Molecular Biology of the Gene - Chapter 12

Chapter 12: Molecular Biology of the Gene - Part 1

• DNA is the genetic material

  • Morgan's group showed genes reside on chromosomes, with DNA and protein being candidates for the genetic material
  • DNA's hereditary role was first discovered through studying bacteria and infecting viruses.
  • Griffith observed transformation (change in genotype and phenotype due to foreign DNA assimilation).
  • Avery, McCarty, and MacLeod identified the transforming substance as DNA

• Evidence that DNA can transform bacteria

  • Frederick Griffith's 1928 research with bacteria strains (pathogenic and harmless) sparked the investigation.
  • When heat-killed pathogenic bacteria were mixed with living harmless ones, some living cells turned pathogenic (transformation).

• Evidence that viral DNA can program cells

  • Bacteriophages (phages) are viruses that infect bacteria.
  • Phages have DNA (occasionally RNA) encased in a protein coat.
  • Phages have been key tools in molecular genetics research.
  • Hershey and Chase experiment:
    • Alfred Hershey and Martha Chase (1952) demonstrated that viral DNA (and not protein) enters the bacteria during infection, establishing that DNA is the genetic material, not protein.
    • DNA was tracked using radioactive isotopes.
  • The experiment involved phage T2.

• Additional evidence that DNA is the genetic material.

  • DNA is a polymer of nucleotides (nitrogenous base, sugar, and phosphate group).
    • Nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C)
  • Erwin Chargaff's (1950) findings revealed species-specific DNA compositions.
  • This diversity further supported DNA as the genetic material.

Chargaff's Rules

• Base composition of DNA varies among species.
• In any species, the number of A and T bases is equal, and the number of G and C bases is equal.

Structure of DNA

• DNA's structure was established using X-ray crystallography data, which provided crucial information about its helical structure.
  • Franklin's X-ray diffraction images were critical in determining the DNA structure.
  • Watson and Crick built a model of the double helix based on existing information, including Franklin's data and the complementary base pairing rule.

DNA Replication and Repair

• DNA replication ensures offspring resemble parents

  • Replication prior to mitosis ensures the faithful transmission of genetic information from a parent cell to daughter cells.
  • Watson and Crick recognized that specific base pairing suggests a copying mechanism for genetic material, thus initiating studies on DNA replication.
  • DNA replication is guided by the principle of complementarity.

• The Basic Principle - Base Pairing to a Template Strand

  • DNA strands are complementary, acting as templates for new strand construction.
  • This results in two exact copies of the original DNA molecule.

• Watson and Crick's Semiconservative Model

  • When DNA replicates, each daughter molecule has one old strand and one new strand

• Experiments by Meselson and Stahl

  • Experiments with isotopes confirmed the semiconservative model,

• DNA Replication: A Closer Look

  • DNA replication is a complex process involving multiple enzymes, remarkable for both speed and accuracy.

• Bacterial DNA Replication Processes

  • Starts at specific origins
  • The replication fork is a region where the parental strands unwind.
    • Helicases untwist the double helix.
    • Single-strand binding proteins stabilize single-stranded DNA.
    • Topoisomerase relieves strain in the double helix by reducing it.

• Synthesizing a new DNA strand.

  • DNA polymerases require primers (short RNA segments).
  • Primase synthesizes RNA primers.
  • Polymerases add nucleotides to the 3' end.

• Antiparallel Elongation

  • DNA polymerases add nucleotides only to the growing strand's free 3' end.

• Leading and Lagging Strands

  • Leading strand synthesized continuously; the lagging strand is synthesized discontinuously as Okazaki fragments.
  • DNA ligase joins the fragments.

• DNA Replication Complex

  • Enzymes work together in a complex.
  • It may be stationary during replication.

• Proofreading and Repairing DNA

  • DNA polymerases proofread newly made DNA.
  • Repair enzymes (e.g., in mismatch repair) correct errors that avoid proofreading.

• DNA Damage and Repair

  • DNA can be damaged by chemical or physical agents (e.g., cigarette smoke, X-rays).
  • Nucleotide excision repair (NER) involves removing and replacing damaged stretches of DNA.

• Replicating the Ends of DNA Molecules

  • Linear DNA chromosomes have telomeres (special nucleotide sequences at the ends) to prevent shortening during each replication.

• Evolutionary Significance of Altered DNA Nucleotides

  • Mutations (changes in DNA) are sources of genetic variation.
  • Natural selection then acts upon these variations contributing to the emergence of new species.

• Eukaryotic Chromosome Telomeres

  • Telomeres protect genes from erosion.