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Biology: Watson and Crick: The Double Helix


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About this Lesson

  • Type: Video Tutorial
  • Length: 13:23
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 144 MB
  • Posted: 02/10/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Final Exam Test Prep and Review (42 lessons, $59.40)
Biology: Genetics: DNA & Replication (35 lessons, $54.45)
Biology Review (19 lessons, $27.72)
Biology: DNA Structure Revealed (2 lessons, $2.97)

In 1953, Watson and Crick published their findings on the structure of DNA in the journal Nature. Knowing the structure of DNA allows us to understand how it works in the body. Professor Wolfe furhter explains their findings and how measurements from the X-ray defraction image helped define the structure of the DNA. These are the 0.34, 3.4, and 2.0 measurements that were observed in the image, but not understood. He also explains the bonding between the purines and pyrimidines. These are hydrogen bonds, formed in the middle of the double-helix. The sugar-phosphate chains are antiparallel. Finally, Professor Wolfe explains the four requirements of DNA, that it is informational, capable of replication, capable of communicating with cells, and capable of change, and how DNA meets all of these requirements.

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.

About this Author

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Molecular Genetics
DNA Structure Revealed
Watson and Crick: The Double Helix Page

The date was April 25, 1953. The journal Nature, and let me just read the introductory sentence. “We wish to suggest
a structure for the salt of deoxyribonucleic acid. This structure has novel features, which are of considerable
biological interest.” How is that for understatement? The article Molecular Structure of Nucleic Acids: A Structure for
Deoxyribonucleic Acid. In that simple sentence, Watson and Crick started what, to this day, is a history literally in the
science that runs our lives, because they took the first step in understanding the true nature of the genetic material,
and the true secret to life. You see the thing here is that we were at the verge of understanding the mechanisms of
genetic inheritance. Funny thing is, ’53, 46 years later we’re still on the verge of understanding those things, because
every verge has another verge. And we’re just constantly walking and saying, “Wow, there’s still more.” But that’s
another story. So what was it they did that was so fabulous? Well you know what they did. They used these
molecular models and unraveled the structure of DNA. And now that we had a structure, we could figure out how it
Let’s take a look at what they figured out. Remember those three numbers, 0.34, 3.4 and 2.0. It all fit together.
Purines and pyrimidines would bond. And what they would do is they would bond across a double helix. And that
double helix would look, say, something like this. Now that double helix therefore would have an adenine and a
thiamin, a thiamin and an adenine, a guanine and a cytosine. And it varied what the sequence was. It clearly matters,
because these are your genes. But in other words, the sequence on one side would determine the sequence on the
other side. The importance of that didn’t escape Watson and Crick, as we’ll talk about in a minute. So here it was,
the structure of a double helix, but there’s a lot more to it than that.
Here’s what else they figured out. These numbers, 0.34, 3.4 and 2.0 plugged into this double helix. How did they
plug into this double helix? Well here we go. Draw the double helix like so. Let’s make believe this is a twisted
ladder. Here’s the thing. If you look at this and you look at one twist of the ladder—and you’ve got to understand they
pieced this together from x-ray diffraction. So they pieced this together from the physical structure of this thing. And
remember I’m limited to a one-dimensional object here. But the bottom line is this, between here and here, is one
twist of the ladder. Look here, and the next time that this strand is back at this position is right here. Position A, and
I’ll call this position A prime, one twist of the ladder. Well if each of these nucleotides represents 0.34 depth, that
means that in one twist of the ladder, there must be ten of them. And that’s where the 3.4 came in; one, two, three,
four, five, six, seven, eight, nine, ten, with a little bit of artistic license. So therefore, this 3.4 represented places where
that x-ray beam ran into the curve of the ladder. And of course, it would be all the way down. Okay, so we’ve got
0.34. We’ve got 3.4. The only way this thing could be possibly connected together is right across here, with a 2.0,
two nanometers. And then the picture was there. That’s not the rest of the whole story, but now we know where this
thing came in.
Well when we take a look at a space-filling model of this, there it is. And once again we take a look and there is one
curve. And if you were to count nucleotides in here, there would be ten of them. Does it say A, G, C and T? No, but
nevertheless, the space filling-model of DNA, and the beautiful picture of the double helix. I ought to have that
hanging on my wall at home. Anyway, let’s move on, because there’s more.
How did these things bond together? And this is where hydrogen bonds come in. What I’m about to do is a baby’s
version of the kind of problems Watson and Crick had. If you’re talking about G and C, say, bonding together,
guanine and cytosine. And you put these together, and you’re looking for a place where hydrogen bonds occur. And
I’m looking at them in my own frame of reference, which is right side up. I’m looking at them saying, “Wow, where are
the bonds going to form. This is kind of difficult.” And if I can manipulate these a little bit, and maybe look at it like
