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Lecture Comments (11)

1 answer

Last reply by: Dr Carleen Eaton
Tue Jun 17, 2014 7:59 PM

Post by Meredith Roach on May 25, 2014

At 5:50 Dr. Eaton says at the 3' end of the DNA strand there is a -OH on the 5' carbon.  Shouldn't this be an -OH group on the 3' carbon?

Also, in the first slide showing bases, if you look at the thymine base, I believe there are 2 hydrogens missing from C1, otherwise the carbon would have a net charge?

0 answers

Post by Bao Vo on February 10, 2014

Transcription and Translation video has the wrong Caption. Fix it please

1 answer

Last reply by: Dr Carleen Eaton
Tue Sep 17, 2013 4:33 PM

Post by Vinit Shanbhag on September 11, 2013

Does a Phosphodiester bond formation require energy?

3 answers

Last reply by: Dr Carleen Eaton
Tue Sep 17, 2013 4:29 PM

Post by Kendrick Miyano on April 16, 2013

Hello, Dr. Eaton.

If the telomeres shorten after each round of replication, does this mean that they will eventually become too short to replicate? Will the telomeres be broken down and then recycled?

0 answers

Post by Dharshini Selladurai on December 6, 2010

3'-OH on 3' Carbon

DNA Synthesis

  • DNA replication is semiconservative. Each strand is used as a template for the synthesis of a complementary strand.
  • DNA replication begins at an origin of replication. DNA Helicase unwinds the double helix, creating a replication bubble.
  • Single-strand binding proteins stabilize the DNA strands and hold them apart.
  • Topoisomerases cut, unwind and religate, the DNA ahead of the replication fork in order to relieve the tension caused by unwinding the double helix.
  • DNA polymerases synthesize the complementary strand of DNA in the 5' to 3' direction and require a primer. An RNA primer is synthesized by primase.
  • DNA is synthesized in one long strand for the leading strand on the template DNA. For the lagging strand, DNA is synthesized in short segments called Okazaki fragments.
  • DNA polymerase possesses a proofreading function. If an incorrect nucleotide is added to the growing strand of DNA, it is removed and replaced.
  • Errors and damaged DNA are also corrected through mismatch repair.
  • Telomeres are repeating nucleotide sequences on the ends of DNA molecules. These segments become shorter with each round of DNA replication.

DNA Synthesis

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Review of DNA Structure 0:09
    • DNA Molecules
    • Nitrogenous Base: Pyrimidines and Purines
  • DNA Double Helix 3:03
    • Complementary Strands of DNA
    • 5' to 3' & Antiparallel
  • Overview of DNA Replication 7:10
    • DNA Replication & Semiconservative
  • DNA Replication 10:26
    • Origin of Replication
    • Helicase
    • Single-Strand Binding Protein
    • Topoisomerases
    • DNA Polymerase
    • Primase
  • Leading and Lagging Strands 16:51
    • Leading Strand and Lagging Strand
    • Okazaki Fragments
    • DNA Polymerase I
    • Ligase
  • Proofreading and Mismatch Repair 22:18
    • Proofreading
    • Mismatch
  • Telomeres 24:58
    • Telomeres
  • Example 1: Function of Enzymes During DNA Synthesis 28:09
  • Example 2: Accuracy of the DNA Sequence 31:42
  • Example 3: Leading Strand and Lagging Strand 32:38
  • Example 4: Telomeres 35:40

Transcription: DNA Synthesis

Welcome to

This is the first in a series of lessons on molecular genetics, and we are going to start out with the discussion of DNA synthesis.0002

Before we talk about the synthesis of DNA, we are going to do a review of DNA structure.0011

And recall that this was covered in detail under the lesson on proteins and nucleic acids.0016

But since synthesis is closely related to the structure of DNA, it is important that you understand that before you proceed.0023

Recall that DNA or deoxyribonucleic acids contain an organism's genetic material.0030

This material is passed from parent to offspring and from cell to daughter cell, and it contains the instructions for the production of proteins.0038

The monomers from which DNA is composed are called nucleotides.0049

RNA is made of nucleotides, as well, although there are some differences between RNA and DNA structure.0053

And it is important that you remember these for the exam.0060

Nucleotide consists of a sugar. In RNA, it is a ribosugar.0065

And in DNA, it is a deoxyribose sugar because one of the oxygens is missing when compared to ribose, so it is deoxyribose; and this is a pentosugar.0072

