14.1 Historical Basis of Modern Understanding

Discovery of DNA as the Hereditary Material

Before Watson and Crick built their famous double helix model, scientists spent decades figuring out a more basic question: what molecule actually carries genetic information? Most researchers assumed it was proteins, since proteins are complex and diverse. A series of clever experiments proved them wrong.

Key DNA Heredity Experiments

Griffith's Experiment (1928)

Frederick Griffith worked with two strains of Streptococcus pneumoniae: a smooth (S) strain that caused disease and a rough (R) strain that was harmless. When he heat-killed the S strain and mixed it with live R bacteria, the R bacteria became virulent. Something from the dead S cells had "transformed" the living R cells, giving them new genetic traits. Griffith called this unknown substance the transforming principle.

Avery, MacLeod, and McCarty (1944)

These researchers set out to identify what the transforming principle actually was. They purified the components of heat-killed S cells and systematically destroyed one molecule at a time using specific enzymes:

• Enzymes that degraded proteins → transformation still occurred
• Enzymes that degraded RNA → transformation still occurred
• Enzymes that degraded DNA → transformation was destroyed

This provided strong evidence that DNA, not protein, carried genetic information. Many scientists remained skeptical, though, because the experiment couldn't completely rule out trace protein contamination.

Hershey and Chase (1952)

Alfred Hershey and Martha Chase used bacteriophages (viruses that infect bacteria) to settle the debate. Their approach relied on a key chemical difference: DNA contains phosphorus but not sulfur, while proteins contain sulfur but not phosphorus.

1. They grew one batch of phages with radioactive phosphorus ($^{32}P$), which labeled the DNA.
2. They grew another batch with radioactive sulfur ($^{35}S$), which labeled the protein coat.
3. They let each batch infect bacteria, then used a blender to separate the phages from the bacterial cells.
4. After centrifuging, they checked where the radioactivity ended up.

The result: $^{32}P$ (DNA) was found inside the bacterial cells, while $^{35}S$ (protein) stayed outside in the supernatant. DNA entered the cell and directed viral replication. This confirmed that DNA is the genetic material.

Chargaff's Rules and DNA Structure

Erwin Chargaff analyzed the base composition of DNA from many different species and found a consistent pattern now known as Chargaff's rules:

1. The amount of adenine (A) always equals the amount of thymine (T).
2. The amount of guanine (G) always equals the amount of cytosine (C).
3. The ratio of $(A+T)$ to $(G+C)$ varies between species but stays constant within a species.

These ratios strongly suggested that A pairs specifically with T, and G pairs specifically with C. This insight was critical for Watson and Crick when they built their double helix model in 1953. Complementary base pairing also explains how DNA can be copied faithfully during replication: each strand serves as a template for building its partner.

DNA Transformation: Techniques and Applications

DNA transformation is the process by which a cell takes up foreign DNA from its environment and incorporates it into its own genome. This happens naturally in some bacteria, allowing them to acquire new traits like antibiotic resistance. In the lab, scientists can induce transformation using several methods:

1. Heat shock: Cells are briefly exposed to high temperature (around 42°C for bacteria), which makes their membranes temporarily more permeable to DNA.
2. Electroporation: A short electric pulse (1–2 kV/cm) creates temporary pores in the cell membrane, allowing DNA to pass through.
3. Chemical transformation: Treating cells with calcium chloride makes them "competent," meaning more receptive to taking up DNA.

These techniques have broad applications in modern biology:

• Genetic engineering: Introducing foreign genes into organisms to create desired traits or study gene function (e.g., pest-resistant crops, fluorescent marker proteins)
• Recombinant protein production: Inserting human genes into bacteria so they produce medically useful proteins like insulin
• Molecular cloning: Amplifying specific DNA fragments by inserting them into plasmid vectors and growing them in bacterial cells
• Gene therapy: Introducing functional genes into a patient's cells to replace defective ones, as in treatments for cystic fibrosis

Foundations of Modern Molecular Biology

The discovery that DNA is the genetic material connects to several broader concepts you'll encounter throughout this course.

The Central Dogma describes the flow of genetic information in cells: DNA is transcribed into RNA, which is then translated into proteins. This principle, proposed by Francis Crick, provides the framework for understanding how genes actually do things in a cell.

Gene expression is the process by which information encoded in a gene gets used to build a functional product, usually a protein. Cells regulate this process at multiple levels, including transcription, RNA processing, and translation. Not every gene is active in every cell at every moment.

The work covered in this section also connects to genetics more broadly. Gregor Mendel's earlier experiments with pea plants established the rules of inheritance, but he had no idea what the physical basis of heredity was. The experiments of Griffith, Avery, and Hershey-Chase filled in that gap by identifying DNA as the molecule responsible for transmitting traits from parents to offspring. Understanding DNA's structure and function also provides a molecular basis for evolution, since changes in DNA sequences (mutations) are the raw material that natural selection acts upon.