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DNA Structure and Replication

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12.1 Experiments Identified 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.Art Quiz
Griffith's Experiment

Art Quiz
Hershey-Chase Experiment
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 DNA is a Double Helix

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.Essential Study Partner
DNA Structure

DNA Double Helix

DNA Packaging

Art Quiz
Base Pairing
6. DNA is highly coiled around proteins, forming nucleosomes.Art Quiz

12.3 DNA Replication Maintains 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.Essential Study Partner
DNA Replication

DNA Replication

Polymerase Chain Reaction (PCR)

Essential Study Partner

Essential Study Partner
Applications of Biotechnology

Art Quiz
Bovine Somatotropin
9. Replication proceeds in a 5| to 3| direction, necessitating that the process be discontinuous in short stretches on one strand. 

12.4 DNA Repair

10. Photoreactivation splits pyrimidine dimers.                 
11. Excision repair cuts out the damaged area and replaces it with correct bases.                Animation
DNA Repair
12. Mismatch repair scans newly replicated DNA for mispairing and corrects the error.                 
13. Repair disorders break chromosomes and raise cancer risk.                 

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