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9.1 Identification of DNA as the Genetic Material

Learning Outcomes

  • Understand the four criteria necessary for genetic material:

    • Information: Provides a blueprint for inherited traits.

    • Transmission: Must be passed from parents to offspring.

    • Replication: Must be copied for cellular division.

    • Variation: Must account for phenotypic variability within a species.

Historical Context

  • In the late 19th century, geneticists like Mendel laid foundational data supporting these criteria.

  • August Weismann and Carl Nägeli proposed a chemical that transmits traits in living cells.

  • The chromosome theory of inheritance was proposed, identifying chromosomes as carriers of genetic material, containing both DNA and proteins.

Griffith's Experiments on Transformation

  • Background: Study on Streptococcus pneumoniae, distinguishing between smooth (type S) and rough (type R) strains.

  • Findings:

    • Type S (smooth) bacteria cause fatal infections in mice due to their capsule, evading immune responses.

    • Type R (rough) bacteria do not cause disease.

    • Heat-killed type S bacteria do not affect mice.

    • Live type R mixed with heat-killed type S killed mice and yielded living type S bacteria, indicating transformation.

    • Termed the phenomenon 'transformation,' allowing R strain to acquire S strain traits.

Avery, MacLeod, and McCarty Experiment

  • Goal: Identify the substance causing transformation.

  • Isolated DNA from S strain and tested its ability to convert R to S strains, achieving positive results exclusively with DNA.

  • Used DNase, RNase, and protease to confirm DNA as the transforming principle, as only DNase treatment inhibited transformation.

Hershey and Chase Experiment

  • Examined the T2 bacteriophage, labeling DNA with radioactive phosphorus and proteins with sulfur.

  • Conclusion: Only phosphorus entered bacterial cells during infection, supporting that DNA serves as genetic material, not proteins.

  • Findings reinforced the understanding that DNA acts as the genetic material across various organisms, although some viruses use RNA.

9.6 Structure of the DNA Double Helix

Molecular Structure

  • Key Features:

    • Composed of two antiparallel strands of nucleotides (deoxyribonucleotides).

    • Complementary base pairing (A-T, G-C) forms the rungs of the helix and stabilizes the structure.

    • Base stacking provides further stabilization by excluding water molecules.

    • Double helix is right-handed, with each turn consisting of ~10 base pairs.

    • Major and minor grooves allow protein binding, affecting gene transcription and DNA functionality.

    • Forms different structural variations in certain conditions (B DNA, Z DNA).

Key Differences Between B DNA and Z DNA

  • B DNA: Right-handed, predominant form in cells.

  • Z DNA: Left-handed structure, less common, influences transcription and chromatin structure.

11.1 Structural Overview of DNA Replication

Learning Outcomes

  • Understand the process of DNA replication and identify key proteins involved.

DNA Replication Basics

  • Template strands separate to allow complementary new strands to be synthesized.

  • Directionality is crucial—DNA polymerases synthesize DNA only in a 5’ to 3’ direction, requiring RNA primers to initiate synthesis.

Meselson and Stahl Experiment: Semiconservative Replication

  • Investigated nitrogen-labeled DNA in E. coli.

  • Found after one round of replication, DNA was half-heavy: one original (heavy) strand, one newly formed (light) strand.

  • Supports the semiconservative model of DNA replication where each daughter DNA contains one old and one new strand.

11.3 Bacterial DNA Replication: Synthesis of New DNA Strands

Required Proteins

  • Key enzymes in replication at the replication fork include:

    • DNA helicase: Unwinds DNA strands.

    • DNA gyrase: Relieves supercoiling.

    • Single-strand binding proteins: Stabilize separated strands.

    • Primase: Synthesizes RNA primers.

    • DNA polymerase: Synthesizes new DNA strands.

    • DNA ligase: Joins Okazaki fragments on the lagging strand.

Distinction Between Leading and Lagging Strands

  • Leading Strand: Synthesized continuously toward the replication fork.

  • Lagging Strand: Synthesized in short segments (Okazaki fragments) away from the fork, requiring multiple RNA primers.

  • RNA primers removed by DNA polymerase I, followed by gap filling and sealing by DNA ligase.