DNA LEC 1

Experiment Tracking Radioactivity

  • Researchers tracked radioactivity to determine components of viruses that infect cells.
    • Key question: Which part of the virus enters the cell?
    • Options: Virus coat or DNA?
    • Correct answer: DNA.

Labeling and Radioactive Tracking

  • DNA was labeled with radioactive phosphorus.
  • Proteins were labeled with radioactive sulfur.
  • Experiment outcome:
    • Viral coat (protein) did not enter the cell; it remained outside.
    • Labeled DNA signal indicated that DNA entered the bacterial cell.
  • Conclusion: DNA is the genetic material because it is injected into the infected cell, not the protein coat.

Griffith's Experiment Clarification

  • Common misconception explained about Griffith's experiment:
    • Not chromosomes that were denatured; instead, it was enzymes necessary for transcription.
    • Chromosomes are condensed and protected from heat.
  • Key outcome: Chromosome transfer from heat-killed pathogenic bacteria to non-pathogenic bacteria allowed for pathogen transcription and cell death.

Hershey and Chase Experiment Significance

  • Question posed: How did Hershey and Chase confirm DNA as hereditary material?
    • Concept of 'pellet' explained:
    • Pellet: Cell components at the bottom after centrifugation.
    • Supernatant: Liquid containing the viral protein coat.
    • Viruses labeled differently to track components.
    • E. coli used for experiments to demonstrate that only DNA entered the cells.

Avery Experiment Connection

  • Avery Experiment linked as a precursor to Hershey and Chase, suggesting DNA was the genetic agent.
  • Avery used various enzymes (nucleases, proteases) to degrade components and determine which entered the cell.

Class Participation Questions

  • Format of participation questions explained:
    • 50% participation points for submitting an answer.
    • Additional 50% correctness points for a correct answer.
  • Reinforcement of learning through pre-class assignments discussed.

DNA as Genetic Material

  • Definition of DNA as the code of life established.
  • Fundamental questions:
    • How does DNA structure enable its function?
    • How is DNA copied during cell division to pass genetic information?

Structure of DNA

  • Rosalind Franklin's X-ray diffraction contributed to understanding of DNA structure.
  • Proposed double helix model by Watson and Crick:
    • Comparison of double helix to a zipper instead of a ladder for dynamic structure.
  • Importance of complementary base pairing in DNA:
    • Sugar: Deoxyribose with five' and three' ends.
    • Bases: Two purines (adenine, guanine) and two pyrimidines (cytosine, thymine).
    • Base pairing:
    • Adenine pairs with Thymine (2 hydrogen bonds).
    • Guanine pairs with Cytosine (3 hydrogen bonds).

Importance of Base Pairing

  • Purines (2 rings) pair with pyrimidines (1 ring) for uniform diameter of the DNA helix.
  • Energetic stability explained:
    • C-G pair more stable (3 H-bonds) than A-T pair (2 H-bonds).

DNA Directionality and Stability

  • DNA has anti-parallel strands:
    • One strand runs 5' to 3', the adjacent strand runs 3' to 5'.
  • Molecular locking and zipping described as contributing to immense stability of DNA structure.
  • Importance of reversible unzipping for DNA replication.

Chargaff's Rules

  • Chargaff’s rules specify:
    • Amount of A equals that of T.
    • Amount of G equals that of C.

Application of Chargaff's Rule Example

  • Hypothetical scenario given:
    • If a species DNA contains 27% A, the percentage of C is derived as follows:
      • A = T = 27%.
      • A + T + G + C = 100% ⟹ 54% (A+T) means G + C = 46%.
      • Since G = C, C = 23%.

RNA Overview

  • Contrast between DNA and RNA highlighted:
    • DNA: Deoxyribonucleic acid.
    • RNA: Ribonucleic acid.
    • RNA typically single-stranded, less stable than DNA due to 2 hydroxyl groups.

DNA Replication Models

  • Three proposed models of DNA replication:
    • Conservative: Original helix conserved, a new helix created.
    • Semi-conservative: Original strand preserved, complementary strand synthesized.
    • Dispersive: Original strands mixed in new helices.

Semi-Conservative Replication Discovery

  • Meselson-Stahl experiment from 1958 demonstrated semi-conservative replication:
    • Growth of E. Coli in nitrogen-15 labeled medium.
    • Transition to nitrogen-14 medium for subsequent generations.

DNA Bands Visualization in Gel Electrophoresis

  • Heavy (N-15) and light (N-14) DNA strands visualized on a gel.
  • After one replication cycle seen: hybrid strands (N-15/N-14) and subsequent results explained for more cycles.

Summary of Outcomes from Meselson-Stahl Experiment

  • After switching from nitrogen-15 to nitrogen-14, the hybrid strand indicates semi-conservative replication structure:
    • Results show continued production of species containing nitrogen-14.

Key Elements of DNA Replication

  • Enzymatic action required for replication, specifically DNA polymerase for copying and helicases for unzipping.
  • Leading and lagging strands in replication explained:
    • Leading strand synthesized continuously.
    • Lagging strand synthesized in segments.

Mitosis vs. Meiosis

  • Mitosis: Cloning somatic cells, producing two identical daughter cells.
  • Meiosis: Reduction division to create four genetically unique haploid cells (sex cells).
  • Distinction between DNA replication cycles for mitosis (1 cycle) and meiosis (1 cycle of replication, 2 cycles of division).

Final Questions on Division Outcomes

  • Unique daughter cells determined from 3 homologous chromosome pairs:
    • Mitosis: 2 identical daughter cells.
    • Meiosis: 4 genetically different daughter cells due to genetic shuffling during division.