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?
Friday, July 27, 2007
Friday, July 6, 2007
Amino Acids and DNA
Let's do an easy one, shall we? Ok! Here's the question:
The cDNA fragment that includes the ricin gene is 5.7 kilobases. If the entire fragment codes for the ricen polypeptide,the approximate number of amino acids in the poly peptide would be: (enter some weird numbers with lots of zeros here).
Well, once again the GRE just loves trying to confuse people with scary names and things. In this case, it throws in that whole ricin thing to throw you off. You can really just take that out of this question, so it reads something like "The cDNA fragment is 5.7 kilobases. How many amino acids does this code for?"
Alright, this is another one of those you-have-to-know-it questions. How much DNA does it take to code for a single amino acid? First, some very basic background. Amino acids are the building blocks of protein, and really what DNA codes for. Remember when we talked about DNA? DNA strands are studded with genes. Genes are simply lengths of DNA that code for certain proteins. Since the lengths of DNA make proteins, parts of the genes must code for the building blocks of proteins, or amino acids.
The next logical question is what percentage of each length of DNA codes for each amino acid? Ok, I'll just tell you: 3 base pairs. Yep, that's it. 3. Once you know how many base pairs are in a gene, then you just divide by three and that gives you the number of amino acids the gene codes for. How many base pairs are in the gene the question is asking about? 5.7 kilobases. Once again, don't be afraid of words here. "Kilo" simply means 1000, while "bases" means, well, bases. So 5.7 kilobases is 5700 bases or base pairs. Divide that by three, and you get the nice round number of 1900. There you go!
The cDNA fragment that includes the ricin gene is 5.7 kilobases. If the entire fragment codes for the ricen polypeptide,the approximate number of amino acids in the poly peptide would be: (enter some weird numbers with lots of zeros here).
Well, once again the GRE just loves trying to confuse people with scary names and things. In this case, it throws in that whole ricin thing to throw you off. You can really just take that out of this question, so it reads something like "The cDNA fragment is 5.7 kilobases. How many amino acids does this code for?"
Alright, this is another one of those you-have-to-know-it questions. How much DNA does it take to code for a single amino acid? First, some very basic background. Amino acids are the building blocks of protein, and really what DNA codes for. Remember when we talked about DNA? DNA strands are studded with genes. Genes are simply lengths of DNA that code for certain proteins. Since the lengths of DNA make proteins, parts of the genes must code for the building blocks of proteins, or amino acids.
The next logical question is what percentage of each length of DNA codes for each amino acid? Ok, I'll just tell you: 3 base pairs. Yep, that's it. 3. Once you know how many base pairs are in a gene, then you just divide by three and that gives you the number of amino acids the gene codes for. How many base pairs are in the gene the question is asking about? 5.7 kilobases. Once again, don't be afraid of words here. "Kilo" simply means 1000, while "bases" means, well, bases. So 5.7 kilobases is 5700 bases or base pairs. Divide that by three, and you get the nice round number of 1900. There you go!
Monday, July 2, 2007
Where do blood cells come from?
Well, conferences are over for the time being, so I'm now able to post daily once again. Here's today's question:
Exposure to high levels of radiation in humans has been demonstrated to cause anemia. The most likely explanation for this is that the radiation damages the: (and then it goes on to list some places).
Ok, this is a relatively simple question that tests your knowledge of some basic anatomy and physiology. First things first: I've noticed that these tests just love making questions seem more complex than they really are. Take this one, for example. The very first line talks about high levels of radiation. I don't know about you, but I studied very little radiation in my biology classes, so when I first read the question, I got a bit worried about what I'm supposed to know. The question is misleadingly complex. If you just take out the radiation bits, you get a question that goes something like: "Damage to what part causes anemia?" That is much, much easier to answer! So I'm just going to skip the explanation about radiation and its dangers, and jump right into the meat of the question: what is anemia?
Anemia is a deficiency of hemoglobin or red blood cells. Hemoglobin deficiency lowers the blood cell's ability to capture and transport oxygen throughout the body (bad, yes?) and a lowered red blood cell count causes basically the same thing. Either way, anemia is bad. Your tissues need oxygen, and the red blood cells are there to get it to them. Without red blood cells, you die. A lot.
Now, I'm sure some of you have been told you need to take iron to prevent or treat mild anemia. This is true, but don't let it confuse you when you go to answer the question. Iron is a precursor to hemoglobin. The most common form of anemia is lack of hemoglobin, so taking iron supplements allows your body to produce more hemoglobin and therefore transport more oxygen. Lack of oxygen can cause lethargy, hence the tired feeling associated with anemia.
This question, however, is referring to the other form of anemia: lack of red blood cells. How do I know? I looked at the answer list! Here's the question again (this time with the answer list present):
Exposure to high levels of radiation in humans has been demonstrated to cause anemia. The most likely explanation for this is that the radiation damages the:
A) Blood vessels
B) Spleen
C) Liver
D) Thymus
E) Bone marrow
I advocate answering the question before you look at the answers, but sometimes the first answer you come up with isn't listed. Here was my thought process as I read this question: "Well, anemia is caused by lack of hemoglobin or red blood cells, so radiation must attack the red blood cells themselves." As you can see, this answer isn't listed. If your top answer isn't there, go through the rest of the answers and see which one makes the most sense.
Blood vessels. While damage to the blood vessels could cause blood leakage into various body cavities and eventually cause anemia due to lack of blood cells circulating, anemia isn't the first worry. I would be much more worried about internal bleeding, which would probably present as pain or death. Tiny amounts of internal bleeding may cause anemia, but that would mean only tiny amounts of radiation damage, and that isn't likely unless the radiation was controlled in some way (as in radiation therapy). I disregard this one right off the bat.
