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Biology: DNA Replication: A Summary

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About this Lesson

  • Type: Video Tutorial
  • Length: 11:16
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 121 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: Events of DNA Replication (5 lessons, $9.90)

In this lesson, Professor Wolfe provides a complete overview of the replication of DNA, beginning with its structure. DNA has a double-helix structure of nucleotides, which made-up of a sugar, a phosphate, and a base. The strands are antiparallel, meaning they run in opposite directions, known as the 5' strand and the 3' strand. DNA replicates by the semi-conservative method of replication.

Professor Wolfe explains how DNA polymerase only reads in one direction, from 3' to 5', and therefore creates ""replication bubbles"" in order to replicate the DNA twice as fast. This creates leading and lagging strands, which require RNA primer and Okazaki fragments added to the lagging strand in order for the process to work properly. At the end of the strands, which lack the free OH molecule needed to complete the process, telomeres are used to protect against a loss of information.

This lesson is perfect for review for a CLEP test, mid-term, final, summer school, or personal growth!

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 http://www.thinkwell.com/student/product/biology. 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

Thinkwell
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Founded in 1997, Thinkwell has succeeded in creating "next-generation" textbooks that help students learn and teachers teach. Capitalizing on the power of new technology, Thinkwell products prepare students more effectively for their coursework than any printed textbook can. Thinkwell has assembled a group of talented industry professionals who have shaped the company into the leading provider of technology-based textbooks. For more information about Thinkwell, please visit www.thinkwell.com or visit Thinkwell's Video Lesson Store at http://thinkwell.mindbites.com/.

Thinkwell lessons feature a star-studded cast of outstanding university professors: Edward Burger (Pre-Algebra through...

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Recent Reviews

Nopic_gry
Life saver
10/27/2012
~ dbangley

I can understand my textbook now and will be prepared for my exam.
This is close to a miracle..

Nopic_dkb
Great
03/14/2012
~ min

He explains very well and it's easy to understand
I wish my professor can teach like him :)

Nopic_blu
This guy is AWESOME!!!!!!!!!!!!!!!!!!!
12/02/2010
~ Heidi8

There ARE many different free videos out there, but they honestly dont compare to this guy!! This vid was worth it. I paid $1.98 and I would have paid $5.

Chelsea_homepage
Another excellent video
09/16/2010
~ Chelsea

I wish this was my biology teacher. He does a great job!

Ht1979_homepage
Love the instructor!
09/16/2010
~ hmt79

This is a great video that explains replication very clearly and concisely...and the guy in it reminds me of a lovable grandfather!

Nopic_gry
Life saver
10/27/2012
~ dbangley

I can understand my textbook now and will be prepared for my exam.
This is close to a miracle..

Nopic_dkb
Great
03/14/2012
~ min

He explains very well and it's easy to understand
I wish my professor can teach like him :)

Nopic_blu
This guy is AWESOME!!!!!!!!!!!!!!!!!!!
12/02/2010
~ Heidi8

There ARE many different free videos out there, but they honestly dont compare to this guy!! This vid was worth it. I paid $1.98 and I would have paid $5.

Chelsea_homepage
Another excellent video
09/16/2010
~ Chelsea

I wish this was my biology teacher. He does a great job!

Ht1979_homepage
Love the instructor!
09/16/2010
~ hmt79

This is a great video that explains replication very clearly and concisely...and the guy in it reminds me of a lovable grandfather!

Nopic_grn
Don't buy it
09/15/2010
~ jnperrone

It's a good video, but no reason to pay for it like I did. You can find the exact same video on YouTube. It's separated into two parts, but it is the same material.

