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Biology: Genetic Mutation: Point Mutation Forms


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  • Type: Video Tutorial
  • Length: 11:37
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 125 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Mendelian Genetics and Mutation (36 lessons, $54.45)
Biology: Genetic Mutation (4 lessons, $6.93)

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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As we start to talk about some of these mutations that can happen in genes, the more we look at them, the more we realize that there's a lot of potential for disaster and, yet, there are some that don't cause any problems at all. Let's talk about some examples of mutation. The one I want to talk about right now is one called base pair substitutions. Now, base pair substitutions is one just like the idea of sickle cell anemia where we substituted an A in for a T. And what's going to happen here in a base pair substitution is, well, for example, let's just say we have a piece of DNA that's reading AATA and, therefore, we should have TTAT. But when the DNA replicates for some strange reason - well, there's many reasons - there may be a base pair that is substituted right here. And what happened there - and so you can see how there's a similarity between this and the sickle cell - what's going to happen here is we are going to have AA and perhaps a G will go there, and TT and, perhaps a C will go here. Well, why is that going to be a problem? Well that's going to be a problem. Let's take a look why. Sometimes it's not a problem. Let me give you an example.
Some of these are what we call silent mutations. When would a mutation be silent? Well, you can think of sometimes, suppose it's in an intron. Well, if a mutation is in an intron and it's going to be coded for and, indeed, it is going to be put into the pre-MRNA transcript; but what do you know about introns? You know that introns absolutely are not coded in that MRNA, at least many introns are not coded in the MRNA, because they are cut out - they are spliced out. And so if there's a mutation in the intron that's not going to be a problem. It might be in something other than an intron. Remember, we highly suspect that the vast majority of our DNA is never turned on and may be evolutionary leftovers. Well, if that's true; not a problem. It may be in one of the repeated sequences, and if it's in one of the repeated sequences and those sequences are merely there, say. to be at the end of a chromosome, well, theoretically it might be silent.
Let me show you another reason it may be silent. This is very cool because there's some evolutionary things happening here. So, what we have in this particular one - in this particular idea of a silent mutation - remember this, this is a genetic code, and this is RNA. These are RNA codons. If you know anything about RNA you know that's what is going to happen is this is going to be the three letter sequences that are on your RNA. Well, let's think about this. Three letter sequences on your RNA. These three letter sequences call for amino acids. So, we know that if we have a piece of RNA, say, with one codon in the ribosome and that codon is CCU. Well, if we look right here we see that CCU codes for the amino acid - you know, its little TRNA is going to come along and bring the amino acid serine. Now the CCU codon was made by DNA that was GGT. The GGT coded for the CCU. Well, suppose we have a base pair substitution in there. Suppose, just when my DNA is replicating, I go to the dentist for an x-ray, the dentist forgets to put the lead vest on me and my germ cell gets mutated and, when that DNA is replicating in my germ cell - I'm going to get right there - instead of a T we'll give it another G.
Well, that's going to change this MRNA to a CCC, and isn't that going to cause this horrible mutation in my children, if not cancer in me? And the answer is no, not this time. Because it just so happened that, our code is degenerate. Our code has redundancy in it. And what that simply means is GGG gives me CCC - CCC also gives me serine. So, the mutation is silent. Sometimes it's not silent but that was a good story. I'll give you a not so good story.
Some of these give you what are called - that one, for example, was a silent mutation. Let me give you another example. So we have silent - here's another one - mis-sense as opposed to nonsense. What does that mean? In this case, the one I just showed you gave me - let's go back to my base pairing here. And now what we might get is we might get a sense - the reason we use sense is for this. If the MRNA is being read and, it's reading right along, it may get to a codon where it's sensing the - the TRNA will come down and actually sense and accurate codon and may put the wrong amino acid in there. So, this is not going to be necessarily silent. You might get something else put in there a miss sensing. So, therefore, if I take say for example a DNA strand that has GAA - GTA, let's do GTA because that's the one I'm looking at. Well, GTA is going to make an MRNA strand that's going to give me CAU - that's my MRNA. And if we look at CANU that's going to give me histidine. So, that's going to bring in histidine. But if all of a sudden I do a base pair switch here and, instead, I make this GTT, now I'm going to be bringing in CAA, and this is no longer a silent mutation. Now, instead of histidine, the silence ends. Now I have glutamine, and glutamine is a different amino acid than histidine. Now, that may not be bad because you know what? Sometimes we find out that these amino acids are so close that it doesn't change the protein shape, and we'd have to get through the whole idea of what that does histidines R group like look, and what does glutamine's R group look like, but you get the idea there depending on how different these two amino acids are that mis-sensing may be silent - that mis-sensing, even though you put the wrong amino acid in may end up not expressing.
Sometimes though bad things happen and that's nonsense. I'll give you a great example of how this is often and always a bad thing. How can you have nonsense? Isn't it always going to put something in there? No. Let me give you another good example of this. Let's take a piece of DNA that is ACT. Now that's going to make a piece of MRNA. Let's do TCG. TCG is going to give me, that's my DNA strand - TCG is going to give me an MRNA strand that's going to read - it's going to give me UGC. So, that's going to give me UGC. Now, after all this, let's see what UGC MRNA is going to give me. UGC MRNA is going to call for cystine. But if I do a base pair substitution and I put a T here instead of the G. let's see what's going to happen. Let's look at that MRNA. That MRNA is going to give me what - UGA. There is no TRNA for that. In fact, that's a stop codon that's nonsense, and if this happens to be in the middle of growing polypeptide chain - there goes your enzyme.
There's a disease actually that this happens, a disease called phenylketonuria - PKU. PKU is one of the diseases we test all newborn babies for. And PKU has a stop code on in the middle of an enzyme. And that enzyme is supposed to do the following. See we have an amino acid that we routinely metabolize. And when we metabolize it, it's supposed to be changed. When we don't change it, because we lack the enzyme - let's just say enzyme A is supposed to be here. If you have a defect, and enzyme A is the one I'm talking about that has the stop code on in the middle of it when it's a mutant - when they have PKU you'll see what happens. We don't do this change. And instead, we make we make a chemical called phenylketones. And phenylketones are not products we should be making, because phenylketones build up and can actually cause nervous system damage through a pathway, but the damage will be there. And the good news is if we can detect this dilemma at birth we can put this person on a diet that is not a normal diet for the rest of their lives, keeping them away from large amounts of phenylalanine and, if they don't have a large amounts of phenylalanine they will never run into this problem because they won't get their build up of phenylketones. So that's why we test new born babies for this disease, because if we can find it then we can keep them from having the nervous system damage that they would get anyway. And in fact, if you look at any diet drink and read it very carefully you will see be aware of phenylketonuriacs this contains phenylalanine. And now you know why we have to warn them about them because of that enzyme.
So there you go. There's just one example of an enzyme dilemma base pair substitutions, but there are more.
Mendelian Genetics and Mutation
Genetic Mutation
Genetic Mutation: Different Forms of Point Mutations Page [2 of 2]

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