1. Molecular Biology of the Gene Chapter 12

2. Outline Genetic Material Transformation DNA Structure Watson and Crick
Eukaryotic DNA Replication

3. DNA is the Genetic Material
Frederick Griffith investigated virulence of Streptococcus pneumoniae
Two forms of the bacteria:
S strain (smooth) has a capsule surrounding the cell, causes mice to die when this strain is injected
R strain (rough) does not have capsule, mice live when this strain injected
Heat-killed S strain, mice live
Combine heat-killed S strain plus live R strain, mice die, live S strain isolated from dead mice
Concluded that some substance needed to make the capsule passed from the dead S strain to the living R strain
Virulence passed from the dead strain to the living strain
Transformation

4. Griffith’s Transformation Experiment

5. Further research by Avery, Macleod, and McCarty
Discovered that DNA is the transforming substance
DNA from dead cell was being incorporated into genome of living cells
Used enzymes:
RNases (breakdown RNA)
Proteases (breakdown proteases)
DNases (breakdown DNA)
Treatment of the transforming substance with DNase prevents transformation

6. Bacteria and Bacteriophages
Viruses consist of a protein coat (capsid) surrounding a nucleic acid core.
Bacteriophages are viruses that infect bacteria

7. Hershey and Chase:
Radioactively labeled the DNA core and protein capsid of a phage.
Results indicated that DNA, not the protein, enters the host.
The DNA of the phage contains genetic information for producing new phages.

8. Hershey and Chase experiments
Determined that DNA is the genetic material
Experiment 1
Viruses with DNA labeled with 32P were incubated with E.coli
Mixed in a blender to remove virus particles attached to cells
Centrifuged so bacteria formed a pellet
Results- viral DNA was inside the bacteria

9. Hershey and Chase experiments
Viral proteins in capsids were labeled with 35S and viruses were incubated with E.coli
Mixed in blender and centrifuged
Results- labeled proteins were washed off with the capsids and were not inside the bacteria

11. Structure of DNA Determined by Watson and Crick Double helix
Composed of monomers called nucleotides
Each nucleotide has 3 parts
Phosphate group
Deoxyribose sugar
Nitrogenous base- 4 possible
Adenine and guanine- purine bases- double ring structure
Cytosine and thymine- pyrimidine bases-single ring structure

12. Erwin Chargaff made key DNA observations that became known as Chargaff’s rule
Purines = Pyrimidines
A = T and C = G
Rosalind Franklin’s ( ) X-ray diffraction experiments revealed that DNA had the shape of a coiled spring or helix

13. X-Ray Diffraction of DNA

14. In 1953, James Watson and Francis Crick deduced that DNA was a double helix
They came to their conclusion using Tinkertoy models and the research of Chargaff and Franklin
James Watson ( )
Francis Crick ( )

15. Watson/Crick Model of DNA

16. Overview of DNA Structure

17. DNA structure- ladder analogy
DNA molecule consists of 2 chains of nucleotides
Arranged in a “ladder” configuration
Alternating phosphate and sugar groups form sides of ladder
Rungs are composed of paired nitrogen bases
Complementary base pairing-purine with pyrimidine
Adenine pairs with thymine
Guanine pairs with cytosine
The “ladder” then coils to form a helix

18. Eukaryotic Replication
The two DNA strands are held together by weak hydrogen bonds between complementary base pairs
A and T
C and G
Each chain is a complementary mirror image of the other
So either can be used as template to reconstruct the other
DNA replication occurs in the nucleus.
Occurs during S phase of interphase of the cell cycle
DNA replication is semi-conservative.

19. Eukaryotic Replication
DNA replication begins at numerous points along linear chromosome.
DNA unwinds and unzips into two strands.
Each old strand of DNA serves as a template for a new strand.
Complementary base-pairing forms new strand on each old strand.
Semiconservative:
Each daughter DNA molecule consists of one new chain of nucleotides and one from the parent DNA molecule.
The 2 daughter DNA molecules will be identical to the parent molecule.

20. Enzymes Involved in DNA Replication
Before replication begins, the 2 strands of the parent molecule are hydrogen-bonded together.
The enzyme DNA helicase unwinds and “unzips” the double-stranded DNA.
The enzyme primase puts down a short piece of RNA termed the primer for daughter strand synthesis. RNA primers are removed and replaced with DNA.
New DNA nucleotides fit into place along divided strands by complementary base pairing.
DNA polymerase- reads along each single strand adding the complementary nucleotide
DNA ligase repairs any breaks in the sugar-phosphate backbone.
Two daughter DNA molecules have now formed that are identical to the original parent molecule.

22. Semiconservative Replication of DNA
Old strand –lavender
New strand- hot pink
Daughter DNA helix Daughter DNA helix

23. Ladder Configuration and DNA Replication

24. Meselson and Stahl’s DNA Replication Experiment

25. Replication Errors
Genetic variations are the raw material for evolutionary change
Mutation:
A permanent (but unplanned) change in base-pair sequence
Some due to errors in DNA replication
Others are due to to DNA damage
DNA repair enzymes are usually available to reverse most errors