1. DNA Structure and Function
AP Biology

2. Big Idea 3
Living systems store, retrieve, transmit, and respond to information essential to life processes.
Heritable information provides for the continuity of life
DNA and in some cases RNA, is the primary source of heritable information

3. The Genetic Material
Scientists confirmed that genes are on chromosomes
Able to map Drosophilia chromosomes
Did not know the composition of genes
Chromosomes contain both proteins and nucleic acids.

4. 3 requirements for genetic material
1. Store developmental, structural and metabolic information
2. Be stable so that it can be replicated
3. Undergo rare changes called mutations.

5. Transformation Frederick Griffith
Trying to develop a vaccine against Streptococcus pneunoniae
Discovered two strains
S – strain ( smooth shiny colonies) mice injected die
R- strain (rough appearing colonies) mice injected live

6. Griffith’s Transformation Experiment

7. The Transforming Substance
Avery
Isolated substances such as carbohydrates, proteins, and nucleic acids, use enzymes to destroy them.
Repeated Griffith’s experiment
If proteins were digested, transformation occurred
If RNA was digested, transformation occurred.
If DNA was digested, transformation did not occur.

9. Reproduction of Viruses
Bacteriophages are viruses that infect bacteria.
Viruses consist of a protein capsid surrounding a nucleic acid core
Viruses use host cells to reproduce
Must inject their genetic material into the host cell

10. Hershey-Chase Experiment
Relied on the chemical difference between DNA and protein
DNA contain P and Proteins contain S
Use radioactive 32P and 35S to label DNA and proteins.

11. Bacteria and Bacteriophages

12. Hershey and Chase Experiments

13. Results of the experiments
Phages with 32P infected bacteria
Phages with 35S infected bacteria
32P was found in cells
Results indicate that DNA is the genetic material

14. Chemistry of DNA Nucleic Acid Contains only 4 nucleotides
Nitrogen containing base
Adenine, Guanine, Thymine, and Cytosine
Phosphate
Pentose – 5 carbon sugar

15. The Structure of DNA DNA Contains Two pyrimidine bases
Single rings
Cytosine and Thymine
Two purine bases
Double rings
Adenine and Guanine

16. Chargaff’s Rules Edwin Chargaff did an analysis of base content of DNA
Found nucleotide content was not fixed, DNA has variability
The % of each type of nucleotide differs from species to species.
In each species the amount of A = the amount of T and the amount of G = the amount of C

17. Variation in Base Sequence
Chargaff’s data suggest A is paired with T and G is with C
Average human chromosome contains about 140 million base pairs
Any 4 possible nucleotides can be present at each position, the total number of possible nucleotide sequence is 4140,000,000.

19. The Double Helix
Using Rosalind Franklin’s X-ray diffraction photo of DNA
Watson and Crick were able to determine the structure of DNA as a double helix
Sugar phosphate backbone
Paired bases on the inside
Purines are paired with pyrimidines

20. R. Franklin’s Photo 51

22. Replication of DNA Process of copying DNA
During replication each old strand of DNA serves as a template for a new strand
Replication is termed semi-conservative replication
One old strand is conserved or present in each daughter DNA.

23. Steps of Replication the old strands are unwound and “unzipped”
The H bonds between base pairs are broken . Helicase enzyme unwinds the molecule.
New nucleotides are positioned by complementary base pairing
Nucleotides are joined together to form new strands
Steps 2 and 3 are carried out by an enzyme complex called DNA polymerase.

25. Messelson & Stahl Experiment confirmed semi-conservative replication
Use N with different densities
Grew bacteria in medium containing heavy 15N.
Then switched bacteria to medium with light 14N

26. After 1 division only hybrid DNA molecules in the cells –intermediate density
After 2 divisions ½ DNA molecules were light and ½ were hybrid

28. Aspects of DNA Replication
DNA strands are anti-parallel to allow for complementary base pairing
One strand of DNA runs from 3’ to 5’ in one direction and the other runs 3’ to 5’ in the opposite direction

29. DNA polymerase synthesizes the daughter strand in the 5’ to 3’ direction.
Join nucleotides to the free 3’ end of th previous nucleotide
DNA polymerase cannot start synthesis of DNA chain
RNA polymerase lays down a short RNA primer complementary to the strand
Then DNA polymerase can join nucleotides to the 3’ end.

31. The leading strand- synthesized continuously toward the replication fork
The lagging strand
The parental strand runs in the 3’ to 5’ direction away from the fork
The daughter strand begins at the fork
Replication begins over and over by new DNA polymerase molecules as DNA winds
Replication of the lagging strand is discontinuous
Results in segments called Okazaki fragments.

32. Completing Replication
RNA primers are removed
While proofreading, DNA polymerase I removes the RNA primers and replaces with DNA nucleotides
DNA ligase joins the fragments

33. The DNA molecule gets shorter with each replication
DNA polymerase cannot replicate the end of the lagging strand after the RNA primer is removed
The DNA molecule gets shorter with each replication
Eukaryotic DNA molecules have a special nucleotide sequence called telomeres
Telomeres do not code for proteins
Series of repeats of short nucleotides sequence such as TTAAGGG

34. Mammalian cells grown in a culture divide about 50 times and then stop
The loss of telomeres signals the cell to stop dividing
Telomeres are added to chromosomes during gamete formation by the enzyme telomerase
In cancer cell telomerase is turned on in error. Contributes to cancer cells ability to divide without limit.

35. Prokaryotic vs. Eukaryotic Replication
Prokaryotic replication
Circular loop of DNA
Replication moves around DNA in one direction
Always occurs in 5’ to 3’ direction
106 base pairs per minute
Eukaryotic replication
Begins at numerous origins
Along the chromosome
Bi- directional
Starts at replication forks

37. Replication Errors Genetic Mutation
Permanent change in the sequence of bases
Mismatched
After proofreading
One mistake per 1 billion bases
DNA damage
Environmental factors
Free radicals in cells
UV radiation
Organic chemicals
Tobacco smoke
Pesticide
Pollutant

38. Genetic mutations need not harm an organism
Some may be beneficial
May cause cancer or genetic disease
Without changes in genetic material, evolution would not be possible.