Microbial Genetics S23 (1)

Chapter 8: Microbial Genetics

Structure and Function of Genetic Material

• Genetics: The study of genes, genetic variation, and heredity in living organisms.

• Genome: The complete set of genes or genetic material present in a cell or organism.

• Central Dogma: The framework in molecular biology that describes the flow of genetic information from DNA to RNA to protein.

Timeline of Key Discoveries in Genetics

• 1865: Mendel documents patterns of heredity in pea plants.

• 1902: Sutton and Boveri propose the chromosome theory of heredity.

• 1927: Muller shows that X-rays induce mutations.

• 1928: Griffith's experiments indicate transformation of non-pathogenic bacteria into pathogenic strains using Streptococcus pneumoniae.

• 1941: Beadle and Tatum describe the "one gene-one enzyme" hypothesis.

• 1944: Avery, McLeod, and McCarty identify DNA as the "transforming principle" responsible for heredity.

• 1952: Hershey and Chase use radioactive labeling to prove DNA is the genetic material.

• 1953: Watson and Crick propose the double helix structure of DNA based on X-ray crystallography images from Rosalind Franklin.

• 1961: Jacob and Monod propose the existence of mRNA.

Historical Experiments

• Griffith's Transformation Experiments (1928)

  • Demonstrated that R (non-pathogenic) cells could be transformed into S (pathogenic) cells when exposed to S cell DNA.

  • Found evidence of DNA as the transforming principle.

• Avery et al. (1944)

  • Conducted experiments to determine that the transforming factor was indeed DNA using a mix of R cells and DNA from S cells.

    • Key Steps:

      1. Mix R cells with DNA extract from S cells.

      2. Allow DNA to be taken up by R cells.

      3. Add antibodies to aggregate untransformed R cells.

      4. Centrifuge to isolate transformed S cells.

      5. Conduct tests using DNase, RNase, and protease to show that transformation occurs only if DNA is present.

• Hershey and Chase (1952)

  • Used radioactivity to demonstrate that DNA is the genetic material by labeling viral DNA and proteins in bacteriophages and tracking the labeled components in infected bacteria.

Structure of DNA

• Watson and Crick (1953) proposed the double helix model of DNA.

  • Contributions from Rosalind Franklin’s X-ray crystallography were pivotal in elucidating the structure.

  • DNA consists of a sugar-phosphate backbone and four nitrogenous bases: adenine, thymine, cytosine, and guanine (A, T, C, G).

  • Base-pairing rules: A pairs with T, C pairs with G.

DNA and RNA Differences

• DNA: Double-stranded, contains thymine (T).

• RNA: Single-stranded, contains uracil (U) instead of thymine.

• Both nucleic acids have a sugar-phosphate backbone and are key players in the storage and transmission of genetic information.

DNA Replication

• Key Points:

  • DNA replication is initiated at the origin of replication.

  • DNA polymerase synthesizes DNA in the 5' to 3' direction.

  • Leading strand is synthesized continuously, while the lagging strand is synthesized in Okazaki fragments.

  • Major Steps:

    • Unwinding of the double helix.

    • Synthesis of RNA primers followed by DNA polymerization.

    • Joining of Okazaki fragments by DNA ligase.

Polymerase Chain Reaction (PCR)

• Function: Amplifies specific DNA sequences.

• Applications: Used in cloning, sequencing, diagnosing genetic diseases, and detecting pathogens.

• Process:

  1. Denaturation: Heating DNA to separate strands.

  2. Annealing: Cooling to allow primers to bind.

  3. Extension: DNA polymerase synthesizes new strands.

  4. Repeat cycles to amplify the target DNA.

Gene Regulation

• Operons: Groups of genes regulated together. Include promoter, structural genes, operator, and repressor.

• Repression: Mechanism by which an operon is inactivated when a repressor protein binds to the operator.

• Induction: Activation of an operon in response to the presence of a substrate (e.g., lactose).

  • Lactose Operon: Involves enzymatic pathways that convert lactose for energy.

Mutations

• Definition: Any random change in the genetic material.

• Types of Mutations:

  • Base substitution (point mutation): Alteration of a single base in DNA.

  • Effects: Can be neutral, beneficial, or harmful. Mutagens are agents that increase mutation rates.

Genetic Recombination

• Process through which genetic material is rearranged to produce genetic diversity. Often occurs during meiosis and can involve transforming elements in bacteria.