1. DNA

2. Searching for Genetic Material
Mendel: modes of heredity in pea plants (1850’s)
Morgan: genes located on chromosomes (early 1900’s)
Griffith: bacterial work (1920’s)
transformation- change in genotype and phenotype due to assimilation of external substance (DNA) by a cell
Avery: transformation agent was DNA (1944)

3. Hershey-Chase Experiment
1952 Experiment with bacteriophages
viruses that infect bacteria
Tested if DNA or protein was the hereditary material
in T2 (a bacteriophage that infects E. coli)
Sulfur (S) is in protein, phosphorus (P) is in DNA
Only P was found in host cell

5. DNA Structure Chargaff
ratio of nucleotide bases (A=T; C=G)
Structure of DNA researched by Pauling and Wilkins/Franklin
Watson & Crick (1953)
Proposed the structure of DNA after viewing an X-ray diffraction photo by Rosalind Franklin

6. The Double Helix
Nucleotides: nitrogenous base (thymine, adenine, cytosine, guanine); sugar (deoxyribose); phosphate group

7. Base Pairing Rules Purines: A & G Pyrimidines: C & T (Chargaff rules)
A’s H+ bonds (2x) with T and C’s H+ bonds (3x) with G
Van der Waals attractions between the stacked pairs

8. DNA Replication
Watson & Crick strands are complementary; nucleotides line up on template according to base pair rules
Meselson & Stahl replication is semiconservative; Expt: varying densities of radioactive nitrogen

9. 3 Replication Models
Meselson and Stahl concluded that DNA follows the semiconservative model

10. DNA Replication in Action
Origin of replication (“bubbles”)= beginning of replication
Replication fork: Y-shaped region where new strands of DNA are elongating
Helicase: catalyzes the untwisting of the DNA at the replication fork
DNA polymerase: catalyzes the elongation of new DNA

11. Antiparallel Nature
Sugar/phosphate backbone runs in opposite directions (Crick)
One strand runs 5’ to 3’, while the other runs 3’ to 5’
DNA polymerase only adds nucleotides at the free 3’ end, forming new DNA strands in the 5’ to 3’ direction only

12. Leading and Lagging Leading strand:
synthesis toward the replication fork (only in a 5’ to 3’ direction from the 3’ to 5’ master strand)
Lagging strand:
synthesis away from the replication fork in small pieces (Okazaki fragments)
joined by DNA ligase

13. The Lagging Strand Primase enzyme attaches a primer
a short sequence of RNA (sometimes DNA)
Small segments replicated 5’  3’
After strand replicates, primer falls off

14. Helpful Proteins

15. Proteins in Action

16. Proofreading
Mismatch repair: DNA polymerase
Excision repair: nuclease

17. Shortening Ends
Because lagging strands need 3’ end, replication cannot extend to the very end of a DNA strand
Chromosomes contain telomeres
Repeating sequences of DNA
Don’t contain genes
Telomerase lengthens telomeres so that they don’t continually get shorter
Chromosomes can divide without losing genes