CellCell Biology Problems

  1. Penicillin was the first antibiotic discovered, and continues to be very widely prescribed throughout the world to stop diseases caused by bacteria. Penicillin prevents bacteria from synthesizing peptidoglycan. How does penicillin "cure" bacterial diseases in humans?

  2. In eukaryotes, cycloheximide inhibits protein synthesis in the cytoplasm, and chloramphenicol inhibits protein synthesis in the mitochondria. In contrast, in prokaryotes, protein synthesis is inhibited by chloramphenicol but not cycloheximide. What do these observations suggest about the origin of mitochondria?

  3. Biochemical study of subcellular components like ribosomes, endoplasmic reticulum, microsomes etc., was greatly facilitated in the early 1940s with the commercial availability of the instrument known as the preparative ultracentrifuge. The preparative ultracentrifuge allowed fractionation of subcellular particles from a homogenate of tissue by centrifuging at different speeds.

    As a UROPer who works in one of the MIT laboratories with modern facilities, you decide to carry out some experiments by using an ultracentrifuge. You obtain a piece of fresh bovine liver from a nearby slaughter house and homogenize it by using your room mate's fruit juice blender. Then you filter the homogenate to remove all the unbroken clumps of cells and connective tissue.

    • a) You take the homogenate and subject it into low-speed centrifugation (1000 x g for 10 mins). You save the supernatant in a refrigerator and take the pellet that accumulates at the bottom of your centrifuge tube. You dissolve the pellet in small amount of a solution that has detergents like Triton X-100 and sodium deoxycholate that can dissolve cell membranes. What you get is a thick, viscous solution that has a gel-like consistency. What subcellular structures were present in your pellet?

    • b) Then you take the supernatant that you saved earlier and subject that into medium-speed centrifugation (15,000 x g for 5 mins). This time, you find that the pellet has enzymes that can hydrolyse proteins effectively. By using immunochemical techniques you demonstrate that the pellet contains a protein that is used in ATP synthesis. There may be other things going on as well, which you haven't detected in these two assays. Assuming that the pellet contains a mixture of subcellular structures, name the organelles you expect to find in it. Explain how you derived your answer from the given information. Can you think of ways to detect more organelles, or prove the presence of the ones you think are in the pellet?

    • c) In the next step, you take the supernatant from the previous step and subject that to high-speed centrifugation (100,000 x g for 60 mins). You separate the pellet and supernatant in the same way as before. This time you find that if you add the appropriate translational factors to the pellet, you can successfully detect protein synthesis. What do you think is the major component of your pellet?

    • d) When you closely examined the protein you synthesized in the above step (c), you find that all your protein is glycosylated. What additional organelle do you think is present in the pellet you obtained in step (c)?

  4. Acetylcholine Receptor Protein is a major transmembrane protein in neurons (nerve cells). In your laboratory you are able to follow the path of acetylcholine within the cell, and see that it is first synthesized, then chemically modified, stored for a while, and eventually transported to the plasma membrane.
    • a) Describe the path Acetycholine Receptor Protein takes in neurons, showing which organelles are involved in each step. Where is it synthesized? Where is it chemically modified? Where is it stored? How does it get from one place to another? How does it end up in the plasma membrane? Draw a colored diagram.

    • b) Describe the synthesis of a lysosome. Where are its transmembrane proteins synthesized? Where are it's internal, digestive proteins synthesized? Where are they modified? How do they get into the lysosome? Where do the lysosome plasma membrane phospholipids come from?

  5. Cilia are important cellular structures exclusively possessed by eukaryotic cells. Some eukaryotes like Paramecium are fully dependent on cilia for motility and food particle retrieval. When examined in ultrastructural detail, cilia appear to have bundles of microtubules arranged in an orderly manner.

    Inside a cell, the length of microtubules can be adjusted according to the needs of the cell by adding or removing the above mentioned protein `monomers' to or from existing microtubules. Your UROP advisor believes that the protein `monomers' are preferentially added to one end of the existing microtubule which she calls the + end of the microtubule. To test if this really is the case, you decide to carry out an experiment. You purify some microtubules and protein `monomers' from a suitable source. Then you label your protein with a red fluorescent dye. In the next step, you incubate your microtubules and fluorescent labelled protein under conditions that favor microtubule assembly. After a certain incubation period, you examine your microtubules under a fluorescent microscope.

