Chapter 16

 Fredrick Griffith – 1928  Studying Streptococcus pneumonia  2 strains – Rough (non- lethal) and Smooth (lethal)  Through a series of experiments discovered transformation

 Oswald Avery – 1944  Used Griffith’s transformation data  Wanted to see which substance transferred  DNA, RNA, or Proteins  Treated each of the three substances sequentially  Treatment inactivated one type of molecule

 Alfred Hershey and Martha Chase – 1952  Were not convinced it was DNA  Used a bacteriophage with radioactive labels  Sulfur – protein  Phosphorus – DNA  Upon analysis found the DNA entered the bacteria

 Erwin Chargaff  Already knew composition of a nucleotide  Sugar (deoxyribose/ribose, phosphate group, and a nitrogenous base  Chemical analysis showed A= T, C= G

 Three groups were racing to discover the 3D shape of DNA  Linus Pauling  Maurice Wilkins/Rosalind Franklin  James Watson/Franklin Crick

 Watson and Crick – 1953  Present their molecular model for DNA  Double helix  Antiparallel  Hydrogen bonding between bases  Nobel prize in 1962

 Watson and Crick also discovered the process of replication  Copying the DNA strand  Predicted that DNA was semiconservative  Each new strand has a parent strand attached

 Replication begins at Origins of Replication  Short, specific DNA sequences  Creates a replication fork  Involves several enzymes  Helicases  Single strand binding proteins  Topoisomerases  Primase

 Each strand serves as a template for new strands  DNA polymerases  Requires a primer of RNA to begin  Strands are antiparallel  5’ – phosphate end  3’ – sugar-OH end  Only add bases to the 3’ end – BUT must go 5’ – 3’ – problem?

 Two strands will exist when replication begins  Leading strand  DNA polymerase III at fork adds nucleotides towards the fork  Requires one primer  Lagging strand  Made as a series of segments

 Lagging Strand  Requires multiple RNA primers  DNA pol III adds nucleotide to RNA primer – Okazaki fragment  DNA pol I adds DNA to replace RNA primer at junction of Okazaki fragments  DNA ligase bonds the pieces of DNA

 Errors in replication occur about 1 in 10 billion  Mismatch repair is when enzymes remove a base that has been incorrectly placed  If the change is not corrected and becomes permanent then it represents a mutation  Sometimes the region where the error is located is excised by enzymes called nuclease and then new DNA place

 Replication leaves the 5’ end uncopied  Telomeres are nucleotide sequences at the 3’ and 5’ ends  Prevent the eroesion of genes located near an end  Become shorter every round of replication  Do not get shorter in germ cells - telomerase

 DNA can be up to 4 cm long in an eukaryote  Combines with histone proteins to become chromatin  When ready to divide the chromatin condenses into packages (chromsomes)