DNA is the information carrying molecule in genetics Griffiths: transformation Griffiths mixed live R strain bacteria with heat killed S (smooth=pathogenic) Streptococcus Pneumoniae bacteria. The mixture was able to kill mice. He concluded that the S bacteria had been TRANSFORMED by something in the killed R bacteria. Avery: transforming agent is DNA Avery repeated Griffith’s experiments, but used purified S strain components. He found that only DNA (not proteins) worked. Luria: damage to nucleic acids, not proteins, affects viral heredity Separate radiation damage to viral components Hershey & Chase: radioactive DNA, not viral proteins, enter cells Viruses labeled separtely with 35 S(protein) and 32 P(DNA) Chargaff’s rules: C=G, A=T (adenine, guanine, cytosine, thyamine) Hold for all DNA extracted from any organism.

Watson Crick Base Pairs Prepared by Guillermo Moyna, 1999.

Models of DNA synthesis

Meselson & Stahl: conservative, semiconservative, or dispersive DNA synthesis? Follow DNA strands with heavy nitrogen isotope. DNA strands can be separated according to density on a cesium chloride gradient, with the position of the bands depending on the ratio of N14 to N15.

## # # #X X# ## ># + # -----> #X + X# ## # # #X X# ## ## XX ## > ## + XX ## ## XX Second generation result showed that this : #X #X XX #X ---> #X + XX #X #X XX happened instead of thi s: #X X# #X #X #X XX #X XX XX #X -----> XX + X# #X ## XX #X XX #X #X XX 1 st generation M-S result: moving from N 15 to N 14 produces a single band halfway between N 14 & N 15, not an N 14 and an N 15 band.

Replication at these forks requires DNA polymerases. In E. coli two enzymes, DNA pol I and DNA pol III are involved. In eukaryotes there are many (more than 10) polymerases. DNA polymerases can’t start from nothing- they need a short bit of complementary DNA or RNA called a primer.

A hard day at the replication fork: just a bit more detail. DNA polymerase III on the leading strand has it easy- it just keeps adding nucletide to the 3’ end. Notice that DNA polymerase III on the lagging strand has to keep copying back away from the fork because it can only add stuff to the 3’ end. This is massively important for all sorts of reasons! The copying stops when it reaches a new RNA primer and DNA pol III falls off. DNA ligase stiches the new bits (Okazaki fragments) together. DNA pol I replaces the RNA primer with DNA.