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Lec 29-30 DNA replication.docx

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BIOL 1010
Igor Kovalchuk

DNA Replication Lecture 29-30 Structure - DNA replication and the cell cycle - DNA is reproduced by Semiconservative replication o Conservation of the original helix o The Meselson-Stahl Experiment o Semiconservative replication in Eukaryotes o Origins, Forks and Units of Replication - Enzymes involved in DNA synthesis in Bacteria o DNA polymerase I o Synthesis of Biologically Active DNA o DNA polymerase II and III DNA is hereditary material - In 1928, Frederick Griffith showed that genetic material is a specific molecule - Two distinguishable strains of Streptococcus pneumonia (a bacterium that causes pneumonia in mammals): with smooth colonies (S) and rough colonies (R) - Cells of the smooth strain were encapsulated with a polysaccharide coat and cells of the rough strain were not - These alternative phenotypes were inherited Transformation of bacteria - The S strain of the bacterium, which was protected from a mouse’s defensive system by a capsule, was pathogenic; - The R strain, a mutant lacking the capsule, was nonpathogenic; - Heat-killed S cells were harmless - A moisture of heat-killed S cells and live R cells caused pneumonia and death - Live S bacteria could be retrieved from the dead mice injected with the mixture - Griffith concluded that molecules from the dead S cells had genetically transformed some of the living R bacteria into S bacteria - In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod discovered that the transforming agent had to be DNA - More evidence came from studies of bacteriophages- viruses that infect bacteria - The Hershey-Chase Experiment - Viral proteins labeled with radioactive sulfur, remained outside the host cell during infection. - 32P labeled viral DNA entered the bacterial cell Additional evidence that DNA is genetic material - A eukaryotic cells doubles its DNA content prior to mitosis - During mitosis, the doubled DNA is equally divided between two daughter cells - An organism’s diploid cells have twice the DNA as its haploid gametes - In 1947, Erwin Chargaff analyzed the DNA content of different organisms: o DNA composition is species-specific o The amount of ratios of nitrogenous bases vary from one species to another - The number of adenine (A) residues approximated the number of thymine’s (T), same was true for guanines (G) and cytosine’s (C) - The A=T and G=C equalities became known alter as Chargaff’s rules Rosalind Franklin’s experiments - DNA is helix with a uniform width of 2 nm - Purine and pyrimidine bases are stacked .34nm apart - The helix makes one full turn every 3.4 nm along it’s length - There are ten layers of nitrogenous base pairs in each turn of the helix Watson and Crick- building models to confirm to x-ray data - To be consistent with a 2nm width, a purine on one strand must pair (by hydrogen bonding_, with a pyrimidine on the other antiparallel helix The double helix - In April 1953- new model for DNA structure, the double helix was published - Watson and Crick’s model of replication - “Two chains unwind and separate. Each chain then acts as a template of the formation onto itself of a new companion chain” - Upon the replication of the double helix each of the two daughter molecules will have one old strand (parental) and one newly made strand. Such a process is called semiconservative replication Semiconservative model Alternative models - Conservative replication- complementary chains are synthesized as for semiconservative, but the two newly created strands then come together and
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