10.1 Using Microbiology to Discover the Secrets of Life

Discovery and Structure of DNA

Understanding how DNA was discovered and characterized is one of the great stories in biology. It took nearly a century of experiments, many of them using microorganisms, to move from isolating a mysterious substance in cell nuclei to building a complete model of the double helix. This section traces that path and highlights why bacteria and bacteriophages were so central to cracking the code.

Steps in Nucleic Acid Discovery

The identification of DNA as a molecule happened gradually, with each researcher adding a critical piece.

• Friedrich Miescher (1869) isolated an acidic, phosphorus-rich substance from white blood cell nuclei and called it "nuclein." This was the first time anyone had extracted what we now call nucleic acid.
• Albrecht Kossel (1880s) identified the four nitrogenous bases in nucleic acids: adenine (A), guanine (G), cytosine (C), and thymine (T). These turned out to be the informational building blocks of DNA.
• Phoebus Levene (1919) showed that nucleic acids are built from repeating units called nucleotides, each made of three parts: a sugar, a nitrogenous base, and a phosphate group.
• Scientists eventually distinguished two types of nucleic acid: deoxyribonucleic acid (DNA), which contains deoxyribose sugar, and ribonucleic acid (RNA), which contains ribose sugar. The missing oxygen on carbon 2' of deoxyribose is what gives DNA its name.

Experiments on DNA Structure

By the 1940s and 1950s, researchers were zeroing in on DNA's physical shape and its role as genetic material.

• Erwin Chargaff (1940s) analyzed the base composition of DNA from multiple species and found a consistent pattern: the amount of adenine always equaled the amount of thymine, and guanine always equaled cytosine. These relationships, known as Chargaff's rules, were a major clue that bases pair in a specific way.
• Rosalind Franklin and Maurice Wilkins (early 1950s) used X-ray crystallography to image DNA fibers. Franklin's famous "Photo 51" revealed that DNA has a helical shape with regular, repeating dimensions, providing physical evidence that was essential for building an accurate model.
• Alfred Hershey and Martha Chase (1952) used bacteriophages (viruses that infect bacteria) to prove DNA is the genetic material, not protein. They labeled phage DNA with radioactive $^{32}P$ and phage protein with radioactive $^{35}S$, then tracked which label entered the bacterial cell during infection. Only the $^{32}P$-labeled DNA entered, confirming DNA carries genetic information.
• James Watson and Francis Crick (1953) synthesized all of this evidence into the double helix model. Two antiparallel strands wind around each other, held together by complementary base pairing (A with T, G with C). This structure immediately suggested a mechanism for replication: each strand serves as a template for a new complementary strand.

Microorganisms in Gene Biochemistry

Bacteria and bacteriophages were ideal for genetic experiments because they grow fast, have small genomes, and are easy to manipulate. Several landmark discoveries came directly from microbial systems.

• Frederick Griffith (1928) demonstrated bacterial transformation using Streptococcus pneumoniae. He showed that heat-killed virulent (smooth) bacteria could transfer a "transforming principle" to live nonvirulent (rough) bacteria, converting them into virulent forms. Something in the dead cells was carrying heritable information.
• Joshua Lederberg and Edward Tatum (1946) discovered bacterial conjugation in E. coli, where one bacterium transfers genetic material to another through direct cell-to-cell contact via a structure called a pilus. The ability to transfer DNA this way depends on a fertility plasmid called the F factor.
• Bacteriophages became powerful model organisms for studying gene structure, replication, and expression. Their genomes are small and well-defined, and their infection cycle can be tracked in real time.
• François Jacob and Jacques Monod (1961) discovered the lac operon in E. coli, the first well-characterized system of gene regulation. When lactose is present, it triggers expression of the genes needed to metabolize it; when lactose is absent, a repressor protein shuts those genes off. This on/off switch concept became foundational for understanding gene regulation in all organisms.

Evidence for DNA's Genetic Role

Multiple independent experiments converged on the same conclusion: DNA is the molecule of heredity.

• Avery, MacLeod, and McCarty (1944) followed up on Griffith's transformation experiment by systematically destroying different macromolecules (proteins, lipids, RNA) in the heat-killed extract. Only when they destroyed DNA did transformation stop, directly implicating DNA as the transforming principle.
• Hershey and Chase (1952) confirmed this with their phage experiment (described above), showing that DNA alone enters the host cell and directs the production of new phages.
• Meselson and Stahl (1958) proved that DNA replication is semiconservative. They grew E. coli in medium containing heavy nitrogen ($^{15}N$) so all DNA was "heavy," then switched the bacteria to light nitrogen ($^{14}N$) medium. After one round of replication, all DNA had an intermediate density, exactly as predicted if each new double helix contains one old strand and one new strand. They separated the DNA by density using cesium chloride gradient centrifugation.
• Francis Crick (1958) articulated the central dogma of molecular biology: genetic information flows from DNA → RNA → protein. DNA stores the instructions, RNA carries the message, and ribosomes translate that message into functional proteins. This framework still guides molecular biology today.

Advancements in DNA Technology

The discoveries above opened the door to powerful technologies that define modern microbiology and genetics.

• Polymerase chain reaction (PCR), developed by Kary Mullis in 1983, allows researchers to amplify a specific DNA sequence millions of times from a tiny starting sample. PCR relies on repeated cycles of heating (to separate strands), cooling (to anneal primers), and extension (by a heat-stable DNA polymerase). It's now used in diagnostics, forensics, and research.
• Genome sequencing techniques have made it possible to read the entire genetic code of organisms, from bacteria to humans. Comparing genomes across species reveals evolutionary relationships and helps identify genes linked to disease.
• Together, these tools have advanced the study of molecular evolution, showing how mutations accumulate over generations and drive the diversification of species. Microbial genomes, because they replicate so quickly, remain some of the best systems for studying these processes in real time.