Discovery of DNA as the Genetic Material

Griffith's Experiment and the Concept of Transformation

In 1928, Frederick Griffith's experiments with bacteria revealed that some unknown genetic "factor" controls the traits of organisms. His work introduced the concept of transformation, where external DNA is taken up by a cell, resulting in a genotypic and phenotypic change.

• Transformation: The process by which genetic material from one organism is taken up by another, leading to a change in phenotype.

• Example: Griffith's experiment showed that non-pathogenic bacteria could become pathogenic when mixed with heat-killed pathogenic bacteria, indicating the transfer of genetic material.

Avery, McCarty, and MacLeod: Identifying the Transforming Substance

Later, Oswald Avery, Maclyn McCarty, and Colin MacLeod identified DNA as the transforming substance, providing strong evidence that DNA is the genetic material.

• Many scientists were initially skeptical, considering proteins as possible candidates for genetic material.

Experimental Evidence for DNA as Genetic Material

The Hershey-Chase Experiment

In 1952, Alfred Hershey and Martha Chase used bacteriophages (viruses that infect bacteria) to confirm that DNA, not protein, is the genetic material.

• Bacteriophage: A virus that infects bacteria, consisting of a protein coat and nucleic acid (DNA or RNA).

• Key Finding: Only the DNA from the phage entered the bacterial cell and directed the production of new viruses, not the protein coat.

Experimental Design:

• Phages were labeled with radioactive sulfur (to label protein) and radioactive phosphorus (to label DNA).

• After infection, only radioactive phosphorus was found inside the bacteria, confirming DNA as the genetic material.

Practice Questions (Sample)

• Which molecule entered the bacterial cell in the Hershey-Chase experiment? DNA

• Which element is found in DNA but not in protein? Phosphorus

DNA Composition and Chargaff's Rules

Chargaff's Discoveries

In the early 1950s, Erwin Chargaff made two key discoveries:

• DNA base composition varies between species.

• In each species, the percentage of adenine (A) is roughly equal to thymine (T), and the percentage of guanine (G) is roughly equal to cytosine (C).

Summary: The percentages of A and T are similar, as are G and C, supporting the base-pairing rules in DNA.

Structure of DNA

Rosalind Franklin, Watson, and Crick

Rosalind Franklin used X-ray diffraction to capture images of DNA, leading to the discovery of its helical structure. James Watson and Francis Crick used this data to propose the double helix model of DNA in 1953.

• DNA Double Helix: Two strands of nucleotides wound around each other, with sugar-phosphate backbones on the outside and nitrogenous bases paired in the center.

• Base Pairing: Adenine (A) pairs with Thymine (T) via two hydrogen bonds; Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

• Antiparallel Strands: The two DNA strands run in opposite directions (5' to 3' and 3' to 5').

Detailed DNA Structure

DNA is a polymer of nucleotides, each consisting of a phosphate group, a deoxyribose sugar, and a nitrogenous base.

• At the 5' end of each strand is a free phosphate group.

• At the 3' end of each strand is a free hydroxyl group (-OH).

• Nucleotides are linked by phosphodiester bonds between the phosphate group of one nucleotide and the 3' OH of the next.

Key Equations:

• Base pairing: $$ \%A = \%T, \quad \%G = \%C $$

• Phosphodiester bond formation: $$ \text{Nucleotide}_{n} + \text{dNTP} \rightarrow \text{Nucleotide}_{n+1} + PP_i $$

Significance of DNA Structure

• The complementary base pairing allows for accurate DNA replication.

• The antiparallel arrangement is essential for the function of enzymes involved in DNA replication and transcription.

Summary Table: Key Discoveries in DNA Research

Practice and Application

• Be able to explain the significance of each experiment in establishing DNA as the genetic material.

• Apply Chargaff's rules to calculate base composition in DNA samples.

• Describe the structure of DNA, including the directionality and base-pairing rules.

Additional info: This guide summarizes foundational experiments and concepts in molecular genetics, focusing on the chemical and structural basis of DNA as the hereditary material. These topics are essential for understanding the molecular mechanisms of inheritance in biology and biochemistry.