Molecular Genetics
Events of DNA Replication
DNA Replication: A Summary

So is there a way to really take this whole DNA story – at least DNA replication – and put into the context of ten or
fifteen minutes? Let’s give it a try – DNA from the beginning all the way down to two DNA strands. Deep breath –
let’s go.
We know that DNA, by the work of many great scientists, is a double helix. We know that it is a double helix of
molecules called nucleotides. Remember that there are four different nucleotides – two, which we call purines. The
purines are guanine and adonine. There’s a way to remember this, and it also tells you a little bit about their structure.
I like to remember it and I tell my students to remember it is PUGA2. The purines are guanine and adonine and they
have a two-ring structure. Let’s take a look at the two-ring structure now. Here you can see adonine, with its two-ring
structure. And we might as well throw this in right now too. Adonine always bonds to the pyrimidine thymine. So let’s
talk about that. We also have pyrimidines. The pyrimidines are thymine and cytosine. Remember that adonine
always bonds to thymine, so here’s a purine pyrimidine bond. How do they bond? Through hydrogen bonds. So far,
so good. Let’s look at the other part of PUGA2, guanine. Guanine, with its two-ring structure, always bonds to
cytosine, okay? And once again, we have the purine/pyrimidine bond. So summary of the summary so far – what do
we have? DNA is a double helix. It is consisting of nucleotides. There are four different nucleotides: the two purines
and the two pyrimidines – adonine, guanine, cytosine and thymine. This isn’t so bad, is it? Remember, that each
nucleotide contains a sugar called deoxyribose, thus the name deoxyribonucleic acid. Remember one more thing.
There’s a phosphate backbone holding this baby together – holding it in its helical structure. So DNA, in a nutshell.
Now that we have DNA in a nutshell, we have to ask the next question. How does DNA replicate? Well it was known
that DNA must replicate – Watson and Crick proposed this – by rupturing – somehow breaking – these hydrogen
bonds – the hydrogen bonds between the GNC and the ANT. All right, that being said, how are we going to do that?
Well, through a series of very clever experiments, it was found out that DNA replicates in what we call the semiconservative
manner. In other words, using a zipper analogy here, the strands will open up and each side will act as
a template for the new side. So this side will act as a template for this side, which brings us to this idea of the 5-prime
side of DNA and the 3-prime side of DNA.
Remember this. Since it’s a double helix and has two sides, we have to think of the following. If we look at the
arrangements of the nucleotides and here is one nucleotide with its group. And we’ll put another one right below that,
with its group. We have this side. And let’s remember to count our carbons – one, two, three, four, five. So this
particular side has the number 5 carbon facing upward, relatively speaking, and the 3 carbon, if you will, at the bottom
of the strand. Well, that’s irrelevant unless we look at the other side, and then we see the relevance of this whole idea
of 5 and 3. The other side, in order to bond properly, has to be anti-parallel to this, which means that, in this situation,
the strands will run the opposite. So if we take a look at this – one, two, three, four and then here comes my
phosphate group there, coming off of the number 5 carbon – this strand will run exactly in the opposite direction, if you
will, as its counterpart on the other side of the DNA molecule. So let’s take a look at this.
If you look at this side, which side faces up, on my paper? Well, here, we have the 3 side up. Here we have the 5
side up. Running in this direction, 5 to 3, running in this direction with the 5 down – 3 to 5. This becomes very
significant when we get to the idea of DNA replication, because of the enzymes that are going to be involved,
particularly the enzyme DNA polymerase. That particular enzyme only reads in one direction. Therefore, it can only
read one side of these strands properly. So let’s take a look at that.
So off we are to semi-conservative replication. What we realize immediately is that simply unzipping a DNA strand is
not good enough. It would take too long. It would take years to replicate all of the DNA in your cells. So what we
have found instead is that DNA polymerase has evolved this very clever structure of making what we call replication
bubbles. And in essence, what happens is you get the beginning of two forks – one here, one here – proceeding in
opposite directions to each other, so that your DNA can form multiple replication bubbles and therefore form multiple
replication sites. And so your DNA can open twice as fast in many different places.
So now we know the structure of DNA. Now we have the formation of replication forks. Let’s take a look at how that
happens. Well, within the replication bubble, DNA “unzips”. It first uncoils by the action of an enzyme called helicase.
The helicase opens the DNA up and the replication forks begin. We have what is called an origin of replication. Now,
here’s the thing. Remember three rules about DNA replication and DNA polymerase. Number one, polymerase reads
the strand in the 3 direction to the 5 prime direction, which means that it adds just the opposite of that. It will add 5 to
3. So as it polymerizes and puts those babies on there, it can only polymerize with a 5 to a 3, which means that it
needs that 3 – look at the DNA molecule, that nucleotide molecule. It needs that OH group hanging off there to
polymerize in that direction. So it adds 5 to 3, reads 3 to 5. And it needs that 3-prime OH group hanging off to add
that next phosphate group onto it.
All right, well that being said, we therefore have a problem at the replication fork. Forget this side for a second and
let’s look this way. We have two strands. We’re going to call one the leading strand, which is the one that DNA
polymerase can read smoothly, and we have the lagging strand, the one it cannot read smoothly. This one, obviously,
it’s going to read 3 to 5 and add 5 to 3. This one, since it’s the anti-parallel of the top one, reads in the opposite
direction. Tough stuff for DNA polymerase, but there is an answer.
First though, how does it all begin? It begins right here by the adding of an RNA primer. We have to add a piece of
RNA there – our primer – by the action of primase. The RNA primer is added there and now, DNA can begin
polymerizing. Because think about it. There was nothing to polymerize to, so the RNA primer is there.
Okay, now DNA polymerase in the leading strand can easily read down that. That would be the end of the story
except the DNA polymerase is going like this and in going like this along the replication fork; it can’t read this bottom
strand. But it does. And the way it does is by the formation of Okazaki fragments. Okazaki fragments – small
portions of DNA, each of which are going to be primed separately by RNA primers. So therefore, we’re going to put
an RNA primer here and add a small group of DNA nucleotides at that replication fork, which of course, was done
when the replication fork was further “upstream” and further “upstream”. And so we have a series of RNA primed
Okazaki fragments that have to be stuck together. Easy!
Along can come DNA polymerase and remove those RNA primers. And then, along can come ligase and “glue” those
okazaki fragments together, making this one smooth continuous strand. So what do we have? When we’re at the
replication fork, we have two things happening. Number one, we have the DNA polymerase moving down, reading 3
prime to 5 prime very smoothly. We have the formation of Okazaki fragments. The Okazaki fragments are first
edited. The RNA primer is taken off and then ligated – put together. When we are done, we have two smooth strands
of DNA template that’s been added.
What’s the rest of the story? The rest of the story is editing. Remember? There are enzymes that can do mismatch
repair, and that’s the DNA polymerase, and excision of any mistakes.
And last but not least, what happens when we come to the end of our DNA strand? Well, what happens when we
come to the end of our DNA strand was a real problem. And that problem was this. Since it’s the end of the DNA
strand – and the beginning – you’re going to run out off places to hook to – to polymerize to. And so you loose
chunks. But chromosomes and DNA is no dummy. What they can do is they have a series of repeated units. And
these repeated units are called telomeres. And telomeres are little safety things, so to speak, so that even though
you’re going to loose a little of DNA, it won’t be coding pieces of DNA. And therefore we have end safety mechanisms
called telomeres.
Well, in a nutshell – now, we could tell you a lot more – in a nutshell, that’s the way DNA replicates. Now we’re going
to have to think about one more thing. If that’s the way it replicates, how does it work? We’ll take a look at that later.

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