Describe the structure of DNA as proposed by Watson and Crick.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Open. DNA is the genetic material. Watson and Crick (1953) showed that it is a double helix.
2 · Draw the double helix. [Diagram: two twisted chains, sugar-phosphate backbone, base pairs A = T and G ≡ C, one turn 34 Å, width 20 Å, arrows for antiparallel chains]
3 · Nucleotides. Each nucleotide has a deoxyribose sugar, a phosphate and a base (A, G, C or T).
4 · Base pairing.
A pairs with T by two hydrogen bonds.
G pairs with C by three hydrogen bonds.
The two chains are antiparallel.
5 · Measurements. One turn is 34 Å with 10 base pairs, 3.4 Å apart; the helix is 20 Å wide.
6 · Importance. DNA carries genetic information, copies itself by semi-conservative replication and controls protein making.
7 · Link to daily life. DNA fingerprinting helps identify people in crime cases.
8 · Close. So the double helix explains how genetic information is stored and copied.
Scoring tip: Write the numbers 34 Å, 10 base pairs, 3.4 Å and 20 Å. They are easy marks.
