BCH 4054                          HOUR TEST 4            NAME _____________________
 December 1, 1995
          
(10) 1.    What type of chemical modification of the DNA
           resulting in a mutation is produced by the
           following mutagens?

           (a)  nitrous acid








           (b) benzopyrene (an intercalating agent)











(6)  2.    Why do cells exposed to visible light following irradiation with
           ultraviolet light have a greater survival rate than cells kept in the
           dark after irradiation with ultraviolet light?














(6)  3.    When proofreading fails to correct a base-pair mismatch, how do the
           repair systems recognize which is the incorrect base in the mismatch? 
           Why is this process called long-patch repair?
           
           
           
           
           
           
           
           
           
           
           
(8)  4.    Describe the role of a glycosylase and an AP endonuclease in excision
           repair. 

















(12) 5.    Several proteins involved in recombination were identified as
           products of "rec" genes.  Describe the properties and function of:

           (a) the recBCD complex









           (b) recA protein









(6)  6.    Why are high rates of mutation observed in regions of DNA that
           contain methylcytosine?
           
           
           
           
           
           
           
           
           
           
           
           
(6)  7.    Initiation of DNA synthesis occurs at a DNA region known as the
           origin of replication.  Initiation of RNA synthesis occurs at DNA
           regions known as promoters.  Describe the general features of a
           prokaryotic promoter site.












(6)  8.    Unlike DNA polymerase, RNA polymerase has no 3'-5' exonuclease
           activity.  Why is this activity not important for RNA synthesis?











(10) 9.    In the splicing of eucaryotic mRNA, introns are removed as lariat
           structures.  
           (a)  Diagram a typical lariat structure, indicating the 3' end, the
                5' end, and the specific chemical linkage forming the branch of
                the lariat.
           (b)  Explain how the small nuclear RNA's U1, U2, and U5 are involved
                in the splicing process.














(6)  10.   How is the splicing of introns from eukaryotic tRNA precursors
           different from that of mRNA?















(10) 11.   Prior to transport out of the nucleus, eucaryotic mRNA is modified at
           both the 3'- and 5'- ends.  Describe how each end is modified.


















(8)  12.   In the regulation of á-galactosidase synthesis in E. coli, two
           classes of constitutive regulatory mutants are found:  I- and Oc.
           (a) What is meant by a constitutive mutant?




           (b) What is wrong with the I- mutant?




           (c) What is wrong with the Oc mutant?






(6)  13.   Explain how cyclic AMP regulates the expression of the á-
           galactosidase gene in E. coli.