Spleen. Anyone who has studied the circulatory system knows that the spleen is involved. The spleen filters worn out red blood cells and sends them to the liver for processing. It also holds a small amount of blood in reserve for times when you need that extra burst of oxygen--like exercising or hiking at high altitudes. This makes your blood system more efficient. However, you can live without this little extra burst without any ill effects. Lacking a spleen doesn't cause anemia. It just like living without a savings account--not the most comfortable way to live, but it doesn't mean your checking account has any less money than it would have otherwise.
Liver. The liver does bunches and bunches of things that you don't need to know about at the moment. One major job is the break down of red blood cells and the recycling of hemoglobin. The liver breaks down the worn out red blood cells and gets rid of the excess material via billirubin. Liver damage would cause major problems in a person, but wouldn't cause anemia.
Thymus. The thymus gland is a place where certain white blood cells go to mature. Don't worry, I'm sure there's a question about white blood cells coming up that I can use to address this issue. Just know that it doesn't cause anemia.
Bone marrow. Ah, we've found it. Bone marrow is what gives rise to all the blood cells circulating in your blood stream. Immature blood cells are formed in the bone marrow, then travel to a variety of places to mature. If the bone marrow gets damaged, it no longer can produce blood cells, which will result in a lowered red blood cell count and eventually anemia. Going back to that bank account example, while the spleen is like your savings account, the bone marrow is like your job. While you can live just fine without a stash of money somewhere, if your income gets cut off then your screwed. Bank account anemia!
So, the answer to this question is "E" bone marrow. Yay!
Exposure to high levels of radiation in humans has been demonstrated to cause anemia. The most likely explanation for this is that the radiation damages the: (and then it goes on to list some places).
Ok, this is a relatively simple question that tests your knowledge of some basic anatomy and physiology. First things first: I've noticed that these tests just love making questions seem more complex than they really are. Take this one, for example. The very first line talks about high levels of radiation. I don't know about you, but I studied very little radiation in my biology classes, so when I first read the question, I got a bit worried about what I'm supposed to know. The question is misleadingly complex. If you just take out the radiation bits, you get a question that goes something like: "Damage to what part causes anemia?" That is much, much easier to answer! So I'm just going to skip the explanation about radiation and its dangers, and jump right into the meat of the question: what is anemia?
Anemia is a deficiency of hemoglobin or red blood cells. Hemoglobin deficiency lowers the blood cell's ability to capture and transport oxygen throughout the body (bad, yes?) and a lowered red blood cell count causes basically the same thing. Either way, anemia is bad. Your tissues need oxygen, and the red blood cells are there to get it to them. Without red blood cells, you die. A lot.
Now, I'm sure some of you have been told you need to take iron to prevent or treat mild anemia. This is true, but don't let it confuse you when you go to answer the question. Iron is a precursor to hemoglobin. The most common form of anemia is lack of hemoglobin, so taking iron supplements allows your body to produce more hemoglobin and therefore transport more oxygen. Lack of oxygen can cause lethargy, hence the tired feeling associated with anemia.
This question, however, is referring to the other form of anemia: lack of red blood cells. How do I know? I looked at the answer list! Here's the question again (this time with the answer list present):
Exposure to high levels of radiation in humans has been demonstrated to cause anemia. The most likely explanation for this is that the radiation damages the:
A) Blood vessels
B) Spleen
C) Liver
D) Thymus
E) Bone marrow
I advocate answering the question before you look at the answers, but sometimes the first answer you come up with isn't listed. Here was my thought process as I read this question: "Well, anemia is caused by lack of hemoglobin or red blood cells, so radiation must attack the red blood cells themselves." As you can see, this answer isn't listed. If your top answer isn't there, go through the rest of the answers and see which one makes the most sense.
Blood vessels. While damage to the blood vessels could cause blood leakage into various body cavities and eventually cause anemia due to lack of blood cells circulating, anemia isn't the first worry. I would be much more worried about internal bleeding, which would probably present as pain or death. Tiny amounts of internal bleeding may cause anemia, but that would mean only tiny amounts of radiation damage, and that isn't likely unless the radiation was controlled in some way (as in radiation therapy). I disregard this one right off the bat.
Spleen. Anyone who has studied the circulatory system knows that the spleen is involved. The spleen filters worn out red blood cells and sends them to the liver for processing. It also holds a small amount of blood in reserve for times when you need that extra burst of oxygen--like exercising or hiking at high altitudes. This makes your blood system more efficient. However, you can live without this little extra burst without any ill effects. Lacking a spleen doesn't cause anemia. It just like living without a savings account--not the most comfortable way to live, but it doesn't mean your checking account has any less money than it would have otherwise.
Liver. The liver does bunches and bunches of things that you don't need to know about at the moment. One major job is the break down of red blood cells and the recycling of hemoglobin. The liver breaks down the worn out red blood cells and gets rid of the excess material via billirubin. Liver damage would cause major problems in a person, but wouldn't cause anemia.
Thymus. The thymus gland is a place where certain white blood cells go to mature. Don't worry, I'm sure there's a question about white blood cells coming up that I can use to address this issue. Just know that it doesn't cause anemia.
Bone marrow. Ah, we've found it. Bone marrow is what gives rise to all the blood cells circulating in your blood stream. Immature blood cells are formed in the bone marrow, then travel to a variety of places to mature. If the bone marrow gets damaged, it no longer can produce blood cells, which will result in a lowered red blood cell count and eventually anemia. Going back to that bank account example, while the spleen is like your savings account, the bone marrow is like your job. While you can live just fine without a stash of money somewhere, if your income gets cut off then your screwed. Bank account anemia!
So, the answer to this question is "E" bone marrow. Yay!
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