A. In 1868, Miescher first isolated deoxyribonucleic acid, or DNA, from cell nuclei.

B. In 1951, Linus Pauling deduced the structure of proteins, which were complicated enough, people thought, to code for hereditary instructions. Remember the 20 amino acids!

C. In 1953, Watson and Crick put together a model of DNA, for which they received the Nobel Prize.

I. Discovery of DNA Function

A. Early work that led to the Discovery of DNA as the Physical & Chemical Material of Inheritance

1. In 1928, Fred Griffith was working with S (smooth=virulent) and R (rough=nonvirulent) strains of a pneumonia-causing bacterium. His experiments are 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 teeming in their blood.

Transformation: Some substance from the S cells had transformed the harmless R cells into cells capable of causing death in mice.

2. Avery (1944) showed that the "Griffith substance" was nucleic acid, not protein as some people had proposed. This was still not universally accepted as the material of inheritance, however.

B. Confirmation of DNA Function

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 by Hershey and Chase to prove which of these (DNA) was the heredity material. How? Look at the experiment in your textbook.

II. DNA Structure

A. What Are the Components of DNA?

1. DNA is composed of four kinds of nucleotides, each of which consists of:

a. a five-carbon sugar (deoxyribose),

b. a phosphate group, and

c. one of four bases&endash;adenine (A), guanine (G), thymine (T), cytosine (C).

2. The nucleotides are similar, but T and C are single-ring bases; A and G are double-ring bases.

3. Edwin Chargaff in 1949 showed that the amount of A = T and 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. Nucleotides are joined along the molecule’s length; sugar-phosphate linkages form "backbones."

B. Patterns of Base Pairing

1. DNA consists of two strands of nucleotides twisted into a double helix.

2. Base pairs are formed by the hydrogen bonding of A with T, and G with C; this is constant for all species.

3. The sequence of bases in a nucleotide strand is different from species to species.

Focus on Bioethics: Rosalind’s Story

IV. DNA Replication and Repair

A. How Is a DNA Molecule Duplicated?

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 (semiconservative).

a. Unwinding requires many kinds of enzymes.

b. DNA polymerases assemble the nucleotides into nucleic acids.

B. Monitoring and Fixing the DNA

1. DNA polymerases, DNA ligases, and other enzymes engage in DNA repair when they "read" the complementary sequence on the other strand and restore it.

2. These same enzymes are responsible for the breaking and reattaching of DNA strands that occurs in crossing over.

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