2. DNADNA Chapter 25

3. Discovery of DNA Function A. Discovery of DNA 1. first isolated from cells in 1868 2. unknown function B. Griffith Experiments (1928) 1. worked with a pneumonia-causing bacteria 2. two strains R strain: non-fatalS strain: fatal

4. 3. experiment: mice injected with a. R (rough) cells - lived b. S (smooth) cells - died c. heat killed S cells - lived d. heat killed S cells + live R cells - mice died !! 4. analysis of last group showed live S cells and no R cells

6. 1.Transformation of R cells into S cells meant that some Genetic Material must have gone from into the R-cells from the S. 2.Source of Genes: protein or nucleic acids

7. Hershey-Chase Experiment 1. Was DNA or proteins the genetic material? 2. Bacteriophages - viruses used a. protein coat b. nucleic acid interior (DNA core)

8. 3. Radioactive labeling of protein & DNA

10. 4. Results showed that DNA directed the formation of new viruses, not the protein coat

11. Discovery of DNA Structure Components of DNA 1. DNA consists of a sugar, base and phosphate group 2. one of 4 possible bases: adenine (A) guanine (G) cytosine (C) thymine (T) Purines Pyrimidines

12. DNA Structure – The Twisted Ladder

13. The Nucleotide: 1.Sugar 2.Phosphate Group 3.Nitrogenous Base Ribose Sugar Phosphate Base

14. Experiments - 1949 1. the amount of A = amount of T 2. the amount of C = amount of G 3. ratios vary by species

15. X-Ray experiments 1.Used X-rays to examine structure of DNA 2.Observed that a. DNA is helical b. DNA exists in long strands c. DNA is highly repetitive

16. Watson-Crick analysis 1. Constructed a model of DNA structure 2. Phosphate groups from backbone 3. Double-stranded molecule 4. Base=pairing of A-T and G-C 5. Helical shape

19. DNA Replication & Repair A. Replication 1. replication is semi-conservative 2. major molecules a. helicase - unwinds DNA strands b. DNA polymerase - copies DNA c. DNA ligases - seal newly formed DNA strands

22. B. DNA Repair 1. DNA polymerase “proofreads” the new strands 2. DNA ligases help to repair mismatched bases

23. DNA Organization A. Chromosome structure 1. DNA is organized into long molecules called chromosomes 2. chromosome are stabilized by proteins called histones 3. histones organize the DNA