Additional specificity of pairing dictated by

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additional specificity of pairing Dictated by structure of bases Each base pair forms a different # of hydrogen bonds Adenine and thymine form 2 bonds, cytosine and guanine form 3 bonds
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N H O CH 3 N N O N N N N H Sugar Sugar Adenine (A) Thymine (T) N N N N Sugar O H N H N H N O H H N Sugar Guanine (G) Cytosine (C) Figure 16.8 H
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Concept 16.2: Many proteins work together in DNA replication and repair
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The Basic Principle: Base Pairing to a Template Strand Since 2 strands of DNA are complementary Each strand acts as a template for building a new strand in replication
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DNA replication Parent molecule unwinds, 2 new daughter strands are built (a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C. (b) The first step in replication is separation of the two DNA strands. (c) Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand. (d) The nucleotides are connected to form the sugar-phosphate backbones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand. A C T A G A C T A G A C T A G A C T A G T G A T C T G A T C A C T A G A C T A G T G A T C T G A T C T G A T C T G A T C Figure 16.9 a–d
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Figure 16.10 a–c Conservative model. The two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix. Semiconservative model. The two strands of the parental molecule separate, and each functions as a template for synthesis of a new, comple- mentary strand. Dispersive model. Each strand of both daughter mol- ecules contains a mixture of old and newly synthesized DNA. Parent cell First replication Second replication Replication is semiconservative 2 new daughter molecules will have one old strand & one new strand (a) (b) (c)
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Experiments performed by Meselson and Stahl Supported semiconservative model of DNA replication Figure 16.11 Matthew Meselson and Franklin Stahl cultured E. coli bacteria for several generations on a medium containing nucleotide precursors labeled with a heavy isotope of nitrogen, 15 N. The bacteria incorporated the heavy nitrogen into their DNA. The scientists then transferred the bacteria to a medium with only 14 N, the lighter, more common isotope of nitrogen. Any new DNA that the bacteria synthesized would be lighter than the parental DNA made in the 15 N medium. Meselson and Stahl could distinguish DNA of different densities by centrifuging DNA extracted from the bacteria. EXPERIMENT The bands in these two centrifuge tubes represent the results of centrifuging two DNA samples from the flask in step 2, one sample taken after 20 minutes and one after 40 minutes.
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