Difficulty: Introductory to Intermediate | Prerequisites: basic understanding of cells, chromosomes, and the central dogma (DNA → RNA → protein).
By the mid-20th century, scientists knew that traits were inherited and that genes sat on chromosomes, but nobody knew what the genetic material was made of or how its physical structure could explain heredity. Determining whether DNA or protein carried genetic information was a decades-long question that required converging lines of evidence from microbiology, biochemistry, and structural chemistry. This topic covers the foundational experiments and discoveries that established DNA as the hereditary molecule and set the stage for Watson and Crick's structural breakthrough. If you are studying molecular biology, genetics, or biochemistry, everything that follows in the course builds on this material.
Scientists spent decades figuring out whether genes were made of DNA or protein. Key experiments by Griffith, Avery, and Hershey-Chase proved DNA was the genetic material. Meanwhile, Chargaff discovered that in any organism's DNA, the amount of adenine equals thymine and guanine equals cytosine, a clue that would later unlock the structure of the double helix.
Nucleotide
The monomer (building block) of DNA. Each nucleotide consists of a phosphate group, a deoxyribose sugar, and one of four nitrogenous bases. Think of it as one rung-and-rail unit of the DNA ladder.
Nitrogenous bases (A, T, G, C)
The four information-carrying components of DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). In simple terms, these are the "letters" of the genetic alphabet.
Chargaff's rules
The observation that in any organism's DNA, the percentage of adenine equals the percentage of thymine, and the percentage of guanine equals the percentage of cytosine (A = T, G = C). In simple terms, the bases always pair off in fixed ratios, which was a major clue that they physically pair together in the molecule.
Transformation (bacterial)
The process by which bacteria pick up and incorporate foreign DNA, thereby gaining new traits. This is how Griffith first showed that genetic information could be transferred between organisms.
Bacteriophage
A virus that infects bacteria by injecting its genetic material into the host cell. Hershey and Chase used bacteriophages to prove DNA, not protein, enters the cell during infection.
Polymer
A large molecule made of many repeating subunits (monomers) linked together. DNA is a polymer of nucleotides, just as a protein is a polymer of amino acids.
Central dogma
The flow of genetic information in cells: DNA is transcribed into RNA, and RNA is translated into protein. Think of it as the one-way route from the master blueprint (DNA) to the working machinery (protein).
Gregor Mendel (1860s) showed that traits are inherited as discrete units, passed predictably from parent to offspring.
Thomas Hunt Morgan (early 1900s) demonstrated that these units of heredity (genes) are located on chromosomes.
Chromosomes were known to contain both DNA and protein. Most scientists initially favoured proteins as the genetic material because proteins are built from 20 amino acids and show enormous structural variety, while DNA appeared too simple.
Friedrich Miescher (1871) first isolated DNA (which he called "nuclein") from white blood cells.
Phoebus Levene identified the three chemical components of each DNA unit: a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (A, T, G, C).
William Astbury (1938) took the first X-ray diffraction images of DNA and correctly determined that the bases are stacked flat, spaced 0.34 nm apart.
Griffith's transformation experiment (1928): Griffith showed that heat-killed virulent bacteria could "transform" harmless bacteria into virulent ones, proving some substance transferred genetic information between cells.
Avery, MacLeod, and McCarty (1944): They identified the transforming substance as DNA. They showed a protein-digesting enzyme did not destroy the transforming ability, but a DNA-digesting enzyme did. Many scientists remained sceptical, partly because proteins still seemed more complex.
Hershey-Chase experiment (1952): Alfred Hershey and Martha Chase used radioactive labels to track DNA and protein separately in bacteriophages. They labelled DNA with radioactive phosphorus and protein with radioactive sulphur. After the phages infected bacteria, only the radioactive phosphorus (DNA) was found inside the cells. This was the most convincing evidence that DNA, not protein, is the hereditary molecule.
Erwin Chargaff (1950) analysed DNA from many organisms and found that the proportion of each base varies between species, but within any single species:
The amount of adenine (A) always equals thymine (T).
The amount of guanine (G) always equals cytosine (C).
These ratios (A = T, G = C) held true regardless of which tissue was sampled. For example, human thymus DNA and human sperm DNA have virtually the same base proportions.
