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Bio 1A03 Study Guide and Practice Questions Mid-Term - II------2

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● DNA is the macromolecule that determines the characteristics of the cell. ● Purines and pyrimidines are paired along the DNA helix. ● Watson and Crick proposed that DNA consists of a pair of template chains which are complementary to each other, and that prior to replication, the hydrogen bonds are broken between these complementary strands, which allows for unwinding and separation. By saying that a strand is complementary to another, what is meant is that each strand contains the information necessary to reconstruct the other. Semiconservative Model for Replication : ● Replication begins with 1 parent DNA and by the end, there would be 2 new helices which are exact copies of the parent strand. ● This occurs in two main steps: (1) – duplex separates; (2) – 2 daughter strands are synthesized from 2 template strands. ● Other researchers hypothesized that perhaps DNA replicated in a more of a conservative fashion, with the two complementary parental strands somehow coming back together after replication. Other researchers imagined that perhaps DNA replication is more dispersive, with all four strands somehow combining into a mixture of old and new DNA strands. Experiment: ● E.Coli was cultured in a medium with nucleotide precursors containing radioactively labeled 15N. 28 of 56 ● E.Coli was then transferred into N14-rich medium which meant that every new strand would have the N14 isotope, if the model is indeed semiconservative. ● Samples were extracted and centrifuged to separate DNA based on different densities (15N DNA is heavier, so it would be in the bottom of the tube). ● DNA from bacteria that had been growing in the media containing the 15N isotope had only one distinct band. In contrast, it was observed that after transfer into and then one round of replication in the medium containing 14N isotope, that the DNA also appeared as a single band, but with lower density and so was positioned higher than the original 15N band in the centrifugation gradient. Therefore this band must contain some hybrid of the 14N and 15N DNA à conservative model rejected since there was no individual

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