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BIOL 205
Ian D Chin- Sang

BIOL205:week 7, lecture 20 DNA Structure and Replication DNA: The genetic material • to fulfill its role, the genetic material must meet several criteria 1. Information: It must contain the information necessary to make an entire organism 2. Transmission: It must be passed from parent to offspring 3. Replication: It must be copied • In order to be passed from parent to offspring 4. Variation: It must be capable of changes • to account for the known phenotypic variation in each species • the data of many geneticists, including Mendel, were consistent with these four properties • however, the chemical nature of the genetic material cannot be identified solely by genetic crosses • indeed, the identification of DNAas the genetic material involved a series of outstanding experimental approaches Frederick Griffith 1928: Experiments with Streptococcus pneumoniae • Griffith studied a bacterium Streptococcus pneumoniae –causes pneumonia in humans- normally lethal in mice • S. pneumoniae comes in two strains ◦ Type S: Smooth ▪ secrete a polysaccharide capsule ▪ protects bacterium from the immune system of animals- LETHAL ▪ produce smooth colonies on solid media ◦ Type R: Rough ▪ unable to secrete a capsule ▪ produce colonies with a rough appearance ▪ NOT LETHAL Transforming R cells into S cells → as expected No longer has protective coat, so the mouse's immune systemcandealw/ it • Griffith concluded that something from the dead type S was transforming type R into type S • he called this process transformation • the substance that allowed this to happen was termed the transformation principle • Griffith did not know what it was • the nature of the transforming principle was determined using experimental approaches that incorporated various biochemical techniques Avery, MacLeod and McCarty, 1944 • Avery, MacLeod and McCarty realized that Griffith’s observations could be used to identify the genetic material • at that time, it was known that DNA, RNA, proteins and carbohydrates are major constituents of living cells • they prepared cell extracts from type S cells containing each of these macromolecules → show DNAis the transforming principle, by destroying it the S strain doesn't kill the mouse Hershey and Chase Experiment with Bacteriophage T2 • in 1952,Alfred Hershey (Nobel prize -1969) and Martha Chase provided further evidence that DNAis the genetic material • they studied the bacteriophage T2 ◦ it is relatively simple since its composed of only two macromolecules: DNAand protein • didn't know what genetic materia phaged used to infect bacteria The phage genetic material is DNA • add radioactive sulfur: incorporates into cystine and methionine (amino acids) • put in blender and centrifuge • most of the sulfur recovered from phage gosts (empty phages) • add radioactive phosphorous incorporate into DNAbackbone • put in blender and centrifuge • most of the phosphorous recovered from bacteria Centrifuges
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