DNA Structure and Replication

Characteristics of Hereditary Material

Hereditary material must possess several essential properties to fulfill its role in genetics and cellular function.

• Localization: Hereditary material is localized to the nucleus and is a component of chromosomes.

• Stability: It is present in a stable form within cells, ensuring consistent transmission across generations.

• Complexity: The material is sufficiently complex to encode information necessary for the structure, function, development, and reproduction of an organism.

• Replication: It can accurately replicate itself so that daughter cells inherit the same genetic information as parent cells.

• Mutability: It is mutable, undergoing a low rate of mutations that introduce genetic variation and serve as a foundation for evolutionary change.

Early Evidence That DNA Is the Hereditary Material

Historical experiments and observations laid the foundation for identifying DNA as the hereditary material.

• Edmund Wilson (1895): Suggested DNA might be the hereditary material after observing equal chromosome contribution from sperm and eggs during reproduction.

• Miescher's Substance: Connection made between Miescher's discovery of nuclein and chromatin in chromosomes.

• Mendel's Principles (1900): Rediscovery of Mendel's hereditary principles reinforced the chromosomal basis of inheritance.

• Sutton and Boveri (1903): Described parallels between chromosome partitioning into gametes and inheritance of genes.

• DNA Localization (1923): DNA was localized to chromosomes, making it a candidate for hereditary material.

Griffith's Transformation Factor and the Discovery of DNA's Role in Heredity

Frederick Griffith's experiments with Pneumococcus bacteria provided key evidence for a "transformation factor" responsible for heredity.

• Strain Identification: Griffith identified two strains: S (smooth, virulent) and R (rough, non-virulent).

• Antigenic Types: These strains occur in four antigenic types (I, II, III, IV) that cannot be altered by mutation alone.

• Mutation and Transformation: A single gene mutation can convert an S strain to an R strain of the same antigenic type, but not to a different type.

• Transformation Experiment: Griffith showed that non-virulent R bacteria could be transformed into virulent S bacteria by exposure to heat-killed S bacteria, indicating the presence of a "transforming factor."

Example: Injecting mice with live R strain and heat-killed S strain resulted in the recovery of live S bacteria, demonstrating transformation.

Griffith's Experiment: The Process of Transformation

Griffith's experiment provided indirect evidence that DNA is the hereditary molecule.

• Transformation Factor: Griffith proposed that the transformation factor carried hereditary information, though he could not identify the molecule.

• Bacterial Transformation: The process described by Griffith is now known as transformation, a method by which bacteria transfer DNA.

• Further Testing: Additional experiments were needed to identify the transforming material as DNA.

Key Terms and Definitions

• Chromosome: A structure within cells that contains DNA and protein, serving as the vehicle for genetic information.

• Mutation: A change in the DNA sequence that can introduce genetic variation.

• Transformation: The genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous genetic material.

• Antigenic Type: A classification based on the specific antigens present on the surface of bacteria.

Summary Table: Griffith's Experiment Outcomes

Additional info: Griffith's work set the stage for later experiments by Avery, MacLeod, and McCarty, which directly identified DNA as the transforming factor, and for the Hershey-Chase experiment, which confirmed DNA as the hereditary molecule in viruses.