RNAs while remaining bound to promoter. Structural blockage occurs because sigma factor region 3.2 loop threads through RNA exit channel, obstructing extension beyond about nine to ten nucleotides. Polymerase synthesizes short oligoribonucleotides two to nine bases that escape into environment as abortive transcripts before productive escape. These ≤10 nucleotide products characteristic of abortive initiation phase. Overcoming this barrier requires conformational change displacing sigma block upon sufficient RNA length and energy, transitioning to elongation competent complex processively synthesizing full-length RNA.
Ref: Watson et al. Chapter 13: Abortive initiation RNA length ≤10 nucleotides; Alberts Chapter 6 Abortive transcripts initiation complex instability
RNA polymerase synthesizes RNA slower than DNA polymerase because
Bacterial RNA polymerase elongates roughly 40-50 nucleotides per second, significantly slower than DNA polymerase III synthesizing DNA near 750-1000 nucleotides per second during replication. Reduced rate attributed to lower processivity because transcription includes frequent pausing, backtracking for proofreading via GreA/GreB, waiting for regulatory factors like NusA, and requirement to unwind DNA continuously. Replication utilizes sliding clamp beta conferring high processivity and topoisomerases cooperating efficiently. Transcriptional pausing facilitates coupling with translation, co-transcriptional folding, attenuation, and regulatory checkpoints absent in highly processive replication optimized for rapid genome duplication.
Ref: Berg Biochemistry Chapter 28: Transcription rate slower than replication processivity; Watson Chapter 13 Elongation speed comparison
Transcription does not require which of the following?
DNA polymerases initiate synthesis only from provided primer supplying 3' hydroxyl because they cannot stabilize initial dinucleotide. RNA polymerase overcomes this through additional contacts: sigma factor, downstream DNA binding, and ability to hold two initiating ribonucleoside triphosphates base-paired to template at transcription start site via base stacking and hydrogen bonds. First phosphodiester bond forms de novo without prior polymer. This primer independent initiation permits transcription to start at defined +1 sites throughout genome. Consequently DNA template, Mg2+, and rNTPs suffice, while exogenous primer unnecessary.
Ref: Alberts Molecular Biology of Cell 7th ed. Chapter 6: De novo initiation without primer; Lehninger Chapter 26 Transcription requirements primer independence
RNA synthesis during transcription occurs in which direction?
Chain growth polarity dictated by chemistry of polymerase active site containing two magnesium ions. Nucleophilic attack by 3' hydroxyl of nascent RNA on alpha phosphate of incoming nucleoside triphosphate extends chain toward distal 5' to 3' direction. Polymerase simultaneously translocates downstream along antiparallel template reading 3' to 5', preserving RNA-DNA hybrid of eight to nine bases. Because synthesis requires free 3' OH, direction fixed unidirectionally analogous to DNA polymerases. This polarity ensures nascent RNA emerges 5' end first from exit channel while template entry determines future transcription downstream trajectory.
Ref: Berg Biochemistry 9th ed. Chapter 28: RNA synthesis 5' to 3' direction mechanism; Watson Chapter 13 Polarity
Transcription is the process of synthesis of
Transcription is enzymatic synthesis of RNA copy complementing segment of one DNA strand. RNA polymerase opens duplex forming transcription bubble about fourteen base pairs, uses exposed template strand to select ribonucleoside triphosphates by Watson-Crick pairing, catalyzing sequential phosphodiester bonds elongating chain 5' to 3'. Resulting transcripts include messenger RNA conveying protein coding information, ribosomal RNAs forming translation apparatus, transfer RNAs delivering amino acids, and regulatory RNAs. Production of RNA rather than DNA distinguishes transcription from replication, representing first directional step of central dogma converting stable genetic storage into transient expressed molecules.
Ref: Lodish Molecular Cell Biology 9th ed. Chapter 8: Transcription synthesis of RNA definition; Alberts Chapter 6 Overview transcription