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Discussion 2 - CELL BIOLOGY(BIO 320 FALL 2009 DISCUSSION 2...

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CELL BIOLOGY (BIO 320) FALL 2009 DISCUSSION 2 – KEY _______________________________________________________________________ _ 1. What are the four weak (noncovalent) interactions that determine the conformation of a protein? Hydrogen bonds, ionic bonds, van der Waals attractions, and hydrophobic force. 2. When egg white is heated, the proteins in it denature and the egg white hardens. The denaturation is irreversible, but hard-boiled egg white can be dissolved by heating it in a solution containing a strong detergent (such as sodium dodecyl sulfate) together with a reducing agent, like 2-mercaptoethanol. Neither reagent alone has any effect. (a) Why does boiling an egg white cause it to harden? (a) Heating egg-white proteins denatures them, allowing them to interact with one another in ways that were not possible at the lower temperature. These interactions form a tangled meshwork of polypeptide chains. In addition, interchain disulfide bonds also form, so that hard-boiled egg white becomes one giant macromolecule. (b) Why does it require both a detergent and a reducing agent to dissolve the hard-boiled egg white? Dissolving hard-boiled egg white requires a strong detergent to overcome the noncovalent interchain interactions and mercaptoethanol to break the covalent disulfide bonds. Together, but not separately, the two reagents eliminate the bonds that hold the tangled protein chains in place. 3. Cro is a bacterial gene regulatory protein that binds to DNA to turn genes off. It is a symmetrical ‘head-to-head’ dimer. Each of the two subunits of the dimer recognizes a particular short sequence of nucleotides in DNA. If the sequence of nucleotides recognized by one subunit is represented as an arrow ( ), so that the ‘head’ of the arrow corresponds to DNA recognized by the ‘head’ of the subunit, which of the following sequences in DNA represents the binding site for the Cro dimer? (a) ( ) (b) ( ) (c) ( ) (d) ( ) (e) could be more than one of the above.
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3. Choice (b) is the only arrangement of DNA binding sites that matches the arrangement of subunits in the ‘head-to-head’ Cro dimer. The DNA that corresponds to such an arrangement is known as a palindrome.
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