1. The History of DNA

2. Erwin Chargaff
Chargaff studied DNA itself, in hopes of providing some clues about its structure.
Discovered that there are always equal amounts of the bases Adenine and Thymine, and equal amounts of Cytosine and Guanine.
Chargaff proposed that these bases pair with one another in some way.

3. Wilkins and Franklin
Rosalind Franklin and Maurice Wilkins worked with X-ray crystalography to find more clues about the structure of DNA.
Franklin’s X-ray images suggested a helical structure.

4. Franklin and Wilkins

5. Watson and Crick
James Watson and Francis Crick were also working on discovering the structure of DNA.
Applied Chargaff’s rule, assumed that A always pairs with T, C with G.
Watson was not entirely convinced of the helical structure that Franklin had suggested, and his critique of her work led her to doubt herself.

6. Watson and Crick
Wilkins consulted with Watson and Crick. Without Franklin’s knowledge, he handed them the data that he and Franklin had worked on.
Watson immediately recognized the significance. He and Crick went to work on a model of DNA.

7. The First DNA Model

8. DNA Replication
Before cells divide, they must double their DNA so that each cell gets identical copies of the DNA strands.
DNA replication helps assure that the bases are copied correctly.
Enzymes carry out the process.

9. Overview of DNA replication
Overview of DNA replication. The hydrogen bonds break to “unzip” the DNA strand. Enzymes guide free nucleotides to the exposed single strands and match the nucleotides.

10. This diagram from your textbook (page 157) shows how enzymes carry out the replication process. DNA Helicase unzips the DNA. DNA Polymerase synthesizes the new strands, using the old strands as templates.

11. DNA Replication Build a DNA Model interactive feature (web)
DNA Replication animation (web)

12. Summary DNA is a nucleic acid made up of nucleotides.
The order of the nucleotides is important, and is maintained by matching of bases across the DNA ladder (A-T, C-G), and by enzymes that patrol the DNA
DNA replication occurs before cell division, and is an orderly, enzyme-driven process.