Molecular Biology: Gene Expression & RNA

1. 📌 Essentials

• Transcription: DNA to RNA synthesis by RNA polymerase.
• RNA processing: Capping, polyadenylation, splicing (removal of introns).
• Translation: mRNA decoding into amino acid chains by ribosomes.
• Promoter sequences: Bacterial (-10, -35); Eukaryotic (TATA box).
• RNA types: mRNA, tRNA, rRNA, snRNA, regulatory RNAs.
• Genetic code: 64 codons, AUG start, UAA/UAG/UGA stop.
• RNA modifications: 5′ cap, 3′ poly-A tail, sp.
• RNA's catalytic role: Ribozymes, self-replication- Gene structure: Exons (coding), introns (noncoding).
• Key differences: Prokaryotic vs eukaryotic transcription and processing.

2. 🧩 Key Structures & Components

• RNA polymerase — enzyme synthesizing RNA from DNA template.
• Promoters — DNA sequences signaling initiation (-10, -35 in bacteria; TATA in eukaryotes).
• snRNPs — small nuclear ribonucleoproteins involved in splicing.
• Ribosome — molecular machine translating mRNA; has A, P, E sites.
• tRNA — adaptor molecules matching codons to amino acids.
• mRNA — messenger RNA carrying genetic info.
• Spliceosome — complex of snRNPs executing splicing.
• Ubiquitin-proteasome system — degrades proteins tagged with ubiquitin.
• RNA modifications — 5′ cap, 3′ poly-A tail, exon junction complexes.
• Genetic code — codon table translating nucleotide triplets into amino acids.

3. 🔬 Functions, Mechanisms & Relationships

• Transcription initiation:
  • Bacteria: sigma factor binds -10/-35 promoter.
  • Eukaryotes: TFIID binds TATA box, recruits RNA polymerase II.
• Elongation:
  • RNA polymerase moves 3′-to-5′ on DNA, synthesizes RNA 5′-to-3′.
• Termination:
  • Bacteria: specific terminator sequences.
  • Eukaryotes: cleavage and polyadenylation signals.
• RNA processing:
  • 5′ capping stabilizes mRNA and aids translation.
  • Poly-A tail enhances stability and export.
  • Splicing removes introns, joins exons.
• Translation:
  • Initiation involves eIFs and initiator tRNA binding start codon.
  • Ribosome moves along mRNA, decoding codons.
  • Polyribosomes produce multiple proteins simultaneously.
• Protein regulation:
  • Post-translational modifications modulate activity.
  • Proteasome degrades ubiquitinated proteins.
• RNA's catalytic activity:
  • Ribozymes catalyze reactions, supporting RNA world hypothesis.
• Gene expression hierarchy:
  • DNA → Transcription → RNA processing → Export → Translation → Protein.

4. Comparative Table: Prokaryotic vs Eukaryotic Transcription

5. 🗂️ Hierarchical Diagram

Gene Expression System
 ├─ DNA
 │    ├─ Promoter
 │    ├─ Exons
 │    └─ Introns
 ├─ Transcription
 │    ├─ Initiation
 │    ├─ Elongation
 │    └─ Termination
 ├─ RNA Processing
 │    ├─ Capping
 │    ├─ Splicing
 │    └─ Polyadenylation
 ├─ Export
 │    └─ Nuclear pore complex
 └─ Translation
      ├─ Ribosome assembly
      ├─ Codon decoding
      └─ Protein synthesis

6. ⚠️ High-Yield Pitfalls & Confusions

• Confusing promoter sequences: -10/-35 (bacteria) vs TATA box (eukaryotes).
• Mistaking introns for exons; introns are noncoding.
• Overlooking RNA modifications: capping and poly-A tail are crucial for stability.
• Assuming all RNA is translated; some RNAs (e.g., miRNA) are regulatory.
• Confusing initiation factors in eukaryotes with bacterial sigma factors.
• Believing splicing occurs after transcription; it is co-transcriptional.
• Misidentifying stop codons: UAA, UAG, UGA.
• Overlooking RNA's catalytic role in early evolution.
• Assuming proteasome degrades all proteins indiscriminately; it targets ubiquitinated proteins.

7. ✅ Final Exam Checklist

• Understand the core steps of transcription in prokaryotes and eukaryotes.
• Recognize key promoter elements and their binding proteins.
• Describe RNA polymerase structure and function.
• Explain RNA processing steps: capping, splicing, polyadenylation.
• Identify snRNPs and their role in splicing.
• Know the genetic code: codons, start/stop signals, degeneracy.
• Describe translation initiation, elongation, and termination.
• Understand ribosome structure and function.
• Recognize post-translational modifications and their effects.
• Comprehend protein degradation via proteasome and ubiquitin.
• Appreciate RNA's catalytic roles and evolutionary significance.
• Differentiate prokaryotic and eukaryotic transcription mechanisms.
• Be familiar with nuclear export signals and processes.
• Know alternative splicing and its contribution to proteomic diversity.
• Recall RNA modifications essential for stability and translation.
• Understand the hierarchical organization of gene expression.

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