1. Reading Assignments Chapter 11, pages 199-209

2. DNA structure and replication
Genetics: the study of how information about the biological characteristics of an organism is inherited from one generation to another, and how that information controls biological structure and function.
Genetic information is encoded in the molecular structure of Deoxyribonucleic Acid (DNA).

3. History
In the early 20th Century, it was unclear whether the genetic material was DNA or Protein (and most bets were on protein).
Classic experiments:
Griffith; Avery, MacLeod, & McCarty
Hershey & Chase
Conclusion: It’s DNA!

5. Chemical Content of DNA
Made of 4 nucleotides, with deoxyribose sugars
purine bases: adenine and guanine (double rings)
pyrimidine bases: thymine and cytosine (single ring)

7. Relative proportions of bases
Chargaff’s rule: In any sample of DNA ---
A = T and G = C
A+G = T+C, i.e. the purine:pyrimidine ratio = 1.0
ratio of (A+T) : (G+C) varies widely, but is fairly consistent for any given species.

9. Chemical Structure of DNA
Maurice Wilkins and Rosalind Franklin used X-ray crystallography to determine that DNA structure was a helix.
James Watson and Francis Crick made models based on the data of Chargaff, Wilkins, Franklin, and others.

12. Important features of DNA
Like a ladder twisted into a corkscrew shape
Uprights of ladder are sugar-phosphate chains
Chains are “antiparallel” – phosphate at the top on one side but at the bottom on other
Rungs of ladder are base pairs.
Adenine always pairs with Thymine Guanine always pairs with Cytosine

13. Replication (copying) of DNA
Two strands separate in a small region.
Free nucleotides move into position according to the base-pair rules.
Phosphodiester linkages (sugar-phosphate bonds) are formed in a chemical reaction catalyzed by DNA polymerase.

14. Replication is Semiconservative
After replication, each double-helix is made of one old strand and one newly formed strand. (Meselson & Stahl)