10.4 The Central Dogma, Strand Nomenclature & Transcription Machinery

10.4 The Central Dogma, Strand Nomenclature & Transcription Machinery

Gene expression converts latent nucleotide sequences encrypted within chromosomal DNA into dynamic functional molecules. While all somatic cells within a multicellular eukaryote share an identical genomic blueprint, differential transcriptional and post-transcriptional regulation directs phenotypic cell specialization, embryological morphogenesis, and environmental adaptation.

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1. The Central Dogma and Strand Nomenclature

Francis Crick framed the Central Dogma as a directional flow of sequential information: DNA → RNA → Protein. Significant biochemical exceptions and nuances exist:

• Reverse Transcription: Retroviruses (such as HIV) and retrotransposons use reverse transcriptase (RNA-dependent DNA polymerase) to synthesize complementary DNA (cDNA) from an RNA template (RNA → DNA).
• RNA Replication: RNA viruses (such as poliovirus, influenza, and SARS-CoV-2) replicate their genomes using RNA-dependent RNA polymerases (RdRP) without a DNA intermediate (RNA → RNA).
• Prions: Infectious proteins (PrP-Sc) that propagate phenotypic alterations by acting as conformational templates to induce misfolding of normal cellular proteins (PrP-C), bypassing nucleic acid templates entirely.

Template (Antisense) vs. Coding (Sense) Strands

Transcription proceeds strictly in the 5' → 3' direction, requiring RNA polymerase to read the parental DNA strand in the opposite 3' → 5' direction:

1. Template Strand (also designated the Antisense, Non-coding, or Minus [-] strand): The actual DNA strand that physical RNA polymerase translocates along and base-pairs with. Its sequence is antiparallel and complementary to the synthesized RNA.
2. Coding Strand (also designated the Sense, Non-template, or Plus [+] strand): The complementary partner DNA strand. Its sequence is identical to the synthesized RNA transcript (running 5' to 3'), with the single difference that uracil (U) in RNA replaces thymine (T) in DNA.

5' - A T G C C G T T A A C G - 3'  Coding (Sense) Strand DNA
3' - T A C G G C A A T T G C - 5'  Template (Antisense) Strand DNA (Read 3'->5')

5' - A U G C C G U U A A C G - 3'  Synthesized mRNA Transcript (Synthesized 5'->3')

MCAT Warning: Exam questions frequently present a DNA sequence labeled as the "coding strand" and ask for the resulting mRNA sequence. Do not waste time computing the reverse complement! Simply rewrite the coding sequence replacing all "T"s with "U"s.

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2. Transcription Machinery: Prokaryotic vs. Eukaryotic RNA Polymerases

Unlike DNA polymerases, RNA polymerases do not require an existing 3'-OH primer to initiate synthesis; they initiate de novo synthesis upon binding specific promoter DNA sequences.

Prokaryotic Transcription

Bacteria utilize a single type of RNA polymerase to synthesize all cellular RNA classes (mRNA, tRNA, rRNA). The catalytic core enzyme comprises five polypeptide subunits: α2ββ'ω.

• The core enzyme can synthesize RNA but lacks sequence-specific promoter affinity.
• Association with a regulatory sigma factor (σ) forms the RNA Polymerase Holoenzyme (α2ββ'ωσ). The sigma factor (most commonly σ⁷⁰) recognizes conserved promoter elements upstream of the transcription start site (+1):
  • -10 Consensus Element (Pribnow Box): 5'-TATAAT-3'
  • -35 Consensus Element: 5'-TTGACA-3'
• Once the holoenzyme binds the promoter, forms an open transcription bubble, and synthesizes an initial ~10-nucleotide abortive transcript, the sigma factor dissociates, allowing the core enzyme to proceed through processive elongation.
• Termination: Occurs via two distinct bacterial mechanisms:
  • Intrinsic (Rho-independent) Termination: RNA polymerase transcribes a GC-rich inverted repeat followed by a poly-uracil tract. The transcribed RNA folds into a stable hairpin loop, causing the polymerase to stall. The weak hydrogen bonding between the downstream poly-U tract and template poly-A sequence promotes spontaneous transcript dissociation.
  • Rho-dependent Termination: A hexameric ATP-dependent helicase (Rho protein) binds an unstructured cytosine-rich sequence on the nascent RNA called the rut (Rho utilization) site, translocates along the transcript toward the paused polymerase, and unwinds the RNA-DNA hybrid to release the transcript.
• Pharmacology: Rifampin (Rifampicin) binds specifically to the β-subunit of bacterial RNA polymerase, sterically blocking the elongating RNA chain without inhibiting eukaryotic RNA polymerases. It is a cornerstone antibiotic in tuberculosis therapy.

Eukaryotic RNA Polymerases

Eukaryotic nuclei feature three distinct RNA polymerases, each compartmentalized and dedicated to specific classes of transcripts (Mnemonic: "Pol I, II, III = R, M, T — Roman numerals in alphabetical order"):

1. RNA Polymerase I (Pol I):

   • Location: Nucleolus.
   • Products: Synthesizes the single 45S pre-rRNA precursor, which is enzymatically cleaved into mature 28S, 18S, and 5.8S ribosomal RNAs (rRNAs).
   • Inhibitor Sensitivity: Completely insensitive to α-amanitin.

2. RNA Polymerase II (Pol II):

   • Location: Nucleoplasm.
   • Products: Transcribes all protein-coding pre-messenger RNAs (pre-mRNAs) and most small nuclear RNAs (snRNAs involved in splicing) as well as microRNAs.
   • Inhibitor Sensitivity: Extremely sensitive to α-amanitin (a cyclic octapeptide toxin produced by the death cap mushroom Amanita phalloides). Ingestion of α-amanitin causes irreversible Pol II arrest, leading to massive hepatocyte apoptosis, acute liver failure, and death.

3. RNA Polymerase III (Pol III):

   • Location: Nucleoplasm.
   • Products: Synthesizes small structural RNAs: all transfer RNAs (tRNAs), the 5S ribosomal RNA (the only rRNA synthesized outside the nucleolus), and the U6 snRNA.
   • Inhibitor Sensitivity: Inhibited by high concentrations of α-amanitin in mammals.

Eukaryotic Promoter Architecture and Initiation

RNA Pol II cannot recognize promoters independently. It requires the assembly of General Transcription Factors (GTFs) at core promoter elements:

• TATA Box (Hogness Box): Located approximately -25 to -30 base pairs upstream of the transcription initiation site, with consensus sequence 5'-TATAAA-3'.
• Initiator Element (Inr): Spans the transcription start site (+1).
• Pre-Initiation Complex (PIC) Assembly:
  1. TFIID binds the TATA box via its TBP (TATA-Binding Protein) subunit, introducing a sharp 80° bend in the DNA.
  2. TFIIA and TFIIB bind, stabilizing the complex and recruiting RNA Pol II complexed with TFIIF.
  3. TFIIE and TFIIH join to complete the pre-initiation complex. TFIIH is a multi-functional enzyme complex possessing ATP-dependent DNA helicase activity to unwind promoter DNA, as well as kinase activity that phosphorylates the C-terminal domain (CTD) of the largest Pol II subunit (on Serine 5 of a conserved heptapeptide repeat). CTD phosphorylation triggers promoter clearance, releasing Pol II from GTFs to enter active elongation.

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The Three Eukaryotic RNA Polymerases

Bacteria manage with a single RNA polymerase directed by exchangeable sigma factors; eukaryotes divide the work across three nuclear enzymes with different products and different drug sensitivities.