BIOL3600 Ch. 13 (1)

The Central Dogma of Molecular Biology

  • Central Dogma: Flow of genetic information from DNA to RNA to protein.

    • Transcription: DNA -> RNA

    • Translation: RNA -> Protein

Expression of Genes

  • Two Steps:

    1. Transcription (DNA → RNA)

      • Language is mostly the same.

      • Concept: "Re-write".

    2. Translation (RNA → Protein)

      • Language is different.

      • Concept: "Translate" or "Decipher".

Transcription Process (DNA → RNA)

  • Requirements for Transcription:

    1. DNA template

    2. Precursors to RNA nucleotides

    3. Protein components

Structural Differences Between RNA and DNA

  • RNA vs. DNA:

    • RNA contains uracil (U) in place of thymine (T).

    • Structure: RNA has a hydroxyl group on 2'-carbon, making it more reactive than DNA.

RNA Nucleotides

  • Composition:

    • Nucleotides consist of a sugar, base, and phosphate.

    • Ribonucleoside Triphosphate: Sugar + Base = Nucleoside.

RNA Secondary Structure

  • The RNA molecule folds into secondary structures due to hydrogen bonding.

    • Significance: Influences RNA function.

  • Comparison: RNA is typically more structurally varied than DNA.

Types of RNA and Their Functions

  1. mRNA (Messenger RNA): Carries genetic information from DNA.

  2. tRNA (Transfer RNA): Brings amino acids during translation.

  3. rRNA (Ribosomal RNA): Crucial in protein synthesis.

  4. Small Nuclear RNA (snRNA): Involved in mRNA processing.

  5. Small Nucleolar RNA (snoRNA): Processes rRNA.

  6. Small Cytosolic RNA (scRNA): Function somewhat unclear.

  7. Micro RNA (miRNA) and Small Interfering RNA (siRNA): Participate in RNA interference (RNAi).

Overview of Transcription Mechanism

  • Transcription requires:

    1. DNA Template

    2. RNA Nucleotides

    3. Transcriptional Enzymes (e.g., RNA polymerase)

Transcriptional Unit

  • Definition: Stretch of DNA that encodes an RNA molecule and required sequences for transcription.

  • Parts of a Transcriptional Unit:

    1. Promoter: Signals where transcription starts.

    2. RNA Coding Region: The sequence transcribed into RNA.

    3. Terminator: Signals where transcription ends.

Transcription in Prokaryotes vs. Eukaryotes

Prokaryotes:

  1. Initiation: RNA polymerase binds to the promoter.

  2. Elongation: RNA polymerase synthesizes RNA.

  3. Termination: RNA polymerase stops at terminator sequences.

Eukaryotes:

  • Differences from Prokaryotes:

    • Multiple RNA polymerases (I, II, III).

    • Use of transcription factors to help RNA polymerase bind to the promoter.

    • Termination is not as straightforward; RNA polymerase continues transcribing beyond coding sequences.

Transcription Terminology

  • Core RNA Polymerase: Catalyzes RNA elongation by adding nucleotides.

  • Holoenzyme: Core RNA polymerase plus sigma factor.

  • Consensus Sequence: Most common nucleotide sequence found at a specific site in DNA.

Key Concepts to Review

  • Directional Nature of Promoter: Determines template strand.

  • Transcription Bubble: The region of unwound DNA where transcription occurs.

  • Polymerase Activity:

    • Eukaryotes have accessory proteins to assist in the binding of RNA polymerase.

  • Termination Mechanisms: rho-dependent and rho-independent termination in prokaryotes; different approaches in eukaryotes.