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?