1. A Quick Review
DNA
The Molecular Basis of Inheritance

2. Scientific History Road to understanding that DNA is the genetic material
T.H. Morgan (1908): genes are on chromosomes
Frederick Griffith (1928)- Mice, S strain and R strain
a transforming factor can change phenotype
Avery, McCarty & MacLeod (1944)
transforming factor is DNA
Erwin Chargaff (1947)
Chargaff rules: A = T, C = G
Hershey & Chase (1952) T2 bacteriophage, S into Proteins and P into DNA
confirmation that DNA is genetic material
Watson & Crick (1953) R. Franklin?
determined double helix structure of DNA
Meselson & Stahl (1958)
semi-conservative replication

4. Different Base Pairs Purines
Nitrogen bases made of double rings of carbon and nitrogen atoms
Adenine (A) and Guanine (G)

5. Different Base Pairs Pyrimidines
Nitrogen bases made of single ring of carbon and nitrogen atoms
Cytosine (C) and Thymine (T)

6. Watson and Crick Purine + purine: too wide Pyrimidine + pyrimidine:
too narrow
Purine + pyrimidine: width consistent with X-ray data
Watson
Crick

7. DNA Bonding Covalent bonds: Sugars to nitrogen bases
Hydrogen Bonds: (A) bonds (2) with (T); (C) bonds (3) with (G)
Van der Waals Bonds
attractions between the stacked pairs
Phosphodiester Bonds
between phosphate of one nucleotide to the sugar of another

8. DNA Replication Leading strand Lagging strand Origin of replication
Overall direction of replication
Leading
strand
Lagging
strand
Origin of replication
Lagging
strand
Leading
strand
OVERVIEW
DNA pol III
Leading
strand
DNA ligase
Replication fork 5¢
DNA pol I 3¢
Primase
Parental DNA
DNA pol III
Lagging
strand
Primer 3¢ 5¢

10. DNA polymerases DNA polymerase III 1000 bases/second! main DNA builder
editing, repair & primer removal
In 1953, Kornberg was appointed head of the Department of Microbiology in the Washington University School of Medicine in St. Louis. It was here that he isolated DNA polymerase I and showed that life (DNA) can be made in a test tube. In 1959, Kornberg shared the Nobel Prize for Physiology or Medicine with Severo Ochoa — Kornberg for the enzymatic synthesis of DNA, Ochoa for the enzymatic synthesis of RNA.
DNA polymerase III enzyme

11. Excision Repair Thymine dimer is a mismatch mutation
caused by adjacent thymines not correctly base-pairing with their complementary adenines
Mismatch repair done by special enzymes
fix incorrectly paired nucleotides.
A hereditary defect in one of these enzymes is associated with a form of colon cancer.
Nucleotide excision repair
Nuclease enzymes cuts out a segment of a damaged strand.
The gap is filled in by DNA polymerase and ligase.

12. Causes of Mutations
DNA constantly subject to potentially harmful chemical and physical agents.
Mutagen - physical and chemical agents that change DNA
Reactive chemicals, radioactive emissions, X-rays, and ultraviolet light can change nucleotides
DNA bases often undergo spontaneous chemical changes under normal cellular conditions.
Mismatched nucleotides that are missed by DNA polymerase or mutations that occur after DNA synthesis is completed can often be repaired.
Each cell continually monitors and repairs its genetic material, with over 130 repair enzymes identified in humans.

13. Telomeres
Repeating, non-coding sequences at the end of chromosomes = protective cap
limit to ~50 cell divisions; Human telomeres sequence is typically TTAGGG repeated 100 to 1,000 times.
Telomerase
enzyme extends telomeres by using RNA as a template
can add DNA bases at 5 end
different level of activity in different cells
high in stem cells & cancers

14. Telomere Extension 11 min introduction **commercial warning**
2 min
1 min animation
3 min
11 min introduction
**commercial warning**
2 min