The Effects of Colchicine

Well, vacation is over, and now it's time to get back to the great subject of biology! Let's jump right into it, shall we?

Today's question:

The addition of colchicine to a culture of actively dividing flagellated eukaryotic cells inhibits all of the following except: (And then there's a list of answers).

Well, I chose kind of a pain in the ass question, didn't I? The GRE loves throwing specific substances/structures/species at you to determine if you know what it is or not. That's fun and all, but what if you have no idea what the bloody thing is? First, I'm going to tell you all about this colchicine stuff and its effects on cells. Then I'll give you some tips to make guessing the correct answer easier.

So, what is going on with this question? First off, notice that the test writers are doing that thing where they fill the question up with lots of multi-syllable words that may or may not confuse the reader. Don't let them win! The two biggest misleading words in this question are "flagellated" and "eukaryotic," neither of which have much to do with the meat of the question, which is "what the heck is colchicine?" If you find yourself getting bogged down in the complexity of the question, just take a moment to define each of the words and decide if they actually have any affect on the question itself. In this case, "flagellated" (the state of having a flagellum, or thing, whip-like tail used to propel and organism) really just gives you more detail about the cells the colchicine is being dumped on, while "eukaryotic" (cells with membrane-bound organelles) gives you even more detail. Fun! Ignore them for now.

Now to the real question: what is colchicine? Colchicine is an organic compound (molecules that contain carbon) that also contains nitrogen as its key component. This particular nitrogen containing organic compound (or "amine" for short) is produced by the Autumn Crocus, a very pretty little plant that you really don't need to know about. What you do need to know is that colchicine is very poisonous, and is therefore used therapeutically by doctors around the globe.

Colchicine causes vomiting and defecation in humans, and is therefore prescribed to combat joint issues such as gout. (For those who don't know, gout is a very painful condition in which uric acid crystals form in the joints of the lower extremities. This condition is related to kidney stones, and flare-ups happen after intake of rich food and drink. It was known as a disease of the rich in days of yore, but is now known as a disease of the unlucky and limping).

On a cellular level, colchicine inhibits the formation of microtubules. It does this by inhibiting tubulin--the substance responsible for making microtubules.

Microtubules are exceptionally important in two areas: growth and structure. Microtubules make up the main structure of the cytoskeleton, which gives the cell its shape. No microtubules, no cytoskeleton. That's a bad thing for new cells. Microtubules are also important during mitosis.

Have we gone over the stages of mitosis yet? I don't think so--that's a long lecture so I'm avoiding it. Well, the short version is mitosis is the process by which a cell reproduces itself. There are several stages of mitosis, during which particular things happen including the copying of DNA,and the relocation of the genome to the new cell. Microtubules are responsible (in the form of spindle fibers) for pulling the DNA from the center of the mother cell into the new daughter cells. If no microtubules form, then the DNA cannot migrate to the new cell, which means no new cells. Growth is inhibited.

This inhibition of growth makes colchicine a great drug for fighting cancer cells. The hallmark of cancer is its unfettered reproduction; since colchicine stops reproduction, flooding cancerous cells with colchicine stops their growth. Good! Of course, it also stops the growth of any healthy cells it touches, so it is only used sparingly and is not a miracle cure.

Ok, now we have an idea as to what colchicine does. Lets get back to the question:

The addition of colchicine to a culture of actively dividing flagellated eukaryotic cells inhibits all of the following except:

A) Movement of flagella
B) Growth of flagella
C) Formation of mitotic apparatus
D) Formation of microtubular cytoskeleton
E) Polymerization of tubulin


The answer seems obvious, right? Hopefully? Since colchicine inhibits tubulin which therefore inhibits microtubule production, all growth is stopped (due to lack of spindle fibers), formation of mitotic apparatus is inhibited (due, once again, to lack of spindle fibers), the cytoskeleton is stunted, and polymerization of tubulin is halted. Basically, everything involving growth and reproduction is stopped. However, this substance doesn't have any effect on already formed microtubules--you see, it only stops the substance that makes up new microtubules, it doesn't break down old tubules. So movement and function of mature cells goes untouched. Movement of flagella, therefore, is unaffected by colchicine. The answer is "A." Yay!

So what happens if you're sitting at the test and have no freakin' clue what colchicine is? You may be able to figure it out with a little bit of effort. Look at the answers given here:

A) Movement of flagella
B) Growth of flagella
C) Formation of mitotic apparatus
D) Formation of microtubular cytoskeleton
E) Polymerization of tubulin

The great thing about a multiple choice exam is the answer is staring you in the face--you just have to recognize it. In this particular example, the question is asking the effects of some substance on some cells. Your first task is to break down the question to its essential parts. Don't go trying to answer a question that isn't even asked! So, what are the effect of this substance? Look at the answers--two of them (B and C) are directly related to the growth and reproduction of a cell. Formation of the cytoskeleton has to do with growth as well, and polymerization is a fancy word for "making" or "putting together" or "growth." So 4 of the 5 answers have to do somehow with growing. Whenever you see a link between most of the answers, and the question asks "which is not like the other" then you have a pretty good idea of the answer. Make sense?