You are a graduate student aiming to develop new antibiotics, small organic molecules that bind to and prevent the activity of proteins involved in processes essential for cell survival or cell division.
The data below was generated after an experiment, where you added a new antibiotic candidate (“drug”) to cultures of bacteria (prokaryotic cells) or cancer cells (eukaryotic cells).
Over a period of 48 hours, you monitored cellular growth (sampling every 3 hours) and determined how many cells (“cancer” or “bacteria”) were present in your cultures (measured in cells/mL) after the addition of your candidate antibiotic (“+ drug”) or water (“no drug”).
Answer the questions below based on your interpretation of the data.
Note that the use of cancer cells as a measure for how eukaryotic cells would respond to a new drug treatment is quite common in molecular and cellular biology.
You will learn more about the use of cancer cells to establish immortalized cell lines in Cellular Biology (TBIOL 303) but for now, assume that the cancerous nature of the eukaryotic cells is irrelevant to the outcome of the experiment.
In other words, any effect observed in these cells will be the result of the drug treatment (and NOT the fact that these are cancer cells).
(1.5 points) State a hypothesis (or the predicted outcome of the experiment) that you could have been testing using this experimental set up.
1.(3 points) Were the eukaryotic cells affected by the drug treatment? Briefly explain how you know this information. You must refer to the graph as part of your answer.
1.(3 points) Were the prokaryotic cells affected by the drug treatment? Briefly explain how you know this information. You must refer to the graph as part of your answer.
1(4 points) You decide to submit your data for publication in the Journal of Bacteriology but Reviewer 2 has requested an additional experiment to confirm your results.
They suggest that you add the drug BEFORE the prokaryotic and eukaryotic cells have reached stationary phase (indicated by the growth curves “leveling out” towards the end of the 48-hour period in previous graph).
In other words, they wanted to see the effect when the drug is added after cellular growth/replication has already started.
Assuming you added your antibiotic candidate mid-way through the experiment (indicated by red arrow), predict what the bacterial and eukaryotic growth curves would look like based on your previous results. Briefly explain your answer.
1.(4.5 points) Below are four different commercially-available antibiotics used to treat various bacterial infections.
Each antibiotic targets a different macromolecule, resulting in a halt of cellular division and/or cell death.
Briefly research each antibiotic and determine what macromolecule is ultimately affected by the drug and why this would be detrimental to the bacterial cell. Portions of this table have been filled out for you.
| Antibiotic Drug | Macromolecule Affected
(Specific Target) |
Effect on Bacterial Cell |
| Ampicillin | Polysaccharide (peptidoglycan) | Antibiotic targets transpeptidase, an enzyme necessary for the synthesis of peptidoglycan (the polysaccharide component of cell wall). Since the bacteria cannot complete the biosynthesis of peptidoglycan, they cannot complete binary fission and lyse as a result. |
| Polymyxins |
|
|
| Ciprofloxacin |
|
|
| Tetracycline |
|
BONUS (1 point): Haemophilus influenzae is a Gram-negative bacterium that infects the upper respiratory track and was once thought to be the causative agent of influenza (back before the influenza virus was identified as the cause of the disease).
Unfortunately, you and your friend James come down with flu-like symptoms over winter break but while James is diagnosed with an H. influenzae infection, you are diagnosed as having influenza.
As you leave the hospital, James is prescribed the antibiotic Cefotaxime and he insists that you take the medication even though you have been sent home with instructions to “rest up and drink plenty of fluids.”
Briefly explain to James why your influenza infection must be treated differently than his H. influenzae infection? In other words, why shouldn’t you take antibiotics to combat viral infections?
2.Mice are often used as model organisms to study the genetics underlying aspects of development and disease owing to their high levels of evolutionary conservation/genetic relatedness to humans.
With that in mind, the data below was generated in an experiment using cells isolated from two different laboratory mice strains (mouse 1 and mouse 2).
The cells that you isolated from the mice were cultured (in a plastic petri dish with appropriate nutrients and growth factors to promote their growth and survival) in the laboratory for several weeks and had been split into several different dishes when you notice a little problem.
Some of the mouse 2 dishes have cells that are almost all dead and other mouse 2 dishes contain cells that are dividing much more rapidly than the cells from mouse 1.
Panel A shows blue-fluorescently stained metaphase chromosomes from the mouse 1 and mouse 2 cells. Panel B shows the relative fraction of cells found at different phases of mitosis.
In a separate experiment (data not shown), you determined that equal fractions of each cell culture were dividing at the same moment in time but determined that they were progressing through mitosis at different rates.
(2 points) What looks different when you compare the chromosomes from mouse 1 and mouse 2 cells (Panel A)?
1.(2 points) Describe the results shown in Panel B. Refer to Chapter 12 of your textbook if you need to review cell cycle and the stages of mitosis.
1.(4 points) How might the difference described in question 2A underlie the results described in question 2B? Discuss the connection between chromosome structure and cell cycle progression as part of your answer.
1.(3 points) Based on the information provided here, provide a hypothesis that may explain why some of the mouse 2 cells are dying off.
1.(3 points) Propose a follow-up question that you could address using these cells that would build on the results shown here and allow you to better understand the relationship between mouse 2 genetic/physical traits and chromosome condensation or progress through mitosis.