It is a 5-carbon sugar, so it is a pentosugar.0082

Attached to the sugar are a nitrogenous base and a phosphate group.0087

Looking at the nitrogenous bases, there are five nitrogenous bases, but remember that uracil is found only in RNA; and thymine is found only in DNA.0093

The other three are found in both.0114

Looking at the two groups, the first group of nitrogenous bases is the pyrimidines - actually pyrimidines right here - and the purines on the top.0116

The pyrimidines consist of a 6-membered, ring and those are cytosine, thymine and uracil.0140

Purines consist of a 6-membered ring fused to a second ring, and those two are adenine and guanine.0148

Recall also that guanine, G, pairs with C, and there are three hydrogen bonds between those two.0156

A pairs with T, and there are two hydrogen bonds between those two.0167

The nucleotides in DNA and RNA are bonded by a phosphodiester bond to form polymers.0175

Another very important difference between DNA and RNA is the fact that DNA consist of a double helix, whereas RNA is single stranded.0184

Two complimentary strands of DNA pair up to form a double helix.0193

In the early 1950s, Watson and Crick proposed this double helix model for DNA.0197

Their work or crucial to their work were actually discoveries by Rosalind Franklin, who was working in the laboratory of Maurice Wilkins.0203

What Franklin did was to perform x-ray crystallography on DNA.0212

The technique of x-ray crystallography involves using purified samples of a material, in this case, DNA.0217

And once those have been crystalized, they are hit with x-rays.0227

The x-rays are scattered by the structure, the DNA structure or diffracted.0231

This creates a diffraction pattern, and the diffraction pattern can be analyzed to try to figure out the actual shape and structure of the molecule.0237

Rosalind Franklin performed this analysis on DNA, it is crystallography analysis, and she showed the picture to Watson and Crick.0249

Looking at this picture, they took that information and then, went on to build the double helix model.0257

It is also important to remember that the work of these scientists was preceded by the0264

work of other scientists who determined that DNA was actually the genetic material in the first place.0269

Earlier in the 20th century, people were not sure if proteins were the genetic material or if DNA was the genetic material.0277

So, it took many years and many experiments to determine that DNA is the genetic material.0284

And then, from there, scientists went on and studied the structure and function of DNA.0290

As you look at the double helix, keep in mind that DNA has a directionality. It has a 5'-end and a 3'-end.0297

Let's say this is a 5'-end, follow it down. 3'-end is here.0305

The other strand is going to be complementary and antiparallel.0310

Since this is an antiparallel, it is orientation will be 3' to 5'.0314

The nucleotide on the 5'-end has a phosphate group on the 5' carbon, whereas on the 3'-end, there is a hydroxyl group on the 5' carbon.0324

There is a definite directionality on the 5' carbon.0340

Just to review base pairing and what we mean by complementary strands, let's use an example.0345

If you have a strand of DNA with the sequence C-C-G-T-A-A-C-T going 5' to 3', the complementary strand,0351

the other half of this double helix will go from 3' to 5', and C pairs with G.0367

C pairs with G. G pairs with C.0378

T pairs with A, A-T, A-T, C-G, A-T, so remember G-C-A-T.0381

These are complementary and antiparallel.0395

And when you are answering questions on the exam,0398

it is important to pay attention not only to the pairing in the nucleotides but also to the directionality 5' versus 3'.0400

This double helix structure suggested to Watson and Crick a mechanism for DNA replication.0411

And it turns out that this structure is very important to the way that DNA is replicated and a perfect example in biology of form and function being related.0419

We are going to start out with an overview of how DNA is replicated and then,0432

break it down into the different steps and into the different enzymes that are necessary.0437

Looking at a piece of DNA 5' to 3' A-G-C-C-T-A, its complementary strand would be 3' T-C-G-G-A-T 5'.0444

The mechanism for replication is such that the DNA double helix is unwound.0463

and each of the strands is used as a template to form the complementary strand.0469

This method of replication is known as semiconservative, so let's look at how this would work.0475

This would be unwound so separated into two different strands and the top strand, 5' A-G-C-C-T-A 3'.0485

This is one strand and the other strand. Here is that first strand.0498

Now, this bottom strand 3' T-C-G-G-A-T 5'.0503

What DNA polymerase can do - that is an important enzyme involved in the replication of DNA -0511

is it will add nucleotides to form the complementary strand using one of these existing strands as a template.0517

