Molecular Biology of the Gene

12 Molecular Biology of the Gene

12.1 The Genetic Material

  • Learning Objectives:
      - Rank knowledge on scale 1 (least) to 5 (expert) pre- and post-lecture.
      - Describe properties of genetic material.
      - Examine historical researchers confirming DNA as genetic material.
      - Explain the chemical structure of DNA according to Watson and Crick.

  • Frederick Griffith's Experiment:
      - Investigated virulence of Streptococcus pneumoniae.
      - Concluded that virulence could be transferred from dead bacteria to live non-virulent strains, known as Transformation.

  • Avery et al. Research:
      - Identified DNA as the transforming substance.
      - DNA from dead cells incorporated into genomes of living cells.

  • Requirements for Genetic Material:
      - Store genetic information.
      - Stable and accurately replicated during cell division.
      - Undergo mutations for genetic variability.
      - DNA fulfills these criteria.

  • Griffith's Transformation Steps:
      - Mice injected with:
        - Live S strain (virulent, kills mice).
        - Live R strain (non-virulent, does not kill mice).
        - Heat-killed S strain (does not kill mice).
        - Heat-killed S strain + live R strain: caused death; live S strain recovered from dead mice.

12.2 Replication of DNA

  • DNA Replication Definition:
      - Copying of a DNA molecule.
      - Semiconservative Replication: Each original strand serves as a template for a new strand in a daughter molecule.

  • Overview of DNA Replication:
      - Enzymes Involved:
        - Helicase: Unwinds the DNA strands.
        - Single-Stranded Binding Proteins (SSB): Stabilize unwound strands.
        - DNA Primase: Adds RNA primers for polymerase.
        - DNA Polymerase: Synthesizes new strands.
        - DNA Ligase: Joins Okazaki fragments on the lagging strand.
      - Creates leading and lagging strands.

  • Prokaryotic vs. Eukaryotic DNA Replication:
      - Prokaryotic: Single circular DNA; replication in both directions from the origin; takes 40 minutes.
      - Eukaryotic: Multiple origins on linear chromosomes; unwinds at multiple points; slower due to complexity.

  • Accuracy of Replication: DNA polymerase makes errors about once per 100,000 base pairs but can correct them.

12.3 Gene Expression: RNA and the Genetic Code

  • Learning Objectives:
      - Explain transcription and translation functions.
      - Describe how mRNA nucleotide sequences determine polypeptide sequences.

  • Flow of Genetic Information:
      - From DNA to RNA to protein to observable traits.
      - One Gene, One Enzyme Hypothesis by Beadle and Tatum via experiments on Neurospora crassa.

  • Major Classes of RNA:
      - Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes.
      - Transfer RNA (tRNA): Transfers amino acids during protein synthesis.
      - Ribosomal RNA (rRNA): Forms ribosomes with proteins.

  • The Genetic Code:
      - Composed of codons (three-nucleotide sequences that code for amino acids).
      - Characteristics:     - 64 codons total; redundant (multiple codons code for the same amino acid); unambiguous (each codon codes for one amino acid only).
        - Example: Codon UCU codes for phenylalanine.

12.4 Gene Expression: Transcription

  • Transcription Overview:
      - A gene is “unzipped” to expose bases; only one strand is used as the template.
      - RNA polymerase synthesizes mRNA in the 5′ to 3′ direction.

  • Stages of Transcription:
      1. Initiation: RNA polymerase binds to promoter; defines transcription start point.
      2. Elongation: RNA polymerase travels down template; elongates RNA strand.
      3. Termination: RNA polymerase encounters stop sequence, releases mRNA transcript.

  • RNA Processing in Eukaryotes:
      - Pre-mRNA modified; includes cap and poly-A tail; introns spliced out.

12.5 Gene Expression: Translation

  • Translation Overview:
      - Occurs at ribosomes; translates mRNA codons into polypeptide sequences.
      - Requires mRNA, tRNA, and ribosomes.

  • Initiation, Elongation, and Termination of Translation:
      - Initiation: Small ribosomal subunit binds mRNA; initiator tRNA (UAC) binds start codon (AUG).
      - Elongation: tRNA brings amino acids to ribosome; peptide bonds form.
      - Termination: Stop codon signals end; release factor triggers polypeptide release.

  • tRNA Structure and Function:
      - Carries amino acids; anticodon region pairs with mRNA codons; wobble hypothesis explains pairing flexibility.

  • Ribosome Structure:
      - large and small subunits; binding sites (E, P, A) facilitate translation.

  • Gene Expression Summary:
      - Successful expression leads to protein production from the corresponding gene in the cell.