Chapter 10: DNA - The Chemical Nature of the Gene

DNA as Hereditary Material

  • Traditional Central Dogma:
    • DNA → RNA → Protein

Key Characteristics of Genetic Material

  • Complex Information: Genetic material must contain detailed information necessary for the development and functioning of organisms.
  • Faithful Replication: Must replicate accurately to ensure genetic continuity across generations.
  • Phenotype Encoding: Must encode traits that can be expressed as phenotypes.
  • Capacity to Vary: Must allow variations to enable evolution and adaptation.

Importance of DNA Structure Discovery

  • Understanding DNA's structure was crucial for grasping how genetic information is encoded and expressed.

Historical Contributions to DNA Understanding

  • 1833: Brown describes the nucleus of the cell.
  • 1869: Miescher discovers nuclein (DNA) in white blood cell nuclei.
  • 1900-1952: Key figures (Mendel, Levene, Griffith, Hershey, Chase, Watson, and Crick) contribute to the understanding of DNA's role in heredity and its structure.
  • Chargaff's Rules:
    • Adenine (A) = Thymine (T)
    • Guanine (G) = Cytosine (C)

Structure of DNA

  • Primary Structure: Sequence of deoxyribonucleotides (nucleotides containing deoxyribose).

    • Each nucleotide consists of:
    • Sugar (deoxyribose)
    • Phosphate group
    • Nitrogenous base (A, T, G, or C)

  • Secondary Structure: DNA forms a double helix, stabilized by hydrogen bonding between complementary bases.

    • Phosphodiester Bonds: Connect the sugar-phosphate backbone.
    • Antiparallel Strands: Two strands run in opposite directions (5' to 3' and 3' to 5').

  • Nucleotide Types and Symbols:

    • A: Deoxyadenosine (dAMP)
    • G: Deoxyguanosine (dGMP)
    • T: Deoxythymidine (dTMP)
    • C: Deoxycytidine (dCMP)

DNA Length Measurements

  • Measured in base pairs (bp), where 1 kilobase (kb) = 1000 bp.

Chromatin Structure

  • Eukaryotic DNA: Closely associated with proteins (histones) forming chromatin.
  • Types of Chromatin:
    • Euchromatin: Less condensed, associated with active transcription.
    • Heterochromatin: Highly condensed, less transcriptional activity,
    • Locations include centromeres and telomeres.
  • Histones: Proteins that organize DNA into nucleosomes.
    • Positive charges attract negatively charged DNA, allowing tight packaging.

DNA Replication and Genetic Information Flow

  • Replication: Semiconservative process within the nucleus.
  • Transcription & Translation: Processes that convert DNA into proteins.

Centromere & Telomere Structure

  • Centromeres: Constricted chromosome regions where sister chromatids connect; important for correct chromosome segregation.
  • Telomeres: Caps at chromosome ends protecting them from degradation, composed of repetitive nucleotide sequences like 5'−TTAGGG−3′.

The Endosymbiotic Theory

  • Proposes that chloroplasts and mitochondria originated from free-living prokaryotes that formed symbiotic relationships with a host cell.
  • This relationship led to gene transfer and eventual incorporation into the eukaryotic genome.

Mitochondria and Mitochondrial DNA (mtDNA)

  • Mitochondria are essential organelles for ATP production, with a structure resembling bacterial cells.
  • mtDNA is inherited maternally and is crucial for energy metabolism in cells.