BCH 4054                          HOUR TEST 3            NAME _____________________
 November 13, 1995
          
(12) 1.    Draw the structures of guanine and cytosine,
           and circle the nitrogen atoms in each that are
           derived biosynthetically from glutamine.









(8)  2.    Inosine monophosphate is the common precursor of AMP and GMP. 
           Identify by giving all of the reactants and products of (structures
           not necessary):
           (a) The GTP requiring reaction between IMP and AMP.






           (b) The ATP requiring reaction between IMP and GMP.







(10) 3.    The enzyme AMP deaminase converts AMP to IMP and NH4+.  If this
           reaction is coupled to the two steps in which synthesis of AMP from
           IMP occurs (i.e., IMP ----> intermediate -----> AMP), the sum of the
           three reactions forms a cycle.  Write out these three reaction steps,
           showing other reactants and products (structures not necessary), and
           then give the overall reaction catalyzed by the cycle.  (This is
           called the purine nucleotide cycle, and is believed to play an
           important metabolic role in muscle.)
           




















(6)  4.    The regulation of nucleotide biosynthesis is complex, with various
           nucleotides serving as regulatory effectors.  Match the nucleotides
           in the list at the right with the enzymes and the effects indicated
           in the list at the left by putting the appropriate number or numbers
           in the blank.  (More than one nucleotide may apply).

     ____  Inhibition of aspartate transcarbamoylase             (1)   ATP
                                                                 (2)   dATP
     ____  Activation of aspartate transcarbamoylase             (3)   GMP
                                                                 (4)   CTP
     ____  Inhibition of ribonucleotide reductase                (5)   AMP
                                                                 (6)   UMP
     ____  Inhibition of IMP ---> XMP

     ____  Inhibition of carbamoyl phosphate synthetase II

     ____  Inhibition of PRPP ---> phosphoribosylamine


(5)  5.    Ribonucleotide reductase has two kinds of allosteric sites.  Explain
           what property of the enzyme each site regulates.  What is the
           rationale for such a complex regulatory mechanism?







(5)  6.    The purine and pyrimidine biosynthetic pathways are complicated by
           the fact that some processes occur at the monophosphate level, some
           at the diphosphate level, and some at the triphosphate level.  For
           each of the following, indicated in the blank whether a nucleoside
           mono-, di-, or tri- phosphate is involved.

           _______   Amination of the uracil ring to form cytosine.

           _______   Decarboxylation of orotic acid.

           _______   Methylation of the uracil ring to form thymine.

           _______   Amination of the hypoxanthine ring to form guanine.

           _______   Reduction of ribose to deoxyribose.
           
(7)  7.    You have prepared DNA from two organisms isolated from the swamps of
           south Georgia, designated culture A and culture B.  DNA from
           culture A contains 24% G, while DNA from culture B contains 30% G. 
           Complete the following table for the expected composition of the
           other purine and pyrimidine bases.

                           %G          %A         %T         %C        Total

           Culture A       24         ____       ____       ____       100%

           Culture B       30         ____       ____       ____       100%

           DNA from which organism will have the higher melting temperature?
           
           
           
           
           
(6)  8.    Denaturing DNA by increasing the temperature is sometimes called
           "melting".  What changes occur upon melting in:
     
           (a) the secondary structure of DNA?




           (b) absorbance at 260 nm?




           (c) viscosity of the DNA solution?




(8)  9.    From the following DNA sequences, write the complementary sequence
           under it (in the 3' to 5' direction), and circle the bases of the
           resulting double stranded DNA which are palindromic sequences at
           least four base pairs in length.

           (a)  5'-GCTTCGAAC-3'            (b)   5'-CTACTACTA-3'
                3'-                              3'-



           (c)  5'-GCGCAACG-3'             (d)   5'-TTATTGCAAG-3'  
                3'-                              3'-



(6)  10.   Suppose that negatively supercoiled DNA with L=23, T=25, and W=-2 is
           acted upon by topoisomerase I.  After one catalytic cycle, what would
           be the values of L, T, and W?



           Suppose that negatively supercoiled DNA with L=23, T=25, and W=-2 is
           acted upon by DNA gyrase and ATP.  After one catalytic cycle, what
           would be the values of L, T, and W?



(6)  11.   Describe an Okasaki fragment.  When and where is it made, and what
           happens to it?











(9)  12.   Explain the roles of helicases, DNA topoisomerase II, and SSB protein
           in DNA replication.



















(6)  13.   What is the function of the 3'-5' exonuclease activity of bacterial
           DNA polymerase III?

















(6)  14.   What is the function of the 5'-3' exonuclease activity of bacterial
           DNA polymerase I.