    • a) What would you expect to observe in terms of the newly added protein `monomers' at the two ends of the microtubules, if your advisor's idea were to be true/false?

    • b) In the above experiment, you add GTP into your incubation mixture in order to get proper microtubule assembly. Can you explain why?

    • c) Colchicine which is once used as an anticancer drug is found to bind the above mentioned free protein `monomers' with high affinity and prevent their assembly into new microtubules. Even though colchicine have no affinity to already formed microtubules, it is observed that when cells are treated with colchicine all the microtubules inside the cell disintegrate. Can you think of a mechanism to explain how this can happen?

    • d) As a laboratory researcher who is working with colchicine you know a few basic facts: colchicine inhibits microtubule formation, and cancer cells grow and replicate much faster than normal cells. What would lead you to believe that colchicine might cure cancer and leave the patient healthy?

  6. The haploid germ cells of a sexually reproducing organism are found to contain 50 femtograms of DNA each. 1 fg = 1 X 10-15 g. Fertilized eggs contain 100 fg before DNA synthesis occurs. How much DNA would you expect to find in the following:
    • a) a somatic cell in G1?

    • b) a somatic cell in G2?

    • c) a germ cell in prophase I of meiosis?

    • d) a germ cell in anaphase II of meiosis?

    • e) a germ cell after telophase II of meiosis?

  7. On p.196 of Purves it says: If gametes were produced by mitosis from a diploid parent...the next generation zygote would be 4n. This is not a tenable situation because subsequent generations would contain more and more chromosomes.

    So what? Why would this be untenable?

  8. Humans have 22 pairs of autosomes and two sex chromosomes. Calculate the following assuming that the maternal and paternal members of each chromosome pair are genetically distinguishable.

    • a) Assuming that homologous chromosomes segregate randomly during meiosis but that crossing over between homologous pairs does not occur, how many genetically unique gametes can an individual produce through meiosis?

    • b) Based on your answer to part (a), how many genetically distinct individuals could one couple hypothetically conceive, again taking into account only random segregation of homologous chromosomes.

    • c) Given your number from b), explain how sexual reproduction enhances the genetic diversity of a species when compared to asexual reproduction.

  9. You are provided with a suspension of cells growing in a nutrient medium. There are many cells per ml. You add a compound "X" to the medium, and are able to measure the rate at which it is taken up by the cells. Describe, using diagrams as well as words, how you could determine if X enters the cell by passive diffusion, facilitated transport, or active transport. Assume you have all the experimental techniques you need.

  10. a) What kind of mechanism allows glucose to enter the intestinal epithelial cell from the lumen of the gut? What is the key energy-requiring step in this process? Explain with diagrams. Using color will help you a lot.

  11. b) Calcium ions, Ca++, are efficiently removed from many living cells, even though this involves moving Ca++ up its concentration gradient. Propose a mechanism to achieve this.

  12. In your studies of some cells, you discover a new protein, esgfun. Esgfun has an extracellular domain A and an intracellular domain C. You notice that esgfun is quite mobile. It can travel the entire length of the plasma membrane in about 15 minutes. Yet no matter how many times you observe the Jane cells, portion A of esgfun always faces the extracellular fluid and portion C always faces the cytoplasm.

    • a) Explain.

    • b) If you were suddenly able to remove all the cholesterol from the plasma membrane, would you expect to see any changes in the movement of esgfun throughout the membrane?

  13. Neurons and other excitable cells have membranes that are polarized: there is a voltage difference across the plasma membrane such that the inside of the cell is negative with respect to the outside. Explain how this polarization occurs in a neuron at rest. (ie, explain the origin of the resting potential.) What are the major ions involved? Are some ions more important than others in determining the resting potential than others? How is the potential created and maintained? How can you measure the potential at any given time?

  14. Based on your knowledge of transport across cell membranes, propose a mechanism by which galactose is transported into the intestinal epithelial cells. Include a diagram of your mechanism. (There are several possible solutions--you only need to propose one.)

    15. Directory Home
    hyperbio@mit.edu