This finding was a crucial clue for Watson: it implied that A and T are physically paired together in the molecule, as are G and C.
Students often think Watson and Crick discovered that DNA is the genetic material. They did not. That conclusion came from earlier work by Avery, MacLeod, McCarty, Hershey, and Chase. Watson and Crick determined DNA's three-dimensional structure.
Students sometimes assume Chargaff's rules mean that A + T = G + C (i.e., all four bases are present in equal amounts). That is wrong. The rules state A = T and G = C separately. The ratio of (A + T) to (G + C) varies between species.
Many students confuse the Griffith experiment (which showed transformation happens) with the Avery experiment (which identified DNA as the transforming substance). Griffith did not know what the transforming material was.
It is a common mistake to think that DNA was immediately accepted as the genetic material after Avery's 1944 paper. In fact, many scientists remained unconvinced until Hershey and Chase provided further evidence in 1952.
⚠️ Be able to list the three components of a nucleotide (phosphate, deoxyribose sugar, nitrogenous base). This is a very common multiple-choice question.
⚠️ Know Chargaff's rules cold: A = T, G = C. Expect questions that give you a table of base percentages and ask you to identify patterns or predict a missing value.
⚠️ Be prepared to compare and contrast the Griffith, Avery, and Hershey-Chase experiments: what each one proved, what technique was used, and why each was important.
⚠️ Understand why proteins were initially favoured over DNA as the genetic material (20 amino acids vs. 4 bases, greater structural diversity).
True or False: A DNA nucleotide contains an amino acid. (False – it contains a phosphate, a sugar, and a nitrogenous base.)
Fill in the blank: Chargaff's rules state that in any organism's DNA, the amount of adenine equals the amount of ____. (Thymine.)
True or False: Griffith's 1928 experiment identified DNA as the transforming substance. (False – Griffith showed transformation occurs, but Avery's team identified DNA as the substance responsible.)
Fill in the blank: Hershey and Chase labelled DNA with radioactive ____ and protein with radioactive ____. (Phosphorus; sulphur.)
True or False: Within a single organism, different tissues have different base compositions. (False – all cells share the same DNA sequence and therefore the same base proportions.)
Q: What are the three chemical components of a DNA nucleotide?
A: A phosphate group, a deoxyribose sugar, and one of four nitrogenous bases (A, T, G, or C).
Q: Why did most scientists initially favour proteins over DNA as the genetic material?
A: Proteins are built from 20 different amino acids and show great structural diversity and specificity, which seemed necessary to encode the wide variety of inherited traits. DNA, with only four bases, appeared too simple.
Q: Describe the key finding of Avery, MacLeod, and McCarty's 1944 experiment. What did it demonstrate?
A: They showed that the "transforming principle" discovered by Griffith was DNA. A protein-digesting enzyme did not destroy the transforming ability, but a DNA-digesting enzyme did, demonstrating that DNA carries genetic information.
Q: In the Hershey-Chase experiment, how did the researchers distinguish between DNA and protein?
A: They used radioactive phosphorus (³²P) to label DNA and radioactive sulphur (³⁵S) to label protein. After phage infection, only the ³²P label was found inside the bacteria, showing that DNA, not protein, entered the cell.
Q: A sample of DNA is found to contain 28% adenine. Using Chargaff's rules, predict the percentages of the other three bases.
A: Thymine = 28% (A = T). The remaining 44% is split equally between guanine and cytosine, so G = 22% and C = 22%.
This material connects directly to the structure of DNA (Part 2 of these notes), because understanding that DNA is the genetic material is the reason Watson and Crick set out to solve its shape. The base-pairing rules discovered by Chargaff are essential for understanding DNA replication, transcription, and translation. The experimental logic used in the Hershey-Chase experiment (radioactive labelling) reappears in the Meselson-Stahl experiment on semiconservative replication.
DNA structure, nucleotide, nitrogenous bases, adenine, thymine, guanine, cytosine, Chargaff's rules, base pairing, Griffith transformation experiment, Avery MacLeod McCarty, Hershey-Chase experiment, bacteriophage, genetic material, heredity, chromosomes, central dogma, polymer, monomer, deoxyribose, phosphate group, molecular biology, the double helix film, HHMI BioInteractive