AP Biology Viral Structure and Genetics

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We will be continuing our discussion of molecular genetics with the topic of viral structure and genetics.0002

Some scientists classify viruses as living organisms, whereas, other scientists classify them as non-living.0012

The reason for this controversy is that while viruses do contain genetic material, they contain nucleic acid.0020

They are not able to replicate independently of a host organism.0028

Some people say "yes, viruses are alive". They contain genetic material either DNA or RNA, and they are alive.0032

Others say "no, these are just a group of chemicals, a very complex group of chemicals.0039

But they cannot replicate on their own therefore, they are not alive".0045

They exist right at that kind of murky border between what is considered living and non-living.0049

The structure of viruses is such that they consist of nucleic acid, which is enclosed in a protein coat called a capsid.0055

And you can see here from these two examples that viral capsids can take different shapes.0064

Going back to the nucleic acid, the genetic material of a virus can be either DNA or RNA.0071

In addition, some viruses contain double-stranded DNA, whereas, others contain single-stranded DNA.0080

RNA viruses can consist of either double-stranded RNA or single-stranded RNA.0089

And viruses, as we will discuss a little later, can be classified according to the type of genetic material that they carry within the capsid.0098

This virus that you see here, this more complex looking one, is an example of a type of phage or bacteriophage, and there are various different ones.0109

This just shows you one example, but bacteriophage or just phage for short, are viruses that infect bacteria, so their host organism is a bacterial cell.0119

Here, you can see that this capsid is more complex than the one shown over here.0133

It has a head as well as what is considered some tail fibers, and within this capsid, though, is packaged the nucleic acid.0138

Here, this virus is a virus that can actually infect humans. This is an example of the influenza virus, and it is just a spherical shape capsid.0150

What you see protruding out here are glycoproteins.0161

Some viruses actually are covered by an envelope.0165

The capsid is covered by an envelope, and this envelope is derived from the host cell. It is derived from the membrane of the host cell.0169

And the glycoproteins that end up on the outside of a virus can be derived from the host cell membrane.0176

They may actually be bacterial glycoproteins produced by the bacteria. Other ones may be viral in origin.0184

Before we get into the various mechanisms of viral reproduction, we are going to just look at an overview of it0196

because there are many different details of the virus life cycle depending on the particular virus.0203

But there is some commonalities these, as well.0208

Again, one commonality is that viruses cannot reproduce independently. They only reproduce inside of another cell.0210

They must infect a host, and then, they actually use the host cell's machinery to make viral nucleic acids and viral proteins.0219

The host range - you can hear the term host range - of a virus is the range of organisms that a virus can infect.0230

Some viruses have a very narrow host range. For example, they may be only able to infect one species such as humans.0249

Other viruses have a broader host range. You might have heard of avian flu or swine flu.0257

Those are viruses that could infect swine or birds, but can also infect humans.0263

Their host range is broader. It includes more than one species.0269

In order to gain entry into a host cell, what a virus does is it binds to specific receptors on the cell surface.0273

As a first step in reproducing, the virus needs to bind to the host cell.0280

This binding is part of what determines the host range or the specificity0286

because the way that the virus binds is the proteins on the surface of the virus recognize specific receptors on the host cell surface.0292

If those receptors are not there, the virus cannot bind.0302

Therefore, this recognition between viral proteins and proteins on the surface of the cell determines the host range.0305

After binding, the virus either injects its nucleic acid, or the entire virus might enter the cell and then, release its nucleic acid.0317

But one way or another, viral nucleic acids enter the cell.0335

And this nucleic acid, it is sometimes just one single linear strand of DNA or RNA, or there are actually may be multiple pieces.0339

There, maybe, more than one piece of genetic material encased in the capsid.0348

The viral nucleic acids enter the cell, and then, at some point, viral nucleic acids and proteins are synthesized using host cell machinery.0357

The virus may actually produce certain enzymes or certain polymerases needed to make nucleic acids or proteins.0377

But many of those enzymes or proteins or factors, raw material, monomers, are just taken from the host cell.0385

Then, the virus will assemble, and viruses undergo what is called cell self-assembly.0397

The nucleic acids have been produced. The capsid proteins have been produced.0404

If there is tail fibers, those are produced, and those all come together within the host cell and assemble into a complete virus.0408

