1. The Molecular Basis of Inheritance
“ The Golden Age of Genetics”

2. Searching for Genetic Material
Mendel: modes of heredity in pea plants
Morgan: genes located on chromosomes
Meischer: isolated DNA
Came from the nuclei he called it nuclein, later changed to nucleic acid
Feulgen: discovered fuchsin dye stained DNA
P.A. Levene: analyzed DNA and found out it had:
cytosine, thymine, adenine, and guanine
a deoxyribose sugar
a phosphate group

3. Frederick Griffith 2 forms (Streptococcus pneumoniae):
S strain (pneumonia-causing)
R strain (didn’t cause pneumonia)
Induced a nonpathogenic strain to become pathogenic.
Transforming factor that caused the change.
Transformation: change in genotype and phenotype due to assimilation of external substance (DNA) by a cell.

4. Delbruck and Luria
Worked with bacteriophages, (virus that attacks bacteria)
Bacteriophages consist of protein coats covering DNA.
They infect a cell by injecting DNA into the host cell.
Turning the host into a viral “factory”.
Host cell bursts, releasing hundreds of new bacteriophage.
Phages have DNA and protein, making them ideal to resolve the nature of the hereditary material.

5. Hershey and Chase
Tried to determine if protein or DNA was the hereditary material.
Labeled DNA and protein with different isotopes.
DNA contains Phosphorous (P) but no Sulfur (S), tagged the DNA with radioactive Phosphorous-32.
Protein lacks (P) but does have (S), tagged it with radioactive Sulfur-35.
Radioactive S remained outside the cell, and radioactive P was found inside the cell, showing DNA is the physical carrier of heredity.

6. DNA Structure Chargaff ratio of nucleotide bases
(A=T; C=G)
Wilkins and Franklin took X-ray diffraction photomicrographs of crystalline DNA extract.
Watson & Crick came up with the Double Helix model
nucleotides:
nitrogenous base (thymine, adenine, cytosine, guanine)
deoxyribose sugar
phosphate group

7. The final product DNA is a double helix
Bases to the center (like rungs on a ladder)
Sugar-phosphate backbone (like the sides of a twisted ladder)
Strands are complementary, large purine pair with small pyrimidine
Purines: ‘A’ & ‘G’
Pyrimidines: ‘C’ & ‘T’
A  T form 2 H+ bonds
G  C form 3 H+ bonds

8. THE CHEMICAL NATURE OF NUCLEIC ACIDS:
Phosphate attaches to 5' carbon
Base attaches to 1' carbon
Hydroxyl (-OH) attaches to 3' carbon
Nucleotides strung together in chains
Phosphodiester bond connects between Phosphate at 5’ C & hydroxyl at 3’ C
Definite direction
Sequences conventionally written in 5’ to 3’ direction

9. Antiparallel nature
Sugar/phosphate backbone runs in opposite directions
One strand runs 5’ to 3’, while the other runs 3’ to 5’

10. DNA Replication
Watson & Crick strands are complementary
Nucleotides line up on template according to base pair rules (Watson)
Semiconservative, each new double helix contains one strand from the parent molecule and one newly synthesized strand. (Meselson, Stahl)
Meselson & Stahl replication is semiconservative; Expt: varying densities of radioactive nitrogen

11. DNA Replication: a closer look
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
Initiation:
Primase: inserts short RNA sequence (Primer) at point of initiation begins the replication process
DNA polymerase: catalyzes the elongation of new DNA
DNA polymerase only adds nucleotides at the free 3’ end, forming new DNA strands in the 5’ to 3’ direction only
Ligase: joins together Okazaki fragments

12. Leading vs. lagging strand
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
Happens multiple times and creates small fragments of DNA called Okazaki fragments.
Fragments joined by DNA ligase

13. Visual Representation of DNA Replication

14. IMPORTANT CONCEPTS
ENZYMES INVOLVED IN DNA REPLICATION WORK SIMULTANEOUSLY TO GET THE JOB DONE.

15. Quick Review DNA Replication
Synthesis of Leading Strand
Priming (Primase)
Elongation (DNA Polymerase)
Replacement of RNA Primer by DNA (DNA Polymerase)
Double helix unwinds, providing single-stranded DNA template. (Helicases and Single –Stranded Binding Proteins)
Synthesis of Lagging Strand
Priming for Okazaki fragments (Primase)
Elongation of fragment (DNA Polymerase)
Replacement of RNA Primer by DNA (DNA Polymerase)
Joining of fragments (Ligase)

16. DNA Repair Mismatch repair: Excision repair: Telomere ends:
DNA polymerase corrects with proper compliment
Excision repair:
Damaged base removed and replaced with correct base by Nuclease
Telomere ends:
Found at the ends of Eukaryotic chromosomes
Little bit lost each time a cell divides if telomere runs out cell ceases to divide
Telomerase replaces what’s lost