Discovery of a “transforming principle”

1. Discovery of a “transforming principle” • Frederick Griffith, 1928 • Pneumonia (Diplococcus pneumoniae) infects mice. • Mice develop pneumonia and die. • Two types of bacteria: • R bacteria rough coat - no pneumonia • S bacteria smooth coat- pneumonia • Coat type is associated with virulence.

4. Avery, MacLeod and McCarty • Isolated DNA from heat killed type S bacteria and injected it along with type R bacteria into mice • The mice died and their bodies contained active type S bacteria • Concluded that DNA passed from type S bacteria to type R, making it lethal

5. DNA is the genetic material • Alfred Hershey and Martha Chase, 1953 • Viruses can infect the E. coli bacteria. • A virus has protein “head” and DNA core. • Infection occurs when virus injects DNA into a bacterial cell.

6. Fig. 9.4

7. Chemical Nature of Nucleic Acids Levene’s work lead to the idea that the structure of DNA was a simple repeating unit of GATCGATCGATC This is why no one thought it could be the genetic material with a structure this simple • Purines - Large organic bases • Adenine and Guanine • Pyrimidines - Small organic bases • Cytosine and Thymine, Uracil (RNA)

8. CHARGAFF’S RULES • In all DNA molecules: • The proportion of adenine is equal to thymine • A = T • The proportion of guanine is equal to cytosine • G = C

9. DNA bases pair via hydrogen bonds • Erwin Chargaff observed: • % adenine = % thymine • % guanine = % cytosine • Complementary bases pair: • A and T pair • C and G pair

20. Replication as a process • Double-stranded DNA unwinds. The junction of the unwound molecules is a replication fork. A new strand is formed by pairing complementary bases with the old strand. Two molecules are made. Each has one new and one old DNA strand.

22. Primase adds short primer to template strand Binding proteins stabilize separate strands Helicase unwinds parental double helix Ligase joins Okazaki fragments and seals other nicks in sugar-phosphate backbone Exonuclease removes RNA primer and inserts the correct bases DNA polymerase binds nucleotides to form new strands Enzymes in DNA replication

23. 3’ 5’ 3’ 5’ 5’ 3’ 3’ 5’ Binding proteins prevent single strands fromrewinding. Primase protein makes a short segment of RNA complementary to the DNA, a primer. Replication Helicase protein binds to DNA sequences called origins and unwinds DNA strands.

24. Overall direction of replication 3’ 5’ 3’ 5’ 3’ 3’ 5’ 5’ Replication DNA polymerase enzyme adds DNA nucleotides to the RNA primer. DNA polymerase proofreads bases added and replaces incorrect nucleotides.

25. Overall direction of replication 3’ 3’ 5’ 5’ Okazaki fragment 3’ 5’ 3’ 5’ 5’ 3’ Replication Leading strand synthesis continues in a 5’ to 3’ direction. Discontinuous synthesis produces 5’ to 3’ DNA segments called Okazaki fragments.

26. 3’ 5’ 3’ 5’ 3’ 5’ 3’ 3’ 5’ 3’ 5’ 5’ Replication Discontinuous synthesis produces 5’ to 3’ DNA segments called Okazaki fragments Exonuclease enzymes remove RNA primers.

27. 3’ 3’ 5’ 3’ 5’ 3’ 3’ 5’ 5’ Ligase forms bonds between sugar-phosphate backbone. Replication Exonuclease enzymes remove RNA primers.

31. PROTEIN PRODUCTION • First step in protein production is transcription • Transcription makes a copy of the DNA called messenger RNA mRNA • Called messenger RNA because it carries the genetic message from the DNA to the protein factory, the ribosomes in the cytoplasm • Transcription is directed by the enzyme RNA polymerase

32. RNA has a slightly different sugar • RNA has U instead of T • RNA is also a nucleic acid Nitrogenous base(A, G, C, or U) Phosphategroup Uracil (U) Sugar(ribose) Figure 10.2C, D

33. RNA • Messenger RNA is synthesized from DNA by a process called transcription • This process is similar to DNA replication in that it depends on complementary base pairing • DNA RNA • Guanine Cytosine • Cytosine Guanine • Thymine Adenine • Adenine Uracil