And then, the virus exits the cell.0416

Exiting the cell can be by budding out, and the cell will still live.0422

Or what we are going to talk about in a minute is a way that the virus leaves the host cell that actually involves killing the host cell.0427

A phage have been very well studied. Again, phage are viruses that infect bacterial cells.0435

And we are going to start out by talking about two types of reproductive cycles in bacteriophage, and those are the lytic cycle and the lysogenic cycle.0440

Remember, this is just a generic discussion of the viral life cycle, viral reproduction.0448

Now, we are going to get into a few specific types starting with the lytic cycle.0454

The lytic cycle is actually simpler than the lysogenic cycle.0458

Here, again, we have a bacteriophage, and here is the bacterial cell with its genetic material.0464

And the first thing that the phage does in the lytic cycle is it attaches to the host cell, so step one: attachment to the host cell.0474

And this is mediated by those specific proteins on the virus and by receptors on the bacterial cell that the viral proteins can recognize.0491

The second step is that the virus injects its DNA, and I am saying DNA here not RNA because most bacteriophage are DNA viruses.0500

Their genetic material is usually DNA.0509

So, we will just stick with talking about DNA viruses for right now. We will talk about RNA viruses shortly.0512

This green is the viral DNA. You see that capsid, which is now empty, just stays outside the bacterial cell.0517

It has no longer got nucleic acid. It is just the left over, empty protein shell.0525

Now, we have the bacterial genome, and there is also the viral genome.0532

The next step is that there is a gene that the virus has as part of its genome that encodes for an enzyme that degrades the host cell DNA.0539

So the host DNA ends up chopped up, OK? It degrades host cell DNA.0550

Here, there is shown multiple nucleic acids, and another step is that in here, we see all the protein components.0565

These are separated out just for clarity, but what happens is viral DNA and proteins are synthesized.0572

Host cell DNA cleaved up, degraded. Viral DNA is replicated, and components of the viral capsid are manufactured.0587

Now, we have all the parts to form a compete virus. The next step is for those to be assembled.0599

Step five involves self-assembly.0607

The virus particles come together. We end up with viral nucleic acids packaged inside the capsids.0614

These are all formed. They are ready to go.0620

The next thing that happens, in order to get out of the cell, is that a viral enzyme is used to lyse the bacterial cell.0623

Bacterial cell is lysed, and that is why this is called the lytic cycle because it involves lysis of the host cell.0634

Once the bacterial cell is lysed, the phage are released, and then, the phage can go and go on0643

These daughter phage can go on and infect another cell, and the cycle starts anew.0655

Again, the lytic cycle involves attachment to the cell, injection of the nucleic acid, degradation of host nucleic acid,0660

formation or replication of viral DNA and proteins, assembly of the virus particles and then, lysis of the bacterial cell and release of the viruses.0671

The second cycle that we are going to discuss is called the lysogenic cycle.0685

And this allows the viral nucleic acids to be replicated without killing the host cell.0689

There are some types of viruses that can undergo, they can reproduce via either the lytic or lysogenic cycles, and these are called temperate phage.0695

One type of temperate phage that you might hear about is called lambda, and it has been particularly well studied once.0704

You might hear that talked about, and that is a temperate phage. Again, temperate phage can reproduce via lytic or lysogenic cycle.0713

Looking at the steps of this process, again, it begins with attachment of the viral cell to the host cell0731

- excuse me - attachment of the virus, not viral cell - attachment of virus to host cell.0739

Again, nucleic acid DNA, the viral DNA injected into the cell, and the empty capsid just stays outside the cell.0751

Once inside this nucleic acid, this DNA actually temporarily forms a circle. The viral genome is normally linear.0770

It becomes circularized, and viral enzymes cleave an area of the bacterial genome.0778

And the viral DNA is incorporated into the bacterial genome via recombination.0786

This is where things are very different than the lytic cycle.0792

Viral DNA is incorporated into the bacterial genome.0795

When two different organisms, DNA, genetic material, are brought together like this, we call this recombination.0806

And we are going to hear more about recombination in some other sections of this course.0814

So this is one type of recombination which integrates viral DNA in the bacterial cell DNA.0819

This form of the virus is called a prophage.0826

We call this form of the virus a prophage when it exists as genetic material integrated into the bacterial cell's genetic material.0830

When the bacterial cell replicates, it is going to duplicate its DNA. When it does that, it is also going to replicate the viral DNA.0841

