BIO Ch.16 Molecular Basis of Inheritence

Introduction to DNA and Genetic Research in the 1950s

  • Overview of historical context (1950s; 75 years ago from present)

    • Focus on experiments leading to understanding hereditary information and DNA structure

The Chromosome Theory

  • Initial knowledge of chromosomes

    • Composition: nucleic acids and proteins

    • Uncertainty about which component is the genetic material

  • Lack of understanding about DNA physical structure and replication

Hershey-Chase Experiment

  • Key finding: DNA as the genetic material

    • Use of bacteriophage to infect bacteria

    • Bacteriophage: a virus that infects bacteria

    • Hypothesis: determining if proteins or DNA were the genetic components

    • Experimental method:

    • Labeling of DNA with radioactive phosphorus

    • Labeling of proteins with sulfur (which is not found in DNA)

  • Findings:

    • After infection, radioactive phosphorus found in bacteria, but no sulfur

    • Conclusion: DNA was injected by the virus, proving it was the genetic material

  • Importance of the experiment:

    • Established a foundation for future genetic research (genetic engineering, genomics)

Chargaff's Rules

  • Focus on nucleotide composition

    • DNA composition varies by species (e.g., human vs. E. coli adenine ratios)

  • Chargaff's conclusions:

    • Ratio of adenine (A) to thymine (T) equates

    • Ratio of guanine (G) to cytosine (C) equates

  • Implications:

    • Supports base pairing and understanding genetic diversity through mutations

Discovery of DNA Structure

Key Figures

  • Rosalind Franklin and Maurice Wilkins

    • Photographic evidence (Photo 51) using X-ray diffraction

    • Key discoveries:

    • Orientation of sugar-phosphate backbone and nitrogenous bases

  • Watson and Crick

    • Integrated previous research to propose double-helix structure

    • Received Nobel Prize, while Franklin did not receive adequate credit

    • Ethical implications concerning recognition of scientific contributions

Structure of DNA

  • Basic components:

    • Four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)

    • Hydrophobic nature of bases vs. hydrophilic sugar-phosphate backbone

    • Directionality of DNA: understanding 5' and 3' ends crucial for replication

  • Base pairing rules:

    • A pairs with T (2 hydrogen bonds)

    • G pairs with C (3 hydrogen bonds)

  • Symmetry and spacing in DNA:

    • Regular distances:

    • 3.4 nm between nucleotide bases

    • Consistency in the structure attributed to purine-pyrimidine pairing

Models of DNA Replication

  • Three models considered prior to consensus:

    1. Conservative model: Parent strands separate, create two new strands while retaining original strands

    2. Dispersive model: Parent and new strands interspersed randomly

    3. Semi-conservative model: Each new DNA molecule comprised of one parent strand and one new strand

  • Validation of semi-conservative model through experiments (i.e., radio-labeled DNA)

Mechanism of DNA Replication

  • Total number of chromosomes and base pairs in humans: 46 chromosomes, 6 billion base pairs

  • Need for efficient and accurate replication

  • Segregation of roles of various proteins/enzyme:

    • Helicase: unwinds DNA strands

    • Single-stranded binding proteins: stabilize unwound DNA

    • Topoisomerase: alleviates tension in DNA strands to prevent breakage

  • Primase: synthesizes RNA primers necessary for DNA polymerase attachment

DNA Polymerase Action

  • Function of DNA Polymerase III:

    • Adds nucleotides in the 5' to 3' direction

    • Requires an initial RNA primer to begin synthesis

    • Distinction between bacterial (2 polymerases) and eukaryotic (11 polymerases) systems

  • Role of DNA Polymerase I and DNA ligase:

    • Polymerase I: replaces RNA primers with DNA

    • Ligase: joins Okazaki fragments on lagging strand

Errors and Repair Mechanisms

  • Initial error rate in replication: 1 in 100,000; significant due to high base pair count

  • Error correction mechanisms:

    • DNA polymerase proofreading ability reduces error rate to about 1 in 10 billion

    • Mismatch repair systems exist post-replication to ensure accuracy

Telomeres and Replication Issues

  • Challenges at chromosome ends:

    • Linear versus circular DNA replication in prokaryotes vs. eukaryotes

  • Function of telomeres: repetitive non-coding DNA protecting genetic information from degradation

  • Telomerase enzyme: maintains length of telomeres to preserve genetic data through successive replication cycles

Conclusion

  • Importance of experiments in the 1950s for modern genetic research

  • Ethical consideration in scientific credit and the implications of replication errors in evolution and stability of genetic information.

Drawing Exercise

  • Practice drawing replication fork and labeling key proteins, directionality, and strand types (leading and lagging).

  • Reinforce understanding of 5' and 3' ends, replication mechanisms, and enzyme functions in DNA processes.