8 DNA Replicaiton Study Guide Answers

8 DNA Replicaiton Study Guide Answers - BS 111 (Kopachik)...

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BS 111 (Kopachik) Answers to Study Guide for DNA Synthesis 1. Why did some biologists believe on theoretical grounds that proteins, not DNA, convey the genetic information? Before the Watson and Crick structure for DNA showed how vast information could be stored in the sequence of DNA bases it was thought by some that proteins had much more variation and information potential which would be required for the genetic material. Proteins were known to be quite variable in size, shape, with 20 amino acid residues in many different sequences and they were also found in chromosomes, known to transmit genetic information whereas DNA seemed to be relatively dull with any different cell or organism having the composition of the same four bases with deoxyribose sugar and phosphate. 2. Hershey and Chase were worried the 20 % of the S-labeled material remaining with cells might be part of the T2 phage genetic material. How could they provide further information that this label material, i.e. protein, was not genetic information? Hershey and Chase did follow up the 20-25 % of 35 S protein into the next generation of phage progeny. That is they took the phage made from the first infection and used them to infect another group of E. coli cells. They found that consistent with the protein not being genetic material the protein was greatly diminished to less than 1% in the new phage from another infection of new E. coli cells. The DNA 32 P counts were, in contrast, almost completely transferred into the next generation of phage. 3. Describe the Meselson and Stahl experiment? State the hypothesis, the results and the conclusion? Be able to predict the results if instead of semiconservative replication, the conservative or dispersive model were correct. Meselson and Stahl devised an experiment to distinguish among three possible versions of DNA replication. In conservative replication the two new strands form one helix and the old template strands remain a helix. In semiconservative replication each new helix is made of one new and one old strand. In dispersive replication all four strands of the two helices are made of a mixture of sequences from both new and old strands. By labeling E. coli DNA for a number of generations to make nearly the entire complement of N in bases with a heavy 15 N isotope in the dNTPs and then shifting to a light 14 N source for the dNTPS for two doublings of the DNA they were able to follow the DNAs from three generations of different densities. The DNAs got density differences from their different masses depending on whether they have the heavy or light N could be separated by the technique of density gradient ultracentrifugation and visualized by their absorption of ultraviolet light. The all H-H DNA (both Watson and Crick strands 15 N) made in the first doubling when replicated to get light N formed every molecule with the composition of H-L DNA ( a result inconsistent with conservative method which would have predicted two type of molecule a H-H and a L-L). Then in the next
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8 DNA Replicaiton Study Guide Answers - BS 111 (Kopachik)...

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