You can see, here is the bacteria. You remember that they reproduce asexually by binary fission, and it has copied its DNA.0851

It has synthesized another bacterial genome for the daughter cell, and it has also replicated the viral DNA.0861

Here, we have the bacterial cell divides, and as a result, the phage DNA is replicated.0870

What, then, happens is this can form the two daughter cells with their DNA including viral DNA. These two can, then, replicate and so on.0888

The viral DNA is going to be present in all of these offspring, and in that way, the viral DNA is reproducing.0901

However, what can happen in the lysogenic cycle is that an environmental trigger can cause the virus to enter the lytic cycle.0909

This environmental trigger could be a chemical that the bacterial cell has been exposed to. It could be ultraviolet light.0920

This viral DNA is going along. It is just quiescent sitting there on the host cell, being replicated along with the host cell.0929

And then, some trigger occurs, so environmental trigger such as ultraviolet light.0936

So, actually, that is something that would occur, hit the bacterial cell, and then, the viral DNA is removed. It is cleaved from the bacterial cell.0950

Once this phage DNA is excised, this cell is going to enter the lytic cycle.0963

The virus will cause the lytic cycle to occur just as we talked about in the previous cell slide.0971

Phage DNA will be reproduced. Phage capsids and other proteins needed to produce a complete virus particle will be produced.0983

The virus will self-assemble. It will lyse the bacterial cell, and then, those viruses will be released.0993

Once this trigger occurs, and this viral DNA is cleaved out of the host DNA, it is just going to go to the same lytic cycle we talked about earlier.1001

And again, a temperate phage is one that can produce through either of these mechanisms.1011

So far, we have been focusing on DNA viruses, and as I mentioned, most viruses that infect bacteria are in fact DNA viruses.1018

However, many animal viruses and plant viruses, viruses that infect animals or plants, are actually RNA.1026

Just reviewing the types of RNA viruses that can exist, one is a double-stranded RNA virus.1036

It has double-stranded RNA as its genetic material. One example is a virus or a set of viruses called the rotaviruses.1044

These are major cause of gastroenteritis - gastric illness - and those are double-stranded DNA viruses.1051

There are also single-stranded - excuse me - it is double-stranded RNA viruses. These are RNA viruses.1058

Single-stranded RNA viruses would include examples such as measles. Prior to vaccination, this is a major childhood illness in the US.1067

Influenza, that is also a single-stranded RNA viruses.1080

There are couple of different types of single-stranded RNA viruses. They can be classed into either positive-sense or negative-sense.1087

A positive-sense single-stranded RNA virus will carry RNA within the capsid, and that RNA can serve directly as mRNA.1098

It can be translated and then, form a protein1112

This can serve as messenger RNA. RNA serves as messenger RNA.1117

Negative-sense single-stranded RNA viruses, within their capsid, they carry a single strand of RNA, as well. Single-stranded RNA is their genome.1130

However, this RNA cannot serve directly as mRNA.1140

Instead, this RNA that is carried within the capsid needs to be used as a template to form a complementary strand, and that will serve as the mRNA.1147

In a negative-sense virus, the RNA that is carried is the complementary strand to the mRNA.1164

Looking at a reproductive cycle of a particular RNA virus, again, there are just some variations on the theme.1173

But just to give you an example of how one cycle could work, here, we have an RNA virus.1181

And it is showing it enveloped with some glycoproteins, and the virus is going to enter the cell.1187

This time, we are not having the genetic material just being injected into the cell, the entire virus is going to enter the cell.1193

Then, once in the cell, this capsid will be degraded or opened up. In some way, the RNA that is packaged inside this capsid will be released.1201

Let's say that this is a positive-sense RNA virus.1210

That means that the RNA that is in here can serve directly as messenger RNA so that when it is released,1216

when this RNA is released, it can be translated, and then, these viral proteins can be formed.1224

Some of these proteins that are found on the envelope might be formed by the virus.1235

And those would have to be eventually sent out to the cell surface via vesicles to be ready for when this virus buds out.1241

In addition, this RNA would have to be used as a template to make more RNA to be packaged inside the capsid. Then, self-assembly occurs.1250

The RNA is put inside the capsid. These glycoproteins all make it out to the cell surface.1259

The virus buds out, and when it does, it ends up with this cell membrane-derived envelope wrapped around it.1265

If this was a negative-sense RNA virus, what would happen is, again, the capsid would enter the cell.1273

The RNA would be released into the cell, but this time, it cannot just directly be used for translation to a protein because it is not mRNA.1286

It is complementary to mRNA.1300

What needs to happen is this is used as a template to make mRNA as mentioned right here.1301

Now, this type of flow of information from RNA to RNA does not typically occur inside the host cell.1309

The flow of information is from DNA to RNA to protein, not RNA to RNA to protein.1315

Therefore, the host cell is not going to be expected to make the enzyme that would be needed to use an RNA template to make messenger RNA.1321

Instead, what is needed is a particular type of RNA polymerase that is called an RNA-dependent RNA polymerase.1335

Or these are sometimes called replicases, these set of RNA-dependent RNA polymerases.1344

A replicase would be used in order to form this second strand that is complementary to the RNA that is carried inside the viral capsid.1352

And this RNA polymerase is usually already found packaged within this capsid so that this enzyme that is needed is already there.1369

The protein is packaged within the capsid. When this RNA is released, this protein is released, as well.1381

This RNA polymerase, it goes ahead. It synthesizes the complementary mRNA strand.1387

Then, those mRNA strands can be used to make proteins, and the host cell machinery can be utilized for that.1395

Notice that this occurs in the cytoplasm.1402

When you look at some DNA viruses in that infected eukaryotic cells, at least part of their life cycle occurs in the nucleus1407

because that is where the machinery to replicate DNA is or to transcribe DNA to RNA.1415

However, since this is an RNA to RNA flow, and the virus is bringing in its own enzyme, the life cycle actually occurs out here in the cytoplasm.1423

RNA viruses actually have a pretty high mutation rate.1434

And if you think about it, it makes sense because we talked about DNA synthesis and that DNA polymerase has a proofreading function.1438

and that proofreading function keeps the mutation rate pretty low.1445

RNA polymerase lacks this proofreading function.1450

So the mutation rate for an RNA virus is going to be higher than for a cell or virus or something that has DNA as a genetic material.1453

The result of these mutations is that RNA viruses in particular can change, and a host cell or a host may have been infected with the type of virus.1464

For example, you might had a flu. You had influenza.1475

Your immune system fought it, made antibodies to it, and then, the next year, you get the flu again.1479

And that is because next year's version of the flu contains slightly different proteins.1484

There is something different about it. Your immune system does not recognize it.1491

And because of this high mutation rate, these changes in the virus are more frequent.1494

This is what can cause what is called a pandemic. A pandemic is just a very widespread, global epidemic.1500

One famous example is the flu pandemic of 1918, which killed 15 million people worldwide.1509

And that is the result of mutation changes, changes in antigen so that the host cell is not recognizing this invader1516

even though you may have had some type of related infection before.1527

There is a particular type of viruses that carries RNA within their capsid, but they are not usually known as RNA viruses.1534

They are actually called retro viruses to distinguish them from other types of viruses that carry RNA as their genome.1542

The reason that retroviruses are set apart although they contain RNA is because they have a DNA intermediate as part of their life cycle.1550

These viruses, retro viruses, therefore, have a flow of information form RNA to cDNA or complementary DNA.1562

And then, form there, they go to messenger RNA and then, on to protein.1573

One well known example of a retrovirus is HIV, the virus that causes AIDS.1580

Let's think about how this would work.1589

Normally, usually, transcription occurs such as that you start out with DNA, and then, transcription occurs, end up with RNA. This is transcription.1591

What is happening here is the opposite. Instead of going from DNA to RNA, this virus is going from RNA to DNA.1604

For that reason, this step is called reverse transcription because it is the reverse of the typical order of things when transcription occurs.1616

Since a typical cell would not use RNA to make DNA, the virus has to bring its own enzyme in to do that job.1630

The cell does not already have a type of enzyme that could catalyze this process.1640

Therefore, the enzyme reverse transcriptase is packaged in the capsid of the retrovirus and allows the retrovirus to make cDNA from an RNA template.1645

This, since it is made by the virus and not by the host cell, is a great target for attacking the retrovirus.1665

And in fact, one treatment for HIV is a group of drugs called reverse transcriptase inhibitors- RTIs.1672

These are reverse transcriptase inhibitors, and they block reverse transcriptase.1682

Different treatments for viruses and bacteria, they target elements of the pathogen that are different from the host cell.1688

That way, we can attack the invader without damaging the host cell.1698

The life cycle of a retrovirus: as usual, the first thing that has to happen is the1704

virus - in this case a retrovirus - attaches to the host cell and enters the host cell.1710

In this case, the entire virus enters. It fuses with the host membrane of the cell and then, enters the cell that way.1722

Once inside, the viral RNA is released, so the capsid releases the viral RNA inside the host cell. Actually, RNA is, then, used to produce DNA.1730

And this DNA strand is complementary to the RNA that served as a template, so it is called cDNA or complementary DNA.1763

The next step is that the DNA enters the nucleus of the host cell and is integrated into the host cell genome.1772

This form, so we have the host cell. We have a nucleus, and we have got our host cell DNA; and then, the retroviral DNA ends up integrated.1798

This probably reminds you what we talked about with the lysogenic cycle, where the phage DNA became integrated into the bacterial genome.1814

And we called that form of the virus a prophage.1822

This form of the virus is called a provirus, the form of the virus whereby the DNA is integrated in the host cell DNA.1826

One very important difference, though, when we talked about the lysogenic cycle, at some point with an environmental trigger,1835

that phage DNA could be excised from the bacterial DNA and then, go off into the lytic cycle.1843

It is removed from the bacterial genome. That does not occur here.1853

Once integrated, this retroviral DNA, this provirus, is permanently a part of the cell's DNA. It does not become excised.1855

It stays in there, and then, using host cell machinery, our messenger RNA can be made.1866

The DNA, the cell's DNA can be transcribed. The viral DNA can also be transcribed.1874

Messenger RNA would be produced, and then, out to the cytoplasm where it is translated, viral particles will be made.1882

These will assemble, and then, they will bud from the host cell.1893

They will be released. They will just bud out from the host cell.1897

Again, virus starts out with RNA inside its capsid. Once inside the host cell, reverse transcriptase is the enzyme used to synthesize cDNA from RNA.1901

That cDNA enters the host cell nucleus, becomes integrated with the host cell genome. It is called a provirus now.1914

That proviral DNA can be transcribed and then, translated to make viral proteins such as reverse transcriptase and capsid proteins.1922

Also, the RNA is made. That is going to be packaged inside these capsids.1933

Viruses assemble. They bud out, and they can go on to infect another cell, so that is the retrovirus life cycle.1942

Now, we are going to talk about infectious agents that are even simpler than viruses.1948

Viruses are composed of a capsid, which is made of protein and then, nucleic acid.1953

There is a couple types of infectious agents. They are composed of even less.1959

One of these is prions. Prions are infectious agents that are composed only of protein.1963

They do not carry nucleic acid. Examples of this, pretty well known examples- mad cow disease.1971

The counterpart of this in humans is a disease called Creutzfeldt-Jakob disease.1982

In sheep, there is a disease called scrapie, and all of these are neurological illnesses.1990

They have very long incubation periods meaning that an individual could be infected by the prion2002

and not have any symptoms for years or even a decade, and then, they begin showing symptoms.2009

It is a degenerative neurological or these are degenerative neurological illnesses. Unfortunately, they are fatal, and they are not treatable.2015

How exactly is a protein causing this?2024

Well, what is thought is that prions appear to be misfolded proteins that are similar to host cell proteins.2026

Prions are composed of these proteins that are not folded correctly, and then, what is thought is that they get into the neurological system.2037

And they cause the normal counterpart inside the neurological system to also misfold, so somehow, they induce similar proteins to misfold, as well.2044

What happens? Well, these misfolded proteins are believed to aggregate, and this clumping up or aggregation of misfolded proteins damages the brain.2059

Prions are actually very hard to inactivate, as well, so disinfecting or getting rid of, killing, deactivating these, is difficult.2072

They are pretty resistant to many of the typical methods of sterilization, and they are tough to destroy.2080

Now, a second type of infectious agent that is very simple is a group called the viroids.2087

Whereas, prions are composed just of protein, viroids are just composed of RNA.2095

These are circular RNA molecules, and they can infect plants, so they can cause plant disease- no capsid, just naked RNA molecules.2099

It is believed that they cause disease in plants by somehow disrupting regulatory systems inside the plant that causes problems with the plant growth.2116

Again, these are both infectious agents that are even simpler than viruses.2129

Alright, today we have discussed viruses as well as prions and viroids.2136

We are going to go ahead and do some examples to review this material.2140

Example one: describe the steps involved in the reproduction of a virus by the lytic cycle.2143

Remember that the first step of infection via the lytic cycle is attachment of the virus to the host cell.2151

And that is mediated by specific proteins on the surface of the virus and receptors on the surface of the host cell.2166

Once the virus is attached, the virus injects its DNA into the host cell.2174

The next thing is that the virus produces an enzyme that degrades the host cell DNA, so a viral enzyme degrades host DNA.2187

After that, the virus uses the host cell's machinery to replicate viral DNA and proteins produced using the host cell machinery.2206

Capsid proteins would be produced and the other proteins that the virus has packaged within the capsid.2224

Also, copies of the genetic material, the viral genome, are made. After that, the virus particles self-assemble.2229

The DNA is placed inside the capsid. Any different parts of the capsid that need to come together do, and then, a viral enzyme lysis the bacterial cell.2241

The viral enzyme lysis the host cell, and virus particles are released.2260

These viral particles, the offspring of the original virus are, then, free to go ahead and infect another cell, so, this is the lytic cycle.2272

How does a retrovirus differ from an RNA virus?2284

Well, recall that with an RNA virus, the flow of information is just RNA to RNA and then, protein. No DNA is involved, so this is a typical RNA virus.2289

The details differ depending on if it is a single-stranded or double-stranded, positive or negative-sense.2305

But overall, the flow of information is just RNA to RNA.2311

In contrast, a retrovirus has a DNA intermediate, so there is a DNA intermediate.2314

The flow of information is from the RNA packaged within the retrovirus that is used as a template to make cDNA,2324

which is then, used to make RNA, and that can be translated to protein.2332

The difference between a retrovirus and a group of viruses that we actually2340

call RNA viruses is that there is a DNA intermediate in the retrovirus life cycle.2343

Example three: what is the difference between a positive-sense RNA virus and negative-sense RNA virus?2351

Again, with the single-stranded RNA viruses, there can be positive or negative-sense viruses.2357

In a positive-sense RNA virus, the genome serves directly as mRNA.2364

That RNA that is packaged within that capsid can be used, can be directly translated into a protein. That is a positive-sense virus.2379

In a negative-sense virus, the genome serves as a template to produce mRNA.2392

In other words, the genome is complementary to mRNA, so the genome can be used in negative-sense.2408

Here, you have the RNA that is used as a genome.2419

That can be used to synthesize - using the particular type of RNA polymerase called a replicase - a complementary strand of RNA.2423

And this is the strand that is an mRNA rather than directly packaging mRNA within the capsid.2434

Example four: put the steps of the lysogenic cycle in the correct order.2442

A: phage DNA is excised from the host cell's genome following an environmental trigger.2447

B: phage DNA is copied along with bacterial DNA when the host cell divides.2454

C: viral enzymes cleave the circular bacterial genome, and the viral DNA is incorporated into the bacterial genome forming a prophage.2459

D: the virus attaches to the host cell using specific receptor proteins.2470

E: the viral nucleic acid is injected into the host cell.2476

Well, recall that the first step of the lysogenic cycle is for the virus to attach to the host cell. That is what happens first.2480

That is right here- attachment to the host cell, so we will put that first.2489

Then, think about what the virus does after it attaches to the host cell.2496

The next thing that the phage does is it injects its genetic material into the host cell, so we need to find that one, which is down here. That is E.2501

After attaching to the host cell, the viral nucleic acid is injected into the host cell- E.2511

With the lysogenic cycle, the next thing that happens,2522

once that nucleic acid is in the cell is that it actually becomes integrated into the bacterial genome.2525

Phage DNA is excised from the host cell's genome- not yet.2535

Phage DNA is copied along with bacterial genome- not yet.2539

Viral enzymes cleave the circular bacterial genome, and the viral DNA is incorporated into the bacterial genome forming a prophage- that is correct,2543

so, attachment, injection of nucleic acid and then, integration into the bacterial genome.2551

After that, the phage DNA is not yet excised.2557

Actually, what happens after integration is that phage DNA is copied along with bacterial DNA when the host cell divides.2561

And that leaves us with the final step.2569

If there is an environmental trigger such as ultraviolet light, the phage DNA can be excised from the host cell's genome and then, enter the lytic cycle.2571

So, this is the correct order- D, E, C, B and A.2580

That concludes this session on viral genetics.2586

Thanks for visiting Educator.com.2590