1. Molecular Basis of Inheritance
AP Biology
Crosby High School

2. Frederick Griffith (1928) Streptococcus pneumoniae
Lethal smooth strain vs. Harmless rough

3. Hershey – Chase Experiment
T2 reprogrammed E. coli
Alfred Hershey and Martha Chase (1952)

4. Additional Support for DNA
Eu. Cells copy DNA exactly before Mitosis
Diploid cells exactly twice as much DNA as Haploid gametes
Chargaff’s rule
%A = %T
%G = %C

5. The Players

6. Watson and Crick James Watson visited Maurice Wilkins at Cambridge
Noticed Rosalind Franklin’s X-ray image of DNA
Took it to Francis Crick who recognized a helix structure

7. Double – Helix Calculations
Width of helix suggested two strands
One complete turn every 3.4 nm
Base pairs .34 nm apart
Purines must be paired specifically with Pyrimidines
Supported Chargaff’s rule

8. The Structure

9. DNA Replication Proposed types 6 billion base pairs copied
Conservative: Original DNA remains intact
Semi-conservative: Half original and half daughter
Dispersive: All four strands have old and new
6 billion base pairs copied
Takes only a few hours
1 mistake out of 1 billion nucleotides

10. Proposed Replications

11. Origin of Replication Recognize specific portion to open molecule
Begins copying in both directions
Number of Origins
Prokaryotic: 1 origin of replication
Eukaryotic: May have hundreds or thousands
Replication fork
Y-shaped region of origin of replication

12. Elongation of New DNA Strands
Catalyzed by DNA Polymerases
Bacteria: 500 nucleotides / sec
Humans: 50 nucleotides / sec
Energy supplied by nucleoside triphosphates
ATP, GTP, CTP, TTP
Nucleotide and phosphate join strand
Pyrophosphate releases energy through hydrolysis

13. Elongation

14. Antiparallel DNA Strands

15. Leading and Lagging Strand
Only elongates in 5’  3’ Direction
Lagging strand contains Okazaki fragments ( )
DNA Ligase joins fragments

16. Primase Begins Replication
Primase: joins RNA together to form a primer
DNA Polymerase adds to the primer
Another polymerase replaces the RNA primer with DNA
Leading strand needs only one primer
Lagging strand needs one primer for each Okazaki fragment

17. DNA Proofreading
DNA polymerase checks for errors and corrects them as it elongates
Mismatch Repair: fixes incorrectly paired nucleotides
Nucleotide Excision Repair
Thymine – Thymine Dimers

18. Excision Repair

19. Telomeres Exist at the ends of DNA Multiple repetitions of TTAGGG
Between 100 – 1,000 repeats
Telomerase replaces telomeres in germ cells
If the repeats run out then the cells die

20. Telomerase