Source: Chapter 4, Lecture slides
Tags: Darwin, Mendel, gene, DNA, double helix, Watson, Crick, Rosalind Franklin, transcription, translation, mRNA, codon, genetic code, Hershey-Chase, Avery, Schrödinger, Delbrück, Bohr, bacteriophage, Oppenheimer, Tolman, loyalty oath
This lecture traces the history of molecular biology from Darwin and Mendel through to the discovery of DNA's double helix. It covers how genetic information is encoded, transcribed, and translated into proteins, and touches on the mid-20th century scientific culture at Berkeley that shaped this field.
Gene
The fundamental unit of heredity. A segment of DNA that encodes the information needed to produce a functional product (typically a protein).
Charles Darwin
Proposed that species arise and develop through natural selection of small, inherited variations that increase an organism's ability to compete, survive, and reproduce. His work On the Origin of Species established the theory of evolution.
Gregor Mendel
Founder of the modern science of genetics. His pea plant experiments (1856–1863) established the laws of Mendelian inheritance, demonstrating that genes underlie heritable traits. Recognised posthumously.
Niels Bohr
Developed the Bohr model of the atom: electrons occupy discrete energy levels and can jump between orbits. His broader insight for this course: acts of observation place fundamental limits on what we are able to know.
Max Delbrück
Proposed that genes are likely large molecules whose atomic configurations could be rearranged by high-energy electromagnetic radiation (such as X-rays). Pioneered the use of bacteria and bacteriophages to investigate the physical properties of heredity.
Erwin Schrödinger
Published What is Life?, drawing attention to Delbrück's ideas and suggesting that studying life at the molecular and atomic level might reveal new physical laws.
Wendell Stanley
Showed that a tobacco virus could be crystallised, leading him to argue that genes must be proteins. (Later disproved by the Hershey-Chase experiment.)
Oswald Avery
Discovered that DNA can carry genetic information from one cell to another. His team struggled to convince others because DNA was considered "too simple" a molecule to encode genetic information.
Hershey-Chase experiment
Demonstrated that DNA, not protein, is the genetic material. Used bacteriophages with radioactively labelled phosphate groups (on DNA) and sulphur groups (on protein) to show that DNA is what confers the ability to produce progeny inside a bacterium.
James Watson and Francis Crick
Proposed the double helical structure of DNA in 1953, one of the founding events of molecular biology. Published in Nature.
Rosalind Franklin
Her X-ray crystallography work was critical to the discovery of DNA's double helical structure, though Watson and Crick are most commonly credited.
DNA (deoxyribonucleic acid)
The molecule encoding genetic information. Consists of two long strands of nucleotide bases (A, T, G, C) joined by covalent bonds to a backbone of deoxyribose sugars and phosphates. The two strands form a double helix held together by hydrogen bonds between complementary base pairs: adenine with thymine, guanine with cytosine.
Nucleotide codon
A sequence of three nucleotides that together form a unit of genetic code in DNA or RNA.
Transcription
The first step of gene expression. A segment of DNA is copied into RNA (especially mRNA) by RNA polymerase. The DNA double helix unwinds, and one strand serves as a template. In RNA, uracil (U) replaces thymine (T).
Translation
The process by which mRNA is decoded to produce a protein. mRNA moves from the nucleus to ribosomes, where transfer RNA molecules match nucleotide triplets in the mRNA with their corresponding amino acids. The amino acids are joined by peptide bonds to form a protein.
Messenger RNA (mRNA)
The RNA molecule that carries genetic information from DNA to the ribosome for protein synthesis.
Bacteriophage
A virus composed only of proteins and nucleic acids that infects and replicates within bacteria and archaea. E. coli and bacteriophages became the system of choice for molecular biology research.
Ouroboros
A circular symbol of a snake swallowing its own tail, representing wholeness or infinity. Served as inspiration for the discovery of the benzene ring structure.