Here, what would be added on the 3' would end up being T-C-G-G-A-T.0526

You can see, this top strand was existing and uses a template to form the bottom strand, so you have an entire DNA double helix back.0538

Looking at when the bottom strand uses a template, what you would end up with is A-G-C-C-T-A, so number one is there.0550

Now, both strands are back.0560

The reason this is called semiconservative is because if you look at the new DNA molecules,0562

you start out with one DNA molecule, these strands were split, and you form two DNA molecules.0568

It is semiconservative because one of the original strands is conserved, only one though, so it is semi. It is partly conserved.0575

Actual conservative replication would be where this either stays intact or separates and then, comes back together.0585

And then, you would end up with a second DNA double helix that is completely formed from fresh nucleotides.0594

If this were to stay together or go back together, that would be conservative replication.0602

Here, it is split apart. The original strand is only partly conserved, so this is semiconservative replication.0609

After having an overview of the process, let's go ahead and look in depth at different steps in components of DNA replication.0618

DNA replication begins at particular areas of the DNA called origins of replication.0629

There are specific nucleotides sequences that are recognized by the enzymes involved in DNA synthesis.0638

Recall that bacterial DNA is circular, and there is only one origin of replication on bacterial DNA.0646

Whereas, in eukaryotes with chromosomes, there can be many, many origins of replication.0652

And that way, synthesis can start at many origins, go along, and then, eventually, these will meet up.0660

And that speeds up the process of the DNA replication quite a bit.0666

The first step in order for replication to proceed is going to be to unwind this double helix.0671

There is a series of enzymes involved in these different steps, and you should be familiar with these.0680

Helicase is the enzyme that unwinds the helix.0685

Replication really cannot occur with this closed because the enzymes need to use these strands as template, so they have to be separated out.0692

This structure formed - let's go ahead and put in the 5' and the 3' here - this is known as a replication bubble.0704

Helicase unwinds this area of DNA at the origin of replication.0718

And another protein called single or set of proteins actually, single strand binding proteins, this tells you what it does.0726

They bind to the areas of DNA that have been separated that are single strand.0741

Single strand binding proteins bind, and they stabilize the unwound DNA. They, kind of, hold them apart, so they stabilize the unwound DNA.0747

Helicase unwinds the helix, then, single strand binding proteins bind to the unwound areas,0760

stabilize them, hold them apart, and this is created a replication bubble.0766

As replication proceeds, you will see replication. Let's say it is starting, and the new DNA is formed.0772

That is the brown, so nucleotides are added. They are added.0783

It goes along, and in front of that, helicase is unwinding the DNA.0787

Well, if you think about if you take a piece of yarn, and you pulled the strands apart, in front of the area you are unwinding, the tension increases.0795

The area in front of where you are pulling apart actually becomes more tightly wound or even tangled.0803

To prevent this tension or to release this tension, there is a set of proteins called topoisomerases.0808

What topoisomerases do is they actually cut, unwind and then, religate or rejoin DNA ahead of the replication fork to release that tension.0820

They cut, unwind and religate or rejoin DNA ahead of the replication fork.0833

So far we have gotten into some of the preparation. The actual replication has not happened yet.0846

A couple more things have to happen before it can occur.0851

The DNA has been unwound. It is being stabilized so that it is held apart.0855

And topoisomerase is keeping this area from being under too much tension ahead of the replication fork.0860

Actual replication is done by a series of enzymes known as DNA polymerases.0867

DNA polymerases add nucleotides in the 5' to 3' direction only. They cannot add the other way.0873

If you have a piece of DNA, it is 5'. It is going along.0889

It is being constructed. DNA polymerase will add to this end.0894

It will add to that 3'-end more nucleotides. It cannot add to the other end.0899

That is one limitation on DNA polymerases.0903

The second one is that it requires a primer.0907

Just briefly, there are multiple different DNA polymerases in bacteria and in eukaryotes.0911

One that you will often hear discussed is DNA polymerase III, which in bacteria, does the main job of DNA synthesis, so just something to be aware of.0920

Alright, DNA polymerase must add nucleotides in the 5' to 3' direction, and it requires a primer.0931

DNA polymerase cannot just pick up a nucleotide, stick a second one on, and go from there.0939

It cannot put that first two together. It has to add nucleotides to a pre-existing strand.0946

