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Microbiology

Critical Thinking

MicrobiologyCritical Thinking

Critical Thinking

63.

A pure culture of an unknown bacterium was streaked onto plates of a variety of media. You notice that the colony morphology is strikingly different on plates of minimal media with glucose compared to that seen on trypticase soy agar plates. How can you explain these differences in colony morphology?

64.

Review Figure 11.4 and Figure 11.5. Why was it important that Meselson and Stahl continue their experiment to at least two rounds of replication after isotopic labeling of the starting DNA with 15N, instead of stopping the experiment after only one round of replication?

65.

If deoxyribonucleotides that lack the 3’-OH groups are added during the replication process, what do you expect will occur?

66.

Predict the effect of an alteration in the sequence of nucleotides in the –35 region of a bacterial promoter.

67.

Label the following in the figure: ribosomal E, P, and A sites; mRNA; codons; anticodons; growing polypeptide; incoming amino acid; direction of translocation; small ribosomal unit; large ribosomal unit.

A long line at the bottom has many letters on it and is labeled G. Each set of 3 letters is circles and labeled D. Attached to this is a large dome (I) and a small oval (3). Attached to one group of 3 letters is a rectangle 9labeled C); the 3 letters at the bottom of the rectangle are labeled H. A circle at the end of the rectangle is labeled B. The rectangle in the center of the dome labeled I has a long chain of circles attached to it labeled A. An arrow at the bottom points to the right and is labeled F.
68.

Prior to the elucidation of the genetic code, prominent scientists, including Francis Crick, had predicted that each mRNA codon, coding for one of the 20 amino acids, needed to be at least three nucleotides long. Why is it not possible for codons to be any shorter?

69.

Below are several DNA sequences that are mutated compared with the wild-type sequence: 3’-T A C T G A C T G A C G A T C-5’. Envision that each is a section of a DNA molecule that has separated in preparation for transcription, so you are only seeing the template strand. Construct the complementary DNA sequences (indicating 5’ and 3’ ends) for each mutated DNA sequence, then transcribe (indicating 5’ and 3’ ends) the template strands, and translate the mRNA molecules using the genetic code, recording the resulting amino acid sequence (indicating the N and C termini). What type of mutation is each?

Mutated DNA Template Strand #1: 3’-T A C T G T C T G A C G A T C-5’
 Complementary DNA sequence:
 mRNA sequence transcribed from template:
 Amino acid sequence of peptide:
 Type of mutation:
Mutated DNA Template Strand #2: 3’-T A C G G A C T G A C G A T C-5’
 Complementary DNA sequence:
 mRNA sequence transcribed from template:
 Amino acid sequence of peptide:
 Type of mutation:
Mutated DNA Template Strand #3: 3’-T A C T G A C T G A C T A T C-5’
 Complementary DNA sequence:
 mRNA sequence transcribed from template:
 Amino acid sequence of peptide:
 Type of mutation:
Mutated DNA Template Strand #4: 3’-T A C G A C T G A C T A T C-5’
 Complementary DNA sequence:
 mRNA sequence transcribed from template:
 Amino acid sequence of peptide:
 Type of mutation:
70.

Why do you think the Ames test is preferable to the use of animal models to screen chemical compounds for mutagenicity?

71.

The following figure is from Monod’s original work on diauxic growth showing the growth of E. coli in the simultaneous presence of xylose and glucose as the only carbon sources. Explain what is happening at points A–D with respect to the carbon source being used for growth, and explain whether the xylose-use operon is being expressed (and why). Note that expression of the enzymes required for xylose use is regulated in a manner similar to the expression of the enzymes required for lactose use.

A graph with time (hours) on the X axis and density of bacteria on the Y axis. An upward slope is labeled A. Next, is a plateau labeled B. Next is an upward slope labeled C. And finally is a plateau labeled D.
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