Лекция 3. Транскрипция, трансляция. Окштейн И.Л., МФТИ

Scientific concepts and nature/biological phenomena mentioned

• Central dogma of molecular biology

  • Information flow: DNA → RNA → Protein
  • Reverse transcription can occur in special cases (e.g., viral proteins), though DNA → DNA replication is part of normal biology.

• Gene expression as cellular “information processing”

  • Three major information-related cellular processes emphasized:
    • Replication (DNA → DNA; “two DNAs from one DNA”)
    • Transcription (DNA → RNA)
    • Translation (RNA → protein)

• Nuclear organization and RNA routing

  • Nuclear pores are passage points through which mRNA (information RNA) leaves the nucleus to the cytoplasm.

• Transcription mechanism (RNA synthesis)

  • RNA polymerase binds to promoter regions and initiates RNA synthesis.
  • Regulatory proteins can bind near promoters/“operator” sequences to either:
    • facilitate recruitment of RNA polymerase, or
    • block it.
  • Terminators end transcription; RNA polymerase detaches after completion.
  • In eukaryotes, the segment being transcribed is called a gene.

• mRNA processing via splicing (in eukaryotes)

  • Transcripts contain:
    • Introns (cut out)
    • Exons (remaining pieces)
  • Splicing markers (splice sites) define what is removed and what is retained.
  • Splice sites are recognized by small nuclear RNAs and associated proteins forming snRNPs / small nuclear ribonucleoproteins.
  • Outcome: generation of a mature mRNA that exits to the cytoplasm.

• mRNA capping and tailing

  • At the 5’ end, a cap is attached (described generally).
  • At the 3’ end, a poly(A) tail is added (roughly ~200 adenine residues mentioned).
  • These features are presented as important for stability/validity of the message.

• Energy coupling during polymer synthesis

  • Nucleotide polymerization uses high-energy nucleotide building blocks:
    • Nucleoside triphosphates (e.g., ATP, GTP, etc.)
  • Energy is stored in macroergic (high-energy) phosphate bonds.
  • Polymer growth is described as:
    • breaking phosphoanhydride bonds (triphosphates),
    • forming new bonds to extend the RNA/DNA chain,
    • with enzyme involvement (e.g., phosphatases described as removing phosphates and helping drive irreversibility).

• DNA replication

  • “Two DNAs from one DNA” is stated as replication, involving replication machinery (e.g., replication forks mentioned).

• Translation / protein synthesis

  • tRNA (transport RNA) carries amino acids and is specific via an anticodon that pairs with the mRNA codon.
  • The ribosome binds mRNA and positions tRNAs at codon sites sequentially.
  • A start codon is referenced; ribosome initiation factors are mentioned.
  • Ribosome functional sites:
    • A site (aminoacyl/acceptor site for incoming tRNA)
    • P site (peptidyl site holding the growing peptide)
    • (An additional decoding/interaction region is implied by the discussion of subunit interactions.)
  • Protein elongation continues codon-by-codon, growing a peptide chain.

• rRNA and ribosome biogenesis in the nucleolus

  • rRNA genes with tandem repeats are transcribed in the nucleolus.
  • Ribosomal RNA undergoes splicing/processing and associates with proteins.
  • Ribosomal subunits assemble and pass through nuclear pores to the cytoplasm.

• Ribosome types and composition

  • Prokaryotic vs eukaryotic ribosome sedimentation coefficients:
    • 70S (prokaryotes) vs 80S (eukaryotes)
    • Subunits like 30S/50S and 40S/60S are referenced.
  • The large subunit contains multiple rRNA molecules; an example claim is that the eukaryotic large subunit contains ~3 rRNAs, with approximate sizes mentioned.

• Ultracentrifugation as a method

  • Used to separate cell components based on their sedimentation velocity.
  • Sedimentation coefficient (“Svedberg units” concept) is mentioned.
  • Includes:
    • the role of particle size/shape,
    • non-additivity of coefficients.

• Evolutionary constraint idea (based on rRNA)

  • Different regions of rRNA show different mutation rates:
    • conserved regions evolve slowly,
    • other regions tolerate changes more.
  • Linked to functional importance.

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Methodology / step-like processes (as described)

Transcription (DNA → RNA)

1. RNA polymerase binds to the promoter.
2. Regulatory proteins act via operator/promoter control, enabling or inhibiting polymerase binding.
3. RNA polymerase synthesizes RNA along the DNA template.
4. Terminators stop transcription.
5. RNA released from polymerase is described as not yet “ready” until processing.

Splicing (mRNA pre-processing)

1. The transcript contains splice markers.
2. Introns are removed between splice sites.
3. Exons are ligated together.
4. The process operates in the nucleus.
5. snRNPs (small nuclear RNAs + proteins) recognize splice sites and catalyze cutting/ligation steps.

3’ end polyadenylation and 5’ capping (mRNA maturation)

• Enzymes add:
  • a 5’ cap
  • a 3’ poly(A) tail (~200 adenines mentioned)
• This maturation supports mRNA functionality/stability (presented indirectly via a “validity period” concept).

Translation (RNA → protein)

1. mRNA enters translation in the cytoplasm.
2. Ribosome initiation:
   • the small subunit + initiation factors bind near the start codon,
   • an initiator tRNA carrying methionine docks.
3. The large subunit joins.
4. Codon-by-codon elongation:
   • incoming tRNA matches the codon via anticodon pairing,
   • peptide bond formation transfers the growing chain,
   • the ribosome advances one codon at a time repeatedly until termination (termination not clearly detailed in the excerpt, but elongation is).

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Researchers / sources featured (explicitly named)

• И.Л. Окштейн (I.L. Okshtein) — lecturer (Okshtейн И.Л., МФТИ).