James Watson is best known as one of the scientists who proposed the double helix structure of DNA in 1953 with Francis Crick at the University of Cambridge. Their model explained how genetic information could be stored, copied, and passed from one generation to the next. The discovery became a turning point in biology because it connected chemistry, heredity, and evolution in one clear structure.
Watson later helped launch modern genomics as the first director of the Human Genome Project.
Key Facts
- Watson and Crick proposed the DNA double helix model in 1953.
- DNA base pairing follows A pairs with T and C pairs with G.
- Chargaff's rules can be written as %A = %T and %C = %G in double-stranded DNA.
- The DNA helix is about 2 nm wide and has about 10 base pairs per turn.
- One full turn of B-form DNA is about 3.4 nm, so spacing per base pair is about 0.34 nm.
- Watson, Crick, and Maurice Wilkins received the 1962 Nobel Prize in Physiology or Medicine for work on DNA structure.
Vocabulary
- DNA
- DNA, or deoxyribonucleic acid, is the molecule that stores genetic instructions in living cells.
- Double helix
- A double helix is a twisted ladder shape made of two DNA strands wound around each other.
- Base pair
- A base pair is a matched pair of DNA bases, either adenine with thymine or cytosine with guanine.
- X-ray crystallography
- X-ray crystallography is a technique that uses X-ray diffraction patterns to infer the structure of molecules.
- Genome
- A genome is the complete set of genetic information in an organism.
Common Mistakes to Avoid
- Saying Watson discovered DNA by himself is wrong because the double helix model was developed with Francis Crick and depended on evidence from Rosalind Franklin, Maurice Wilkins, and others.
- Forgetting Rosalind Franklin's role is wrong because her X-ray diffraction image known as Photo 51 provided crucial evidence about DNA's helical shape and dimensions.
- Matching A with C or G with T is wrong because DNA base pairing is specific: A pairs with T, and C pairs with G.
- Thinking the DNA model was just a drawing is wrong because it was a physical and chemical model constrained by measurements, bond geometry, base pairing, and diffraction data.
Practice Questions
- 1 A segment of B-form DNA contains 80 base pairs. Using 0.34 nm per base pair, what is the approximate length of the segment in nanometers?
- 2 A double-stranded DNA sample has 28% adenine. What percentages of thymine, cytosine, and guanine should it have according to Chargaff's rules?
- 3 Explain why Photo 51 was important for building the double helix model, and describe one reason it is important to credit Rosalind Franklin in the history of DNA.