Unit 4: Central Dogma & Protein Synthesis

Syllabus Reference: Central Dogma, Genetic code, DNA replication, Transcription (Prokaryotes/Eukaryotes); Lac/Trp operon; Split genes (introns/exons); mRNA processing; Translation; [cite_start]Post-translational modifications [cite: 515-518].

1. The Central Dogma

Proposed by Francis Crick. It describes the flow of genetic information:

DNA → (Transcription) → mRNA → (Translation) → Protein

2. DNA Replication

Mechanism is Semi-conservative (Meselson & Stahl experiment). Occurs in S-phase.

• Enzymes:
  • Helicase: Unzips DNA.
  • Primase: Synthesizes RNA primer.
  • DNA Polymerase III: Main replicating enzyme (adds bases 5' → 3').
  • DNA Ligase: Joins Okazaki fragments on the Lagging strand.
• Process: Starts at Origin of Replication (Ori). Forms a Replication Fork. Leading strand is continuous; Lagging strand is discontinuous.

3. Transcription & Regulation

Synthesis of RNA from DNA template.

• Prokaryotes: Occurs in cytoplasm. Single RNA Polymerase (with Sigma factor for initiation).
• Eukaryotes: Occurs in nucleus. 3 RNA Polymerases (Pol I for rRNA, Pol II for mRNA, Pol III for tRNA).
  Split Genes: Eukaryotic genes contain Exons (coding) and Introns (non-coding).
  Processing: Splicing (removal of introns), 5' Capping, 3' Poly-A Tailing.

Gene Regulation (Operon Model)

• Lac Operon (Inducible): In E. coli.
  When Lactose is absent: Repressor binds Operator → No Transcription.
  When Lactose is present: Lactose binds Repressor → Repressor detaches → Transcription ON.
• Trp Operon (Repressible):
  When Tryptophan is high: It acts as a corepressor, binding the repressor to block transcription.

4. The Genetic Code

• Triplet: 3 bases code for 1 amino acid (Codon).
• Universal: Same in bacteria and humans.
• Degenerate: Multiple codons can code for the same amino acid.
• Start Codon: AUG (Methionine).
• Stop Codons: UAA, UAG, UGA.

5. Translation

• Initiation: Ribosome assembles around mRNA. tRNA carrying Methionine binds to AUG.
• Elongation: Ribosome moves along mRNA. New amino acids added via Peptide bonds.
• Termination: Stop codon reached. Release factor binds. Polypeptide released.
• Post-translational Modifications: Folding, glycosylation, or phosphorylation to make the protein functional.