The enzyme that makes this primer to get things started is called primase, and the primer is actually an RNA primer, so it makes an RNA primer.0956

The RNA primer...let's look at this top strand.0970

I will talk about this bottom strand in a minute. It is a little more complicated there.0975

Let's look at this top strand. We are going from 3' to 5'.0978

What primase will do is it will make a little RNA primer, so that is the little black area; and then, the brown line, those are DNA so RNA primer.0983

And then, DNA can be added to that by DNA polymerase.1000

Let's look at the difference between synthesis of these two strands because there are several important differences.1005

This strand that you see at the top is called the leading strand.1014

The bottom is the lacking strand, and synthesis of the leading strand is more straight forward.1026

You see that the exposed end of DNA, when this helix was initially separated, is initially pulled apart, what you ended up with is this is 3’ and free.1033

Before any of this was synthesized, it was just 3' to 5' and this 3' and this free.1048

Primase comes along. It makes the RNA primer, and then, DNA polymerase adds nucleotides, the complementary nucleotide one at a time, going from 5' to 3'.1054

It is adding to this free 3'-end, and therefore, it goes along.1065

It adds helicases unwinding at the replication fork.1074

Synthesis continues in one long continuous strand on the leading strand.1078

Synthesis, you will see 5' to 3', is moving towards the replication fork. Fine.1083

The problem on the other strand is that to move towards the replication fork, DNA polymerase would have to add 3' to 5'.1091

And it cannot do that because the free end here is 5'.1104

The complementary strand is going to be 3', and that would force a DNA polymerase to add this way, which it cannot do.1107

It cannot do that, so how is this going to work because DNA polymerase wants to start here, 5', and go towards 3'.1115

But if it does that, it is going away from the replication fork, and things will quickly come to a halt.1126

Well, the solution is to synthesize the DNA in a bunch of short segments called Okazaki fragments after the scientist who discovered this.1132

These are Okazaki fragments. These are short segments of DNA that are in the 5' to 3' direction, constructed 5' to 3'.1143

What happens is the RNA primer is made, and then, an Okazaki fragment is made.1154

Primase makes the RNA primer, and then, DNA polymerase adds nucleotides going in that 3' direction; then, it comes to the end.1166

Another RNA primer will be made towards the replication fork, closer not towards but closer to replication fork.1178

We saw 5' to 3', and then, DNA polymerase will add nucleotides.1189

Then, another RNA primer will be made, and nucleotides DNA will be added, 5' to 3'.1195

What we end up with is a series of short segments of DNA. The only thing is, now, these need to be attached.1203

There is a separate set of enzymes that is responsible for that.1208

There is another polymerase. DNA polymerase III is doing the work of adding the nucleotides.1212

DNA polymerase III, or you will see it called DNA Pol III.1219

DNA polymerase I/DNA Pol I, removes the RNA primer on the Okazaki fragments and replaces them with DNA.1223

And it is able to do that because if you look at these two fragments right here, remember that DNA polymerase must add to a pre-existing nucleotide strand.1242

Well, in order to remove this primer here, what DNA polymerase can do is it can attach1252

around here and add nucleotides to the end of the Okazaki fragment in front of it.1258

It adds nucleotide, nucleotide. It comes to the RNA.1265

It can remove the RNA, add nucleotide, nucleotide, nucleotide, DNA.1267

And then, another enzyme called ligase, DNA ligase, joins adjacent Okazaki fragments together after the primer has been removed.1273

Just continuing on, DNA polymerase would, then, grab on to this area, add a nucleotide to the end of this fragment1297

all the way removing the primer and the fragment behind it and adding nucleotides, and that will continue on.1306

The only time there is a problem is that these very 5'-ends, and we will talk about that in a minute,1314

because the ends, the 5'-ends of the DNA, are a problem because there is nothing for DNA polymerase to add on to in front of it.1321

When you get to the very end, how are you going to replicate that when you need a primer?1329

DNA synthesis is actually an incredibly accurate process. The error rate after synthesis is only about one in a billion.1339

The accuracy is a result of proofreading, and there are also repair mechanisms.1348

If you looked at DNA right when it was synthesized, right when DNA polymerase added a nucleotide, the error rate would be higher than one in a billion.1355

However, the final product is much more accurate because what DNA polymerase does is after it adds a nucleotide,1363

it checks to make sure that the nucleotide is correct, and if it is not, it removes it and replaces it.1374

