Transcription & RNA

Transcription & RNA Processing

Transcription is the synthesis of RNA from a DNA template, catalyzed by RNA Polymerase. It follows the Central Dogma: DNA → RNA → Protein.

General Mechanism

  1. Initiation: RNA polymerase binds to Promoter sequence (consensus: TATA box in eukaryotes at -25, CAAT or GC box at -75). In prokaryotes: σ (sigma) factor recognizes -10 (Pribnow box) and -35 boxes. In eukaryotes: General Transcription Factors (GTFs) + Mediator + RNA Pol II assemble at promoter.
  2. Elongation: RNA pol moves along template strand (3'→5'), synthesizes RNA in 5'→3' direction. No primer needed. Ribose nucleotides used (not deoxy). U replaces T in RNA.
  3. Termination: Prokaryotes: Rho-independent (hairpin + poly-U sequence) or Rho-dependent (Rho helicase chases RNA pol). Eukaryotes: Cleavage at poly-A signal (AAUAAA) → polyadenylation.

RNA Polymerases (Eukaryotes)

  • RNA Pol I: Nucleolus; synthesizes pre-rRNA (28S, 18S, 5.8S). Inhibited by α-Amanitin (high dose — mushroom toxin)
  • RNA Pol II: Nucleoplasm; synthesizes pre-mRNA (→mRNA), snRNA, miRNA. Inhibited by α-Amanitin (low dose — most sensitive)
  • RNA Pol III: Nucleoplasm; synthesizes tRNA, 5S rRNA, snRNA. Inhibited by α-Amanitin (very high dose)

Pre-mRNA Processing (Eukaryotes)

  • 5' Capping: 7-methylguanosine cap added co-transcriptionally. Protects from 5' exonuclease; required for ribosome binding (translation initiation); required for nuclear export.
  • 3' Polyadenylation: ~200 A residues added to 3' end after cleavage at AAUAAA signal. Protects from 3' degradation; required for nuclear export; ↑mRNA stability.
  • Splicing: Removal of introns (non-coding) and joining of exons. Site: Spliceosome (snRNPs: U1, U2, U4, U5, U6). Branch point A (within intron) attacks 5' splice site → lariat intermediate → 3' splice site attacked → intron released as lariat → exons joined. Alternative splicing: one pre-mRNA → multiple protein isoforms. Mutations in splice sites → human diseases (e.g., β-thalassemia).

Types of RNA

  • mRNA (~5%): Carries genetic code; poly-A tail; translated on ribosomes
  • rRNA (~80%): Most abundant; ribosomal scaffold + catalytic function (peptidyl transferase activity of 23S/28S rRNA = ribozyme)
  • tRNA (~15%): Aminoacid adapter; 73–93 nt; CCA-3' end; anticodon loop; L-shaped tertiary structure
  • snRNA: In spliceosome
  • miRNA: ~22 nt; base-pairs with 3'UTR of target mRNA → mRNA degradation or translation repression. Important in development and cancer.
  • siRNA: Gene silencing (RNAi); therapeutic applications
  • lncRNA: >200 nt non-coding; chromatin remodeling, X-inactivation (Xist)

Ribozymes

RNA molecules with catalytic activity. Examples: Peptidyl transferase (23S/28S rRNA), Ribonuclease P (tRNA processing), Group I and II self-splicing introns, Hepatitis Delta Virus ribozyme. Important evidence for RNA World hypothesis.

Reverse Transcription

Retroviruses (HIV): ssRNA → dsDNA via Reverse Transcriptase (RNA-dependent DNA polymerase). RT lacks proofreading → high mutation rate. NRTIs (Zidovudine/AZT) and NNRTIs target RT. Telomerase is also a reverse transcriptase.

Quiz - Exam Preparation Strategy

When studying Quiz for your final board exams, it is critical to focus on the core concepts and fundamental formulas. Relying strictly on NCERT textbook solutions and practicing previous year questions (PYQs) is the proven methodology for scoring high marks. Avoid rote memorization and instead focus on the logical application of the theories presented in this chapter.

⚠️ Common Mistakes to Avoid

❓ Frequently Asked Questions

How can I quickly memorize the concepts of Quiz?

The most effective way is to create short, handwritten revision notes and continuously test your knowledge using our interactive Mock Tests. Spaced repetition and active recall are much better than passive reading.

What type of questions are most commonly asked from Quiz?

Board exams tend to favor conceptual application questions and direct formula-based derivations from the NCERT syllabus. Ensure you have solved every single exercise in the official textbook.

Is reading the NCERT book enough for this chapter?

Yes, the NCERT textbook is the absolute gold standard for board exams. However, to improve your speed and accuracy during the actual exam, you must supplement your reading by solving timed mock tests and objective questions.