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Biology: DNA Fingerprinting


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About this Lesson

  • Type: Video Tutorial
  • Length: 9:26
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 101 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Biotechnology (16 lessons, $23.76)
Biology: More Techniques in Biotechnology (5 lessons, $8.91)

In this lesson Professor Wolfe explains how tandem repeats are repeating sequences of DNA (such as the sequence 5’ CATCATCATCAT 3’). Variable number tandem repeats (VNTRs) are repeating sequences of DNA that vary in their number of repeating units between individuals.
VNTRs can be used to differentiate between the DNA of individuals in a process known as DNA fingerprinting. In a typical DNA fingerprint, several VNTR markers are compared in order to decrease the odds that more than one person will have identical DNA fingerprints.

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

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One of the cooler things about this whole biotechnology issue is it allows us to bring all your astute knowledge of some of the stuff you knew about in molecular genetics and make it make actual concrete sense. It becomes so much more than just, oh, let me memorize another thing. It really makes it applicable. Let me give you a good example of something like that.
Remember, if you've ever read anything about molecular genetics or listened to anything about molecular genetics, did you ever hear the term "tandem repeats?" I think you have, particularly in the aspect of something like satellite DNA. What were tandem repeats? Tandem repeats were repeating sequences of DNA - Well, they still are - they are repeating sequences of DNA that run one right after the other. And it turns out that in the human genome, which is what I want to talk to you about now, although it is certainly present in other ones, we have these things called variable number tandem repeats, or VNTR's.
Think back to what you know about RFLP's, restriction fragment -linked polymorphisms. What made those things so great as markers? What made them is so great as markers, was the fact that a restriction site cut those things in different lengths so we could use them as an identifier of something. The fact that one gene or one piece of DNA had a restriction fragment linked polymorphism or had a site where it would cut it, and the other one did not have that site, gave you different lengths of DNA. Well, it's even cooler with VNTR's and let me explain what this means. I'm going to use a simple one.
Most VNTR's are more than three base pairs, but I'm going to use three because I don't want to write a lot. So let's just say the sequence CAT. Let's just say that is a tandem repeat. So, it's going to be CAT, CAT, CAT, CAT, CAT, get it? I could keep going on. And so, you can literally have many, many CATs if you will, in your genome. But here's the thing. I might have four - it's more probably like I might have one hundred and eighty-seven - and you might have twelve CATs in this given segment of DNA. So if I could somehow cut my DNA - if I could somehow find a restriction site, there and there, or somewhere, on either end of that CAT sequence, I could tell your DNA from my DNA. Why? Because these tandem repeats vary in the number that there are. These tandem repeats vary in the number that they are in the human gene population. So, like I said, I might have 187 of these CAT sequences, you might have 12. So how are we going to use this as a DNA fingerprint?
Watch this. It's so easy. Suppose I was at a crime scene and I found a glove. And on that glove was a few drops of blood and I said, "Wow, I wonder if this glove and the blood on there is any relation to the blood of the victim, because there's a dead guy next door, or could it be the blood of a so-called perpetrator?" So, here's what I'm going to do. I'm going to take that glove and I'm going to take the blood off the glove. So we take the glove. And we have a little blood on the glove. And we're going to take that blood and we're going to put it in a test tube. But that's not very much DNA. Can you think of a way that I might be able to make more of that DNA? Well, if you're good students, you know that what I could do is I could take this and if I happen to have a marker by - oh, how cool would this be - if I know that there is a CAT sequence and one particular CAT sequence, or VNTR has, and I know a little bit of DNA upstream of that and perhaps a little bit of DNA downstream of that, well what could I do? I could take this and I could amplify it with PCR.
Now, I have a lot of the DNA that was on the glove, don't I? Because, I've amplified it. And what DNA have I amplified? Not all of it. Only the DNA that has the CAT on it. So, they say, "You know what, George? I think you're the criminal." Take my blood. Please, take some of my blood and let's check it out. So here's what we do. We take my blood and we amplify the CAT sequences. What do you think we're going to do? Well, let's just say I have twelve CAT sequences. Let's say you have three CAT sequences. Let's see what happens. So, we run a gel. We run an electrophoresis gel. Here's me. Here's you. And let's see, we have to have the blood on the glove. I have twelve, you have three, so whose is smaller? Yours. So yours is going to run further. Mine is going to be here. And then we take the blood from the gloves and it ends up right there. And you say, "Gotcha." And I say, "Wait a minute, time out." Granted, I have the same number of CATs as was on that glove. But that's only one VNTR, guys and my studies have shown that that runs in one out of forty people who have that same CAT, that same VNTR. That means that there are 4 billion people on the planet. That only means that that one out of forty of us could have done it. So you take a roomful of 41 people, two of us are going to have that CAT. You say, "Good point. Let's find another VNTR." And so you run another VNTR. And you amplify it from the glove blood and it comes out and let's just say that this is not CAT, this is TAG. And the TAG comes here. I do yours. Well, you're out of it already, and mine comes here. "George, this isn't looking good for you." Wait a minute. That TAG VNTR runs in one out of forty people (just to keep the numbers the same). That only gives it a one in sixteen hundred chance of being me. Okay, let's do a third one. And so we do a third one. Boom, boom. Wait a minute. That only runs in one out of one hundred people.
Get where we're going? By the time we're done, to convict someone on a criminal case, we've got to do at least six to twelve of these. Do the numbers. Even if it is one out of forty chance, do the numbers. You end up with more chances than there are people on the planet. It gets into the billions real fast when you start going one out of forty. I'll tell you what, one out of forty to the sixth power is a little bit over 4 billion, which is about the number of people on the planet. So, by matching up six of these VNTR's, each of which has about a one out of forty chance, which we can get by doing human population studies, we can determine pretty much that the odds are that you are the only person on the planet with that DNA fingerprint. DNA can be used to identify you down to the nucleotide. We could sequence all of your DNA. What's quicker? A few VNTR's, or your whole sequence? Certainly, this is. So you see that this whole idea of amplifying DNA and amplifying sequences that may be not part of your genome, part of your express genes, but part of maybe your junk DNA. It can free you or it can send you to jail. Don't leave your blood on the glove.
More Techniques in Biotechnology
DNA Fingerprinting Page [1 of 2]

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