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Molecular Genetics

Molecular Genetics - UNIT II MOLECULAR GENETICS History...

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UNIT II: MOLECULAR GENETICS History: We’ve said all along that DNA is the genetic material and that DNA encodes all of the information needed for the organism to function throughout life. This was not always known. For many years, biologists thought that proteins were the genetic material, because they thought that nucleic acids, with only 4 different nucleotides, were not complex enough to code for all this information; that proteins with 20 different amino acids would be a better molecule for this purpose. A key experiment to show that DNA was the hereditary material was done by Hershey and Chase . ____________________________________________________________________ Bacteriophage (“phage” ) = “virus” that infects bacterial cells. It: Multiplies in cells Makes many new phage particles, which burst out of the cell and then infect new cells. Made up only of a protein coat and a DNA core. DNA core Protein coat ____________________________________________________________________ Experiment : Radioactively labeled two separate phage preparations: 35 S amino acids for protein coat (with non-radioactive DNA core) 32 P nucleotides for DNA core (with non-radioactive protein coat) Steps in the experiment : 1. Let labeled phage attach to 2 separate bacterial cell cultures, wait long enough for infection to be initiated (a few minutes), and then use blender to separate cells from any external phage components. 2. Use centrifuge to separate cells from their surrounding liquid; any part of the phage that entered the cells will stay with the infected cells and be part of the cell pellet at the bottom of the centrifuge tube, whereas external components will stay in the liquid above the pellet. 1
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3. Measure radioactivity in cell pellet and in liquid in both preparations – where is the label? **Whatever part of the phage stays with the cells must be the hereditary material. Results : 35 S - protein label stayed in the liquid; not found in the cell pellet 32 P - DNA label was found in the cell pellet, not in the liquid Conclusion : phage attached to the cell, injected its DNA into the cell; protein coat remained outside of the cell and was only important for initial phage attachment, not heredity. DNA is molecule of heredity. ____________________________________________________________________ DNA STRUCTURE Deoxyribonucleic acid (full name) Made up of covalently linked subunits (monomers) called NUCLEOTIDES. Each nucleotide contains a central 5-carbon sugar, deoxyribose , and is attached to a phosphate group on one side and a nitrogen-containing base on the other side. **Watson and Crick came up with the structure in the 1950’s. OH 5' HO – P – O – CH 2 base phosphate O O 4' sugar 1' 3' 2' OH H deoxy 4 kinds of nucleotides in DNA corresponding to the 4 kinds of bases: Adenine (A) Guanine (G), Thymine (T) Cytosine (C) The first two of these (purines) are twice as large as the second two (pyrimidines). For illustrations, see Figure 13.3, p. 254, Krogh text.
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Molecular Genetics - UNIT II MOLECULAR GENETICS History...

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