Chapter 13.2

1. Chapter 13.2 The Discovery of DNA’s Structure AP Biology Fall 2010

2. DNA’s Building Blocks • DNA consists of 4 kinds of nucleotides, each of which consists of: • A five carbon sugar: deoxyribose • A phosphate group • One of four bases

3. DNA’s Building Blocks • Adenine (A) • Guanine (G) • Thymine (T) • Cytosine (C)

4. DNA’s Building Blocks • The nucleotides are similar • T and C are single ring pyrimidines • A and G are double ring purines

5. DNA’s Building Blocks • In 1949, Edwin Chargaff noted two critical bits of data • The 4 kinds of nucleotide bases making up DNA molecule differ in relative amounts from species to species • The amount of A = T and the amount of G = C

6. DNA’s Building Blocks • Rosalind Franklin used X-ray diffraction techniques to produce images of DNA • X-ray diffraction uses a beam of electrons directed at a molecule which scatters the x-ray in a pattern • Franklin used x-ray diffraction with a “wet” form of DNA • DNA exists as a long, thin molecule of uniform diameter • The structure is highly repetitive • DNA is helical

7. DNA Structure

8. Patterns of Base Carriers • The following features were incorporated into their models • Single ringed thymine was hydrogen bonded with double-ringed adenine, and single ringed cytosine with double ringed guanine, along the entire length of the molecule • The backbone was made of chains of sugar-phosphate linkages • The molecule was double stranded and looked like a ladder with a twist to form a double helix

9. DNA Structure

11. Patterns of Base Carriers • The base pairing is constant for all species, but the sequence of base pairs in a nucleotide strand is different from one species to the next • This is the basis for life’s diversity