this, all of a sudden I can see where bonds would form. For example, now I’m going to show you a finished version of
this, and then I’ll come back to these. So if I take a look at guanine and cytosine, I can see that we will form three
places where bonds will form. Remember we’re looking for hydrogen bonds, and HO attraction there, and HN
attraction there, because remember hydrogen bonds in biological systems, we’re mostly concerned with hydrogennitrogen
and hydrogen-oxygen bonds, HO, HN, HO. And so you see, when Watson and Crick worked with this thing,
they had to manipulate these things around to figure out, “Okay, I’m convinced 2.0 across there, but where are the
bonds going to form?” So they had to twist and turn and move them around, and finally they were able to do that.
And it was the same thing with A and T. A and T, we had the same dilemmas. So if you take A and T and you work
with the same thing, you’re saying to yourself, “Wow, how am I going to get these babies to bond?” And you’re
looking, “Well, could it be that I have to turn this around? And, golly gee whiz, could this hydrogen be involved with
this oxygen right here? That’s kind of cool. And if that’s true, well gee, where are these nitrogens going to bond?”
Molecular Genetics
DNA Structure Revealed
Watson and Crick: The Double Helix Page [2 of 2]
And so eventually they came up with the fact that A and T form two hydrogen bonds, an HO and an HN. You see?
So putting that together, they came up with this picture of the double helix.
But there was one more thing they had to worry about. And I can explain this one with my artistic talents. Let’s take a
look at this. Here’s my house with my swimming pool and my garage. That’s how I like to remember these. I want to
remind you about something. Remember your organic chemistry. We number carbons. Just because this is the end
of the lesson, don’t think this is unimportant. What I’m about to show you is going to come back lesson after lesson
after lesson. So get this. Remember how we number our carbons, one, two, three, four, five, and the phosphate
group. This is an oxygen. Now if we propose that this molecule is going to be a chain of these things, watch what we
have here. If I wanted to link another one of these right here, and make a hydrogen bond here, I’m going to draw one
just like I just did. Watch. Now remember the double helix. If you’re going to fit this in here, and the phosphate group
is not going to go in the center, can you see a dilemma that I have with this fitting in here? The answer is I have to flip
it over. I have to roll that baby around so that it’s upside down. So instead now what we’re going to have is this is
going to be upside down relative to this one right here. So what’s going to happen in this case is something like this.
And see, let’s number these carbons so you know what I’m doing here. There’s my oxygen, one, two, three, four, five.
And so, you’re going to see that the strands of DNA are going to be what we call anti-parallel to each other. Man is
that going to loom huge. I’ve got one more picture I want to show you of this. But these are anti-parallel. And what
that means is this. One side will, at its top, have the number five carbon. And remember all of these are gone. This
side is going to line up like this. So we are saying that this side has, we’re going to call it, the five prime end. And this
end will be called the three prime end, because the bottom one way down here is going to have its number three
carbon at the bottom. On the other hand, this side will have its three end at the top, and it’s five at the bottom.
Therefore they are anti-parallel to each other. They, in essence, run in different directions. And that is going to
present a huge problem when we start talking about the way enzymes work with this thing.
Here it is folks, the anti-parallel nature of DNA. Up here we have the five prime end, moving down, and there’s the
number three carbon. And take a look right here. What’s up on top? The number three carbon is on top here, and
it’s running in the five.
So to summarize I have one question for you. What are the four things that you need to have in the genetic material?
It has to carry information. Does DNA carry information? The code is there. It has to be able to replicate. Is the
replication ability there? Let me read to you from Watson and Crick. “It has not escaped our notice that the specific
pairing we have postulated immediately suggests the possible copying mechanism for the genetic material.” They
were no dummies. Does it give you a way to talk to the cell? Well, you know because you’re a 90s kind of biologist
that DNA makes RNA, and RNA makes protein. Does it give you the ability to change? Let me read you two more
quotes. “If the actual order of the bases on one of the pair of chains were given, one could write down the exact order
of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the compliment of the
other. And it is this feature which suggests how the DNA molecule might duplicate itself.” So there’s how it might
reproduce. And last but not least, “We imagined that prior to duplication the hydrogen bonds are broken, and the two
chains unwind and separate.” So now we’re figuring out the mechanism of breaking this thing apart. And last but not
least, “Our model suggests possible explanations for a number of other phenomena. For example, spontaneous
mutation may be due to a base occasionally occurring in one of its less likely forms.” They’re already thinking about
mutation. And the ultimate irony in itself, “For the moment, the general scheme we have proposed for the
reproduction of DNA must be regarded as speculative.” Speculate they did. How does DNA replicated? You’ll see.

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