Gene Expression

Central Dogma of Molecular Biology
  • Information flow: DNA → RNA → Protein
  • Steps involved:
    • Transcription: Process of making RNA from DNA.
    • Requirements:
      • DNA template
      • RNA polymerase
      • Ribonucleotides (A, U, G, C)
      • Transcription factors
      • Promoter region (specific DNA sequence where RNA polymerase binds)
    • Translation: Process of making proteins from RNA.
    • Requirements:
      • mRNA
      • tRNA
      • Ribosomes
      • Energy
DNA Strands
  • Sense Strand: Non-template strand; has the same sequence as the mRNA after transcription but is not used for transcription.
  • Antisense Strand: Template strand; used by RNA polymerase to synthesize RNA.
    • Comparison with Terminology: Sense = Non-template, Antisense = Template.
  • Sense RNA: Also refers to mRNA, which is produced from the DNA and can include antisense RNA that binds to sense RNA to block translation (gene regulation).
RNA Polymerase
  • Role: Enzyme responsible for synthesizing mRNA from the DNA template during transcription. Without mRNA, translation to make proteins cannot occur.
Comparison of DNA and Transcription Bubbles

  • DNA Replication Bubble:

    • Both strands of DNA serve as templates.
    • Bidirectional replication (moving in both directions).
    • Enzymes involved: DNA polymerase.

  • Transcription Bubble:

    • Only the antisense (template) strand is used for RNA production.
    • Unidirectional (RNA is synthesized only in one direction).
    • Enzyme involved: RNA polymerase.
Ribosome Structure and Function
  • Composition: Ribosomes are macromolecular particles composed of rRNA and proteins.
  • Binding Sites: Includes A (Aminoacyl), P (Peptidyl), and E (Exit) sites.
  • Function in Elongation Cycle:
    1. Codon recognition: tRNA enters A site, matching anticodon to mRNA codon.
    2. Peptide bond formation: Ribosome catalyzes bond between amino acids.
    3. Translocation: Ribosome shifts one codon forward.
  • Antibiotics: Target bacterial ribosomes, e.g., they affect prokaryotic ribosomes (70S) without harming eukaryotic ribosomes (80S).
Differences in Protein Synthesis: Prokaryotes vs. Eukaryotes

  • Prokaryotes:

    • Simpler, no mRNA processing.
    • Transcription and translation happen simultaneously.
    • RNA polymerase binds directly to the promoter at binding sequences (e.g., TATAA).

  • Eukaryotes:

    • More complex, with extensive regulation and mRNA processing before translation.
    • Initiation of transcription requires transcription factors that bind to the promoter region.
Key Terms
  • Promoter: Specific DNA sequence where RNA polymerase binds to start transcription.
  • Transcription Bubble: Temporary unwound section of DNA where RNA polymerase transcribes RNA.
  • Codon: A sequence of three ribonucleotides that codes for a specific amino acid or serves as a start/stop signal.
  • Anticodon: A sequence of three bases on tRNA that are complementary to a codon on mRNA.
Types of RNA and Their Functions
  • mRNA (messenger RNA):
    • Single-stranded, serves as a template for protein synthesis.
  • tRNA (transfer RNA):
    • Adapter molecule delivering amino acids to ribosomes; contains anticodon loop for codon recognition.
  • rRNA (ribosomal RNA):
    • Long strands that combine with ribosomal proteins to form ribosome subunits, ensuring proper alignment of mRNA and tRNA.
Genetic Code Characteristics
  • Triplet Code: Codons are made up of three nucleotides.
  • Redundant Code: Multiple codons can code for the same amino acid.
  • Unambiguous Code: Each codon codes for only one amino acid.
  • Universal Code: The genetic code is consistent across different organisms.
Translation Process
  • Initiation: Ribosome reads mRNA starting at the start codon.
  • Elongation: During this cycle, polypeptide chain grows as ribosome reads mRNA, with codons recognized and peptide bonds formed.
  • Termination: The stop codon signals the end of the translation process, completing the protein synthesis.
Polyribosomes (Polysomes)
  • Definition: A group of ribosomes translating a single mRNA simultaneously.
  • Significance: Allows for increased speed and efficiency in protein production, essential for mass synthesis of proteins.