Microbial Genetics Exam          Fall 1999

Your Name:____________________                                     

1.     a) What is an allele?

        b) Bacteria genomes are haploid. (i) Define haploid, then (ii) explain how bacteria can have alleles, and (iii) describe how allele behavior can be studied in bacteria, even though bacteria are haploid and reproduce by binary fission.

2.     You have isolated five (5) mutant strains of a bacterium.  The wild-type strain is a prototroph, and the mutants are auxotrophs that cannot grow unless provided with the amino acid joline.  You label your mutants jol-1 through jol-5.

        a) What does the term complementation mean?  What do you conclude if two of your jol mutants complement each other?

        b) fill in the complementation results, with + and – symbols, in the table below, assuming that there are only three jol genes, and assuming that jol-1 is identical to jol-3 and jol-5.

3.     Study the following nucleotide sequences, and indicate whether they, when found in a double helix, would be a palindrome or not.

        5’GGGGGGG 3’

        5’ GGATCC 3’

        5’ GACT 3’

        5’ AAATTT 3’

 

4.     Describe penicillin enrichment, and explain how it makes it easier to isolate auxotrophs.

5.     You mix two diauxotrophic strains: A: his^- leu^- pro⁺ lys⁺, and B: his⁺ leu⁺ pro^- lys^-, and plate 10⁹ cells on minimal media agar plates.  You get 1,000 colonies. a) What would Lederberg have called this phenomenon, back in the early 1950s?

        b) Can you rule out reversion as the explanation for the (large) number of colonies?  Explain.

        c) How would you determine whether the mechanism at work was conjugation, transduction, or transformation.

6.     Describe the steps of a typical natural transformation event, beginning with live donor and recipient cells and ending with a recipient having a new genotype.

7.     You have done a series of interrupted mating experiments with a newly isolated bacterium whose genome has not been characterized (as linear or circular) or mapped.  Would you guess, before analyzing the data, that the genome is linear or circular, and why?

        Here are the results, with 4 Hfrs

Hfr strain	Order of transfer First                                                                        Last
1	phe	fum	apr	mal
2	may	glc	pho	mal
3	fum	phe	try	may
4	glc	may	try	phe

        Is the genome circular or linear?

         Draw a map of the genome, which should include the genes in their correct order, along with arrowhead symbols (►) indicating the position of the 4 Hfrs, as well as the direction of transfer of genes by each Hfr.

8.     In the Hershey-Chase experiment, why were ³⁵S and ³²P chosen as the radioisotopes?  What was the blender used for?

9.     Write out the sequence of an RNA molecule that could form a stem-loop with 8 nucleotides in the stem, and 5 nucleotides in the loop.  Label the 5’ and 3’ ends.  Do not draw the stem-loop.  Just write the primary sequence.