Microbial Genetics Exam          Fall 1999

Your Name:____________________                                     

1. (10 points) What base is formed when cytosine is deaminated? …when 5-methylcytosine is deaminated.  Which event is more mutagenic, and why?

2 (15 points) Gene regulation in eukaryotes involves the activation/deactivation of transcription factors, just as with prokaryotes.  The difference between prokaryotic and eukaryotic gene regulation lies in the mechanisms used for activation/deactivation.  List three (3) mechanisms used by eukaryotes to modulate the activity of transcription factors, and choose one of the following– steroid hormone/receptors, AP1, or CREB (from your text) as an example to illustrate one of the mechanisms.

3. (10 points) In the fluctuation test experiment of Luria and Delbrück, what was the significance of the wide variation in the number of colonies obtained by plating the small cultures on T1-containing agar plates?  What is the significance of the “jackpot” culture?

4. (10 points) a) What is the effect of cytosine methylation on eukaryotic gene expression?

b) What are the two mechanisms of cytosine methylation, and how do these mechanisms combine to influence the fate of developing cells in eukaryotes?

5.  (10 points) What are nonsense suppressors?  Nonsense suppressor strains (sup strains) of E. coli tend to grow poorly, compared with wild-type E. coli.  Why do you think this is?

 

 

6.  (15 points) Insertion sequences and transposons cause mutations when they jump into and disrupt other genes or operons.  Everybody knows this.  However, these mobile genetic elements also cause deletions and inversions, at high frequencies.  Using diagrams, describe the mechanisms that are probably involved in these gross (hint hint) rearrangements.

7. (15 points) You have determined the sequence of a large region of a bacterial genome.  You label the genes in this region A B C D E F G H I.  A search of the GENBANK nucleotide sequence database reveals that some of these genes are carried by a bacteriophage that is known to infect your bacterium.  The linear genome of that phage looks like this: cos E F G B C D cos.  You conclude that your chromosomal region contains a prophage.  Draw the circular form of the phage genome, and indicate where the attachment site (attP) is located.  Also, draw the sequence of the bacterial genome that would remain after the prophage excised (and be sure to include the attB).

8. (15 points)

This diagram illustrates a mechanism of homologous recombination at double strand breaks in DNA.  In the space below, describe (using your own diagrams) what is happening, at the biochemical (enzyme) level, in steps 1 and 2.  In addition, describe in words what is occurring in steps 3 and 4.  What enzymes are involved in steps 3 and 4, and what do we call those Xs in the diagram at step 4?