DNA Structure and Replication, Gene Mutation and DNA Repair

  • Historical Experiments:

    • Griffith's Experiment: Showed transformation from nonvirulent to virulent bacteria via heat-killed virulent strain.

    • Avery, MacLeod, and McCarty's Findings: Identified DNA as the transforming principle.

    • Hershey and Chase: Demonstrated DNA (not protein) entered bacterial cells, causing genetic changes.

  • Contributions to DNA Structure:

    • Rosalind Franklin: X-ray diffraction images suggested a helical DNA structure.

    • Maurice Wilkins: Shared Franklin's data with Watson and Crick.

    • Watson and Crick: Developed the double helix model using Franklin's data and Chargaff's rules.

  • Nucleotide Structure:

    • Composed of:

    • Phosphate group

    • Deoxyribose sugar

    • Nitrogenous base (A, T, C, or G)

    • Nucleotides linked by phosphodiester bonds form a DNA backbone.

  • Antiparallel Nature of DNA:

    • One strand runs 5’ to 3’, the other 3’ to 5’.

    • Proper directionality critical for replication and transcription.

  • Semi-Conservative Replication:

    • Each new DNA has one original strand and one new strand.

    • DNA synthesized from 5’ to 3’ direction.

  • Key Players in Replication:

    • DNA Polymerase: Enzyme adding nucleotides to growing strand.

    • Helicase: Unwinds DNA double helix.

    • Primase: Creates RNA primers for DNA polymerase.

    • Ligase: Joins Okazaki fragments on lagging strand.

  • Base Pairing Rules:

    • Adenine (A) pairs with Thymine (T) (2 hydrogen bonds).

    • Cytosine (C) pairs with Guanine (G) (3 hydrogen bonds).

  • Gene Mutation and Repair:

    • Mutations: Changes in DNA sequence affecting protein function.

    • Types of Mutations:

    • Somatic: Not passed on.

    • Germline: Can be inherited.

  • Effects of Mutations on Fitness:

    • Categories:

    • Lethal: Causes death.

    • Harmful: Reduces fitness.

    • Beneficial: Increases fitness.

    • Neutral: No effect.

  • Consequences of DNA Damage:

    • Can lead to cell malfunction, cancer, genetic disorders.

    • Cells have repair mechanisms to maintain genetic integrity.

    • Apoptosis can prevent passing on damaged DNA.

  • Spontaneous and Induced Mutations:

    • Spontaneous: Occur naturally via replication errors.

    • Induced: Caused by chemicals, radiation, or viruses.