Darwin's theory of evolution by natural selection: inherited variations that improve fitness are selected for over generations
Mendel's pea plant experiments established that discrete units (genes) underlie traits
These two frameworks, evolution and genetics, converged in the 20th century to create molecular biology
Bohr's atomic model: discrete energy levels, limits on observation
Delbrück: genes are probably large molecules, rearrangeable by X-rays; used bacteria and phages to study heredity
Schrödinger's What is Life? popularised the idea that molecular-level study of life could reveal new physics
Stanley: crystallised a tobacco virus, proposed genes are proteins (later shown to be incorrect)
Avery: showed DNA carries genetic information between cells, but the scientific community was sceptical because DNA seemed "too stupid"
Hershey-Chase experiment: definitively showed DNA (not protein) is the genetic material
Labelled DNA with radioactive phosphorus (³²P) and protein with radioactive sulphur (³⁵S)
Only the ³²P entered the bacterial cell and directed new phage production
Watson and Crick proposed the double helical structure of DNA in 1953
Rosalind Franklin's X-ray crystallography was essential to this discovery
Published in Nature; considered one of the most impactful publications in biology
The structure immediately suggested how heredity works: genetic information encoded in the linear sequence of A, T, C, G
Two strands of nucleotides, each with a deoxyribose-phosphate backbone
Base pairs: adenine (A) pairs with thymine (T), guanine (G) pairs with cytosine (C)
Strands held together by hydrogen bonds between complementary bases
The double helix wraps around itself in a right-handed spiral
Transcription:
DNA double helix unwinds
RNA polymerase copies one strand into a complementary RNA molecule
In RNA, uracil (U) replaces thymine (T)
The product is an RNA copy of the gene's information
Translation:
mRNA travels from the nucleus to ribosomes in the cytoplasm
Transfer RNA (tRNA) molecules match each three-nucleotide codon in the mRNA to its corresponding amino acid
Amino acids are joined by peptide bonds to build a protein
Codons: three-nucleotide sequences that specify individual amino acids (the genetic code)
Robert Oppenheimer: theoretical physicist at UC Berkeley, "father of the atomic bomb" (Manhattan Project)
Frank Oppenheimer: also worked on the Manhattan Project, later created the Exploratorium museum in San Francisco (1969)
Edward Tolman: famous Berkeley psychologist who refused to sign the University of California Loyalty Oath during the McCarthyist era. Led resistance, sued, and the California Supreme Court overturned the oath in 1955 (Tolman v. Underhill)
⚠️ Know the base pairing rules: A-T (DNA), A-U (RNA), G-C.
⚠️ Be able to distinguish transcription (DNA to RNA) from translation (mRNA to protein).
⚠️ Understand the Hershey-Chase experiment: what was labelled, what entered the cell, and what conclusion followed.
⚠️ Know who actually did the X-ray crystallography that informed the double helix model (Rosalind Franklin).
⚠️ Understand what a codon is (three nucleotides = one amino acid).
⚠️ Avery's discovery was initially rejected because DNA was thought to be too simple. This is a good example of scientific bias.
Q: What did the Hershey-Chase experiment demonstrate, and how?
A: It demonstrated that DNA, not protein, is the genetic material. They labelled phage DNA with radioactive phosphorus and phage protein with radioactive sulphur. Only the phosphorus-labelled DNA entered bacteria and directed new phage production.
Q: What are the base pairing rules for DNA?
A: Adenine pairs with thymine (A-T), guanine pairs with cytosine (G-C), held together by hydrogen bonds.
Q: What is the difference between transcription and translation?
A: Transcription copies a DNA sequence into an RNA molecule (using RNA polymerase). Translation decodes that mRNA at ribosomes to build a protein from amino acids.
Q: What is a codon?
A: A sequence of three nucleotides that specifies a single amino acid in the genetic code.
Q: Why was Avery's discovery that DNA carries genetic information initially rejected?
A: Because DNA was considered "too simple" (or "too stupid") a molecule to carry genetic information. Proteins, with their greater structural complexity, were the favoured candidate.
Q: Who contributed the X-ray crystallography data critical to the discovery of the DNA double helix?
A: Rosalind Franklin.
Gene, heredity, Darwin, natural selection, evolution, Mendel, Mendelian inheritance, Niels Bohr, Max Delbrück, Schrödinger, What is Life, Wendell Stanley, Oswald Avery, Hershey-Chase experiment, bacteriophage, DNA, double helix, Watson, Crick, Rosalind Franklin, transcription, translation, mRNA, tRNA, ribosome, codon, genetic code, RNA polymerase, uracil, adenine, thymine, guanine, cytosine, peptide bond, Oppenheimer, Tolman, loyalty oath, ouroboros, benzene, MCB C61, UC Berkeley