9 A virus that grows in bacteria (bacterial viruses are called protein bacteriop
ID: 80356 • Letter: 9
Question
9 A virus that grows in bacteria (bacterial viruses are called protein bacteriophages) can replicate in one of two ways. In the prophage state, the viral DNA is inserted in to the bacterial chromosome and is copied along with the bacterial genome each ne Cino gene time the cell divides. In the lytic state, the viral DNA is released from the bacterial chromosome and replicates many times in the STATE cell. This viral DNA then produces viral coat proteins that together with the replicated viral DNA form many new virus particles that burst out of the bacterial cell. These two forms of growth are controlled by two transcriptional regulators, called cl Cro protein and Cro, that are encoded by the virus (see figure to the right). In the prophage state cl is expressed; in the lytic state, Cro is Co gene eigene expressed. In addition to regulating the expression of other genes, cl represses the Cro gene, and Cro represses the cl gene. LYTIC NO clGENE When bacteria containing the phage in the prophage state are briefly irradiated with UV light, cl protein is degraded. A. What will happen next? B. Will the change in (A) be reversed when the UV light is switched off? C. Why might this response to UV light have evolved? 10 The structure of a lipid bilayer is determined by the particular properties of its lipid molecules. What would happen if... A. Phospholipids had only one hydrocarbon tail instead of two? B. The hydrocarbon tails were shorter than normal, say, about 10 carbon atoms long? C. All of the hydrocarbon tails were saturated? D. All of the hydrocarbon tails were unsaturated? E. The bilayer contained a mixture of two kinds of phospholipid molecules, one with two saturated hydrocarbon tails and the other with two unsaturated hydrocarbon tails?Explanation / Answer
Answer:
9. A. UV light throws the switch from the prophage to the lytic state: when cI protein is destroyed, Cro is made and turns off the further production of cI. The virus produces coat proteins, and new virus particles are made.
B. When the UV light is switched off, the virus remains in the lytic state. Thus, cI and Cro form a gene regulatory switch that "memorizes" its previous setting.
C. This switch makes sense in the viral life cycle: UV light tends to damage the bacterial DNA, thereby rendering the bacterium an unreliable host for the virus. A prophage will therefore switch to the lytic state and leave the "sinking ship" in search for new host cells to infect.
10. A. You would have a detergent. The diameter of the lipid head would be much larger than that of the hydrocarbon tail, so that the shape of the molecule would be a cone rather than a cylinder, and the molecules would aggregate to form micelles rather than bilayers.
B. Lipid bilayers formed would be much more fluid, as the tails would have less tendency to interact with one another. The bilayers would also be less stable, as the shorter hydrocarbon tails would be less hydrophobic, so the forces that drive the formation of the bilayer would be reduced.
C. The lipid bilayers formed would be much less fluid. Whereas a normal lipid bilayer has the viscosity of olive oil, a bilayer made of the same lipids but with saturated hydrocarbon tails would have the consistency of bacon fat.
D. The lipid bilayers formed would be much more fluid. Also, because the lipids would pack together less well, there would be more gaps and the bilayer would be more permeable to small, water-soluble molecules.
E. If we assume that the lipid molecules are completely intermixed, the fluidity of the membrane would be unchanged. In such bilayers, however, the saturated lipid molecules would tend to aggregate with one another because they can pack so much more tightly and would therefore form patches of much-reduced fluidity. The bilayer would not, therefore, have uniform properties over its surface.
F. The lipid bilayers formed would have virtually unchanged properties. Each lipid molecule would now span the entire membrane, with one of its two head groups exposed at each surface.
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