1. The Molecular Basis of Inheritance
AP Biology
Ch. 16

2. Life’s Operating Instructions
Nucleic Acids are unique in their ability
Direct their own replication
Hereditary info is encoded in the DNA
DNA program directs biochemical, anatomical, physiological and some behavioral traits.

3. The Search for the Genetic Material
T.H. Morgan demonstrated that genes are located on the chromosomes
Chromosomes contain two chemical components
Proteins and DNA
Scientists debated over Proteins or DNA as the genetic material

4. DNA can Transform Bacteria
Frederick Griffith
Studying 2 strains of Streptococcus pneumoniae
Heat killed pathogenic strain was a mixed with living nonpathogenic strain, injected into mouse caused pneumonia
Some chemical changed the genotype and phenotype of the nonpathogenic bacteria

6. Avery 14 year study to id transformation agent
Isolated purified molecules (Carbohydrates, Lipids, proteins, nucleic acids), from pathogenic bacteria
Transformation occurred only with DNA.

7. Hershey and Chase Bacteriophages are viruses that infect bacteria
Composed of DNA and protein
Viruses are capable of reprogram cells to produce viruses
Used Phosphorus-32 and Sulfur-32 (radioactive isotopes) to label proteins and DNA
Infected bacteria and then traced the markers to identify which molecule was incorporated into the bacterial cells.
Results showed proteins remained outside the cells, but the DNA entered the cells
Concluded that DNA was the genetic material.

9. Edwin Chargaff DNA is a polymer of nucleotides
Nucleotides contain nitrogenous base, phosphate group and a pentose sugar called deoxyribose
Nitrogen base can be adenine(A), thymine(T), guanine(G) or cytosine(C)
Chargaff analyzed the base composition of DNA from several organisms.
He found that the percentage of A was equal to the percentage of T and % of G = % of C
Chargaff’s Rules
A=T and G=C

11. The Structural Model of DNA
Challenge to determine the structure of the molecule.
Its structure could account for its role in inheritance
Scientists working on this challenged included Linus Pauling in California, Maurice Wilkins and Rosalind Franklin at Kings College in London
The winners were James Watson and Francis Crick at Cambridge University

13. The Double Helix
Wilkins shows x-ray diffraction image of molecule, made by Franklin, to Watson and Crick.
Image revealed that molecule is helical in shape
Two strands with the nitrogenous spacing between the them.

14. The structure Two strands, sugar and phosphate backbones
Nitrogen bases stick out sideways
Hydrogen bonds form between the purines and pyrimdines
A forms 2 hydrogen bonds T and G forms 3 hydrogen bonds with C

15. DNA Replication Two strands are complementary,
each strand serves as a template for a new strand.
Semiconservative model of replication

16. Meselson and Stahl Labeled nucleotides with isotopes of nitrogen.
Bacteria cultured in two different mediums
Concluded that DNA replicates using semi-conservative replication.

18. Replication
Begins at origin of replication at each end is a replication fork
Elongation is catalyzed by enzymes called DNA Polymerases.

19. Antiparallel Elongation
Two strands are oriented in opposite directions

20. DNA polymerases add nucleotides only to the free 3’ end of a growing DNA strand, never to the 5’ end.
The DNA strand can only elongate in
the 5’ ’ direction.
Leading strand is synthesized in this direction
Lagging strand is synthesized in a series of segments. These segments are called Okazaki Fragments.

21. DNA ligase is an enzyme that joins the sugar-phosphate backbones of the Okazaki Fragments.
DNA polymerase can not start synthesis of a polynucleotide it can only add to an existing chain.
Primase is an enzyme that starts an RNA chain which will become the primer.