DNA Structure and Replication
12.1 How Did Experiments Identify and Describe the Genetic Material?
1. DNA encodes the information necessary for a cell’s survival and specialization and must be able to replicate for a cell to divide.
2. Many experiments described DNA and showed it to be the genetic material. Miescher identified DNA in white blood cell nuclei. Garrod connected heredity to symptoms resulting from enzyme abnormalities.
3. Griffith determined that a substance transmits a disease-causing trait to bacteria; Avery, MacLeod, and McCarty showed that the transforming principle is DNA; Hershey and Chase confirmed that the genetic material is DNA and not protein.
4. Levene described the proportions of nucleotide components. Chargaff discovered that A and T, and G and C, occur in equal proportions. Wilkins and Franklin provided X-ray diffraction data. Watson and Crick combined these clues to propose the double helix conformation of DNA.
12.2 What Is the Three Dimensional Structure of DNA?
5. The rungs of the DNA double helix consist of hydrogen-bonded complementary base pairs (A with T, and C with G). The rails are chains of alternating deoxyribose and phosphate, which run antiparallel to each other.
6. DNA is highly coiled around proteins, forming nucleosomes.
12.3 How Does DNA Replication Maintain Genetic Information?
7. Density shift experiments showed that DNA replication is semiconservative, and not conservative or dispersive.
8. To replicate, DNA unwinds locally at several origins of replication. Replication forks form as hydrogen bonds break. Primase builds a short RNA primer, which is eventually replaced with DNA. Next, DNA polymerase fills in DNA bases, and ligase seals the sugar-phosphate backbone.
9. Replication proceeds in a 5¢ to 3¢ direction, necessitating that the process be discontinuous in short stretches on one strand.
12.4 How Does DNA Repair Itself?
10. Photoreactivation splits pyrimidine dimers.
11. Excision repair cuts out the damaged area and replaces it with correct bases.
12. Mismatch repair scans newly replicated DNA for mispairing and corrects the error.
13. Repair disorders break chromosomes and raise cancer risk.