Chapter 13 DNA Structure and Function
I. Cardboard Atoms and Bent-Wire Bonds
A. Linus Pauling discovered the helical structure of proteins in 1951.
B. In 1953, Watson and Crick discovered the structure of the master molecule of life—DNA.

II. Discovery of DNA Function
A. In 1868, Miescher first isolated deoxyribonucleic acid, or DNA, from cells in pus and from fish sperm. No one knew its function.
B. A Puzzling Transformation
1. In 1928, Fred Griffith was working with S (virulent) and R (nonvirulent) strains of a pneumonia-causing bacterium.
2. He performed four experiments summarized here.
a. Inject mice with R cells; mice lived.
b. Inject mice with S cells; mice died; blood samples contained many S cells.
c. S cells were killed then injected into mice; mice lived.
d. Live R cells plus heat-killed S cells were injected into mice; mice died; live S cells were found in the blood.
3. Some substance from the S cells had transformed the R cells.
a. Both proteins and nucleic acids were candidates.
b. Oswald Avery showed that the substance was DNA.
C. Bacteriophage Studies
1. Viruses called bacteriophages use bacterial cells for reproduction.
2. Because they consist of only a protein coat and a nucleic acid core, these viruses were used in experiments (Hershey and Chase) to prove which of these was the hereditary material.
a. 35S-labeled proteins in the bacteriophage coat did not enter the bacteria and thus were not participating in providing directions for new virus assembly.
b. 32P-labeled DNA in the viral core did enter the bacteria and direct new virus assembly.

III. DNA Structure
A. Components of DNA
1. DNA is composed of four kinds of nucleotides, which consist of:
a. a five-carbon sugar—deoxyribose,
b. a phosphate group,
c. one of four bases—adenine (A), guanine (G), thymine (T), cytosine (C).
2. The nucleotides are similar, but T and C are single-ring pyrimidines; A and G are double-ring purines.
3. Edwin Chargaff, in 1949, noted two critical bits of data.
a. The four kinds of nucleotide bases making up a DNA molecule differ in relative amounts from species to species.
b. The amount of A = T, and the amount of G = C.
4. Rosalind Franklin used X-ray diffraction techniques to produce images of DNA molecules.
a. DNA exists as a long, thin molecule of uniform diameter.
b. The structure is highly repetitive.
c. DNA is helical.
B. Patterns of Base Pairing
1. Watson and Crick used numerous sources of data to build models of DNA.
2. The following features were incorporated into their models.
a. Single-ringed T was hydrogen bonded with double-ringed A, and single-ringed C with double-ringed G, along the entire length of the molecule.
b. The backbone was made of chains of sugar-phosphate linkages.
c. The molecule was double stranded and looked like a ladder with a twist to form a double helix.
3. The sequence of base pairs in a nucleotide strand is different from one species to the next.

IV. DNA Replication
A. Assembly of Nucleotide Strands
1. First, the two strands of DNA unwind and expose their bases.
2. Then unattached nucleotides pair with exposed bases.
3. Thus, replication results in DNA molecules that consist of one “old” strand and one “new” strand.
B. A Closer Look at Replication
1. Origin and Direction of Replication
a. The two strands of the double helix unwind at one “origin” (viral and bacterial) or many origins (eukaryotic).
b. Unwinding and strand assembly proceeds simultaneously in both directions at replication forks.
2. Energy and Enzymes for Replication
a. Several kinds of enzymes participate in the unwinding process.
b. DNA polymerases assemble the nucleotides into nucleic acids and “proofread” the new bases for mismatched pairs, which are replaced with correct bases.
c. The free nucleotides exist as triphosphates, which supply energy to the polymerases when one phosphate is cleaved.

V. Organization of DNA in Chromosomes
A. Each eukaryotic chromosome contains one long DNA molecule.
B. The DNA of humans and other eukaryotes is highly organized to prevent tangling.
1. Some histones (a type of protein) act as spools to wind the DNA into units called nucleosomes.
2. Another histone (H1) stabilizes the arrangement and allows the beaded chain to form looped regions.
3. Other proteins serve as structural scaffolds for the loops.