Proofreading is done by DNA polymerase.1383

DNA polymerase checks the newly added nucleotide and replaces it if it is incorrect, and it can check because the template is right there.1389

It has been forming the new strand based on this template and it can double check off the template.1407

Now, mistakes can still be slipped through even with proofreading.1414

There is a second mechanism that ensures the accuracy of DNA replication, and this is mismatch repair.1421

If the wrong nucleotide is added somehow, and it is not caught by DNA polymerase,1428

then, these set of enzymes can actually clip out the incorrect nucleotide and replaces it with the correct one- replaces incorrect nucleotides.1435

This is also a way to fix DNA if damage has occurred.1451

And we will talk about mutations more in the next lecture when we talk about transcription and translation.1455

But changes in DNA sequence are called mutations, and if they are not fixed, they will be passed on when DNA is synthesized; and they can be harmful.1463

We talked about correcting these nucleotides that are incorrect by cutting out the wrong one with mismatch repair and replacing the correct one.1473

Just to be aware, that enzymes that cut DNA are called nucleases.1485

The ends of DNA, as I said, the 5'-end, presents a problem in terms of replication because you have got your 5'-end and your 3'-end.1499

Here is the replication fork 3', 5'. Let's look at the leading strand- it is simpler.1513

5' and there is the primer. Nucleotides are added, added, added, added, no problem.1521

We get all the way to here. Now, how are we going to get rid of this primer?1531

There is nothing to add on to in front of it. Actually, we cannot.1535

What ends up happening is this segment, this could be degraded.1542

What you would end up with is this segment of DNA that has not been replicated.1548

The same will happen on the lagging strand, but it will end up happening down here,1555

that this very end of the DNA cannot be replicated because there is no way to replace that RNA primer with DNA because there is no 3'-end to add on to.1560

This would obviously cause a huge problem if there is some important genes at the end of the DNA.1569

The way nature has gotten around that is the ends of DNA contain what is called telomeres.1577

And these are repeating nucleotide sequences that do not code for proteins.1584

They are noncoding regions. A common sequence is T-T-A-G-G-G, and this sequence is repeated many times on the end of DNA.1589

There will be a whole section of repeats of this on the ends, and that has a couple of functions. It protects the ends.1604

And when replication occurs, these ends are going to become shorter because this little part will not be replicated; then, you have a shorter piece of DNA.1615

Next round of synthesis, a little more will not be replicated and so on.1624

With each round of replication, these segments become shorter.1628

As an organism ages, the telomeres become shorter, and this is an interesting and active area of research.1632

What role do telomeres play in the aging process?1639

Another interesting area of research is cancer research.1647

It is thought that when the telomeres get to a certain length - they are shorter and shorter with each round of replication -1650

it may be a signal to the cell to stop replicating.1659

Remember that cells usually just replicate a certain number of times, and then, they stop, whereas cancer cells, we often describe them as immortal.1662

They will just keep dividing, dividing, dividing, dividing.1670

And it could be that a defect in the telomere is one factor that allows them to become immortal and continue dividing.1674

Now that we have talked about DNA synthesis, we are going to do questions to review the material.1684

Example one: describe the function of each enzyme during DNA synthesis, and it is definitely important to know these enzymes for the exam.1690

The first one is DNA ligase.1700

Remember ligate is "to join", and the job of DNA ligase is to join the ends of adjacent Okazaki fragments.1703

Okazaki fragments are the short segments of DNA that are formed on the lagging strand.1715

DNA polymerase I removes the RNA primer between those adjacent strands and replaces them with the correct DNA sequence.1727

And then, DNA ligase joins those adjacent fragments.1734

Primase: notice that enzymes end in ase, and the rest of the word often tells you the function; so this is an enzyme that has to do with primer.1740

This is the enzyme that actually makes an RNA primer, so it synthesizes the RNA primer used by DNA polymerase.1749

Recall that DNA polymerase can only add nucleotides to a pre-existing strand, and therefore, to get synthesis of DNA started, there needs to be a primer.1769

Helicase: helicase has the job of unwinding the DNA double helix.1782

In order for synthesis to occur of DNA, the double helix needs to be separated.1796

And then, each of those strands serves as a template for the synthesis of a new strand,1805

a new complementary strand of DNA, in the process of semiconservative replication.1809

