9.1 Experiments Identify and Describe the Genetic Material
1. DNA, the genetic material, contains information the cell requires to synthesize proteins and replicate itself.
2. In 1871, Miescher first isolated DNA and named it "nuclein."
3. In 1902, Archibald Garrod linked heredity to enzyme (protein) defects which he called "inborn errors of metabolism."

DNA is the Hereditary Molecule
1. In 1928, Griffith identified a "transforming factor" that transmitted infectiousness in bacteria.
2. In 1944, Avery, MacLeod and McCarty showed that the transforming factor was DNA.

Protein is not the Hereditary Molecule
1. In 1950, Hershey and Chase, using their famous "blender" experiment, confirmed that DNA, not protein, is the genetic material.
2. The experiment involved using radioactive labeled proteins and nucleic acids. Proteins were labeled with radioactive sulfur, and the DNA was labeled with radioactive phosphorous.

Discovering the Structure of DNA
1. The work of chemist Phoebus Levene showed that DNA includes deoxyribose, nitrogenous bases, and phosphates, in equal proportions.
2. Chargaff's data showed that the number of purines equals the number of pyrimidines.
3. Franklin's X-ray diffraction patterns demonstrated that DNA was a helix.
4. Pauling believed DNA was a triple helix.
5. Using Franklin's data, Watson and Crick correctly deduced that DNA is a double helix.

9.2 DNA Structure
1. The gene is a segment of DNA containing the information to specify a sequence of amino acids in a protein.
2. The DNA double helix is a ladder of alternating deoxyribose and phosphate groups, with rungs formed by complementary base pairs.
3. Adenine (A) and guanine (G) are purine bases; cytosine (C) and thymine (T) are pyrimidine bases. A pairs with T and G with C.
4. The DNA double helix is directional, the two strands are antiparallel.
5. DNA is highly coiled and wound tightly about histones, forming nucleosomes, which wind into chromatin.

9.3 DNA Replication - Maintaining Genetic Information
Replication is Semiconservative
1. Meselson and Stahl in 1957 showed that DNA replication was not conservative or dispersive, but semi-conservative.
2. Using heavy nitrogen as a label, they determined that the two parental strands separate and each is a template for assembling new daughter strands.

Steps of DNA Replication
1. Replication occurs simultaneously at several points on each human chromosome. Each site is called a replication fork.
2. At each initiation site, primase builds a short RNA primer. DNA polymerase builds a new strand from the 5' end of the DNA to the 3' end. Bases in the template and complementary bases in the new strand hydrogen bond to form the new double helix.
3. The new strand of DNA is checked for errors and the RNA primers removed and replaced with DNA bases. Ligase joins the sugar-phosphate backbone.
4. DNA replication is discontinuous because DNA polymerase can't readily replicate strands that run in opposite directions.