Module 2 | Source: A Brain-Mind Odyssey, UC Berkeley, Ch. 4
Tags: genes, DNA, heredity, molecular biology, Hershey-Chase experiment, double helix, transcription, translation, nucleotide codons, genetic code, central dogma
The concept of the gene evolved from Mendel's abstract unit of heredity to a physical molecule, DNA, confirmed through landmark experiments by Avery and Hershey-Chase. Watson and Crick then described DNA's double-helical structure, and the mechanism by which genetic information flows from DNA to mRNA to protein (transcription and translation) became the foundation of molecular biology.
Gene
The fundamental unit of heredity. A segment of DNA that encodes the instructions for building a protein.
Codon
A triplet sequence of nucleotides in mRNA that corresponds to a specific amino acid. There are 64 possible triplet combinations but only 20 amino acids, so the genetic code is redundant.
Transcription
The process by which one strand of DNA serves as a template for synthesising messenger RNA (mRNA), which carries the genetic message to the ribosome.
Translation
The process occurring at the ribosome where transfer RNA (tRNA) matches nucleotide triplets in mRNA with corresponding amino acids, which are joined by peptide bonds into a protein chain.
Nucleotide bases
The four chemical letters of DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). Joined by covalent bonds to a sugar-phosphate backbone.
Double helix
The three-dimensional structure of DNA proposed by Watson and Crick: two complementary strands wound around each other, held together by hydrogen bonds between base pairs.
Charles Darwin (1809–1882) proposed that all life is related and that biological diversity results from variation and natural selection.
Gregor Mendel (1822–1884) showed that traits are passed from parent to offspring in orderly, predictable patterns through his work with pea plants.
Niels Bohr (1885–1962) and Erwin Schrödinger (1887–1961), both quantum physicists, influenced thinking about the physical basis of life.
Max Delbrück (1906–1981) proposed that genes are large molecules whose atomic configurations could be rearranged by high-energy radiation such as X-rays.
Oswald Avery (1877–1955) demonstrated with pneumococcal bacteria that DNA can transfer genetic information from one cell to another.
The Hershey-Chase experiment provided decisive evidence:
Used bacteriophage T2 grown on two different radioactive media: one with radioactive sulfur (labels protein), one with radioactive phosphorus (labels DNA).
Logic: cysteine and methionine (amino acids in protein) contain sulfur; DNA contains phosphorus but no sulfur; proteins contain no phosphorus.
After phages infected E. coli, radioactive phosphorus appeared in the bacterial pellet, while radioactive sulfur did not.
Conclusion: it is viral DNA, not protein, that enters the bacterium during infection. Genes are made of DNA.
DNA is a double helix of two long strands, each a sequence of nucleotide bases (A, T, G, C) linked by covalent bonds to a backbone of deoxyribose sugar and phosphate groups.
Complementary strands are held together by hydrogen bonds between bases (A pairs with T, G pairs with C).
Transcription: one DNA strand acts as the template for synthesis of mRNA. The mRNA then travels to the ribosome.
Translation: at the ribosome, tRNA molecules read mRNA codons (three-nucleotide sequences) and deliver matching amino acids. Amino acids are linked by peptide bonds into a polypeptide chain, forming a protein.
The genetic code has built-in redundancy: 64 codons code for just 20 amino acids.
Base-pairing rule: A–T, G–C (held by hydrogen bonds)
Codon table: 4 bases taken 3 at a time = 4³ = 64 possible codons, coding for 20 amino acids + stop signals
Central dogma flow: DNA → (transcription) → mRNA → (translation) → Protein
⚠️ The Hershey-Chase experiment is a classic exam question. Know which radioactive label tracks which molecule (sulfur = protein, phosphorus = DNA) and what the pellet result means.
⚠️ Understand the difference between transcription (DNA → mRNA, in the nucleus) and translation (mRNA → protein, at the ribosome). These are frequently confused.
⚠️ The redundancy of the genetic code (64 codons for 20 amino acids) is commonly tested. Redundancy does not mean ambiguity: each codon specifies only one amino acid, but multiple codons can specify the same amino acid.
Q: In the Hershey-Chase experiment, why was radioactive sulfur used to label protein and radioactive phosphorus to label DNA?
A: Sulfur is found in the amino acids cysteine and methionine (components of protein) but not in DNA. Phosphorus is found in the sugar-phosphate backbone of DNA but not in protein. This allowed the researchers to track each molecule independently.
Q: What is the difference between transcription and translation?
A: Transcription is the synthesis of mRNA from a DNA template. Translation is the assembly of a polypeptide chain at the ribosome, guided by the mRNA sequence and carried out by tRNA molecules delivering amino acids.
Q: Why is the genetic code described as redundant?
A: There are 64 possible three-nucleotide codons but only 20 amino acids, so more than one codon can code for the same amino acid. Each codon still specifies only one amino acid, so the code is unambiguous.
Q: What holds the two strands of the DNA double helix together?
A: Hydrogen bonds between complementary base pairs (adenine with thymine, guanine with cytosine).
heredity, inheritance, Mendel, Darwin, natural selection, Hershey-Chase, phage T2, Avery, pneumococcal transformation, Watson and Crick, double helix, DNA structure, nucleotides, adenine, thymine, guanine, cytosine, deoxyribose, mRNA, tRNA, ribosome, peptide bond, codon, genetic code, redundancy, central dogma, transcription, translation, molecular biology