DNA polymerase: this is actually a group of enzymes that are multiple DNA polymerases such as DNA polymerase I and DNA polymerase III that we discussed.1816

The job of DNA polymerase is to add nucleotides to either a primer, or it is starting out as the RNA primer and then, to the growing chain of DNA.1824

OK, it added nucleotides to form a new strand of DNA, and this is a strand that is complementary to the template strands.1845

Recall that it only works in the 5' to 3' direction. It only synthesizes DNA in the 5' to 3' direction, and it requires a primer.1857

Single strand binding proteins bind to the separated DNA strand meaning it is no longer a double helix in that region.1871

It has been pulled apart to stabilize them.1885

After helicase separates those strands of DNA, single stranded binding proteins hold them apart, keep them separate, keep them stable.1890

Example two: what are two mechanisms by which the accuracy of DNA sequence is maintained?1905

Recall that the error rate is only about one in a billion. How can that be?1911

Well, two main mechanisms: one is proofreading, and the other is mismatch repair.1917

Proofreading is done by DNA polymerase. DNA polymerase checks to make sure that the correct nucleotide has been added.1930

If it is incorrect, it replaces it with the correct nucleotide.1938

In mismatch repair, a series of enzymes checks the DNA. If it finds a mismatch then, the incorrect nucleotide is clipped out, and the correct one is replaced.1943

Example three: how is the method by which DNA is synthesized from the leading strand different than the method used for the lagging strand?1959

Recall that when a replication fork is formed, you are going to end up with two.1969

The DNA strands separated. Each can act as a template.1977

These strands are complementary and antiparallel.1980

Since DNA polymerase has to work in the 5' to 3' direction, the strand where the 3'-end is exposed, no problem.1987

The RNA primer is formed. There is a little 3'-end exposed.1998

DNA polymerase comes along as nucleotides, 5' to 3', and it moves towards the replication fork.2003

Helicase is unwinding at the area of the replication fork, and DNA polymerase just moves along adding nucleotides.2012

The DNA in the leading strand is synthesized in one continuous strand- one long continuous strand.2021

Lagging strand is a different situation. The exposed end is 5'.2045

Well, we cannot just start going 3' to 5'. That would not happen.2050

That cannot happen. Instead, what happens is DNA is synthesized in a series of short segments.2054

The lagging strand is down here and the DNA is synthesized in a series of short segments.2065

There is the primer. These are called Okazaki fragments- synthesis via Okazaki fragments.2071

This requires a couple of extra steps because now you have a bunch of just separate small pieces of DNA.2083

After synthesis of the Okazaki fragments, the RNA primer is removed, and adjacent Okazaki fragments are ligated.2091

Later, adjacent fragments are ligated by DNA ligase.2103

The essential difference is that in a leading strand, DNA is synthesized as one long continuous piece.2114

In a lagging strand, DNA is synthesized in short segments called Okazaki fragments.2122

And then, DNA polymerase I replaces the primers, the RNA primers between these, and they are ligated together by DNA ligase.2128

Example four: what are telomeres?2141

How do they change with each round of DNA synthesis, and why does this change occur?2144

First, what are telomeres?2148

Remember that telomeres are repeating nucleotide sequences found on the ends of DNA molecules- repeating sequences, and they are noncoding.2150

They do not code for a gene, and they are found at the end of a DNA molecule.2168

They become shorter with each round of DNA synthesis.2181

The first question, what are telomeres, we answered that.2185

How do they change with each round of DNA synthesis? They become shorter with each round.2188

Why is this?2203

Recall that since DNA polymerase has to have an existing strand at nucleotides II, so we have a strand of DNA 5' to 3', and then, there is an RNA primer.2207

The DNA is synthesized 5' to 3'. There is no way to remove and replace...well, there is a way to remove this primer.2229

There is no way to replace it.2237

Once this is removed, there is no free 3'-end to add nucleotides to, so this little segment ended up not getting replicated.2239

Telomeres become shorter with each round of replication, and that answers why this change occurs.2250

This change occurs because there is no free 3'-end to add nucleotides to at one end of the DNA.2257

And DNA polymerase can only add nucleotides to an existing strand.2271

OK, telomeres are repeating noncoding sequences found in the end of DNA.2302

They become shorter with each round of replication, and this is because there is no free 3'-end at nucleotides II at the very ends of the DNA.2307

That concludes this lecture on DNA synthesis. Thanks for visiting