AP Biology Bacterial Genetics and Gene Regulation

Welcome to Educator.com.0000

We will be continuing the topic of molecular genetics with the discussion of bacterial genetics and gene regulation.0002

We will start out with the review of the bacterial genome, which was covered initially in the cell structure and function lecture.0009

Recall that the bacterial genome is a single, circular molecule of DNA.0018

This genome is located in what is called the nucleoid region because prokaryotic cells do not have a true membrane-bound nucleus.0023

Bacteria sometimes also have additional genes that are carried on what is called a plasmid.0033

This is the main genome, the main bacterial genetic material.0040

However, right here, what we have is a small segment of circular DNA that can carry additional genes called a plasmid.0047

This is found outside the nucleoid region, and plasmids are important in biotechnology. We will be discussing that later.0059

And what this piece of DNA can do is by providing additional genes for the bacteria, they can confer on them their traits such as antibiotic resistance.0068

Bacteria reproduce asexually through binary fission.0084

The bacterial DNA is replicated, and one copy of the genome is passed to each of the two daughter cells; and those are identical.0088

However, bacteria do have ways of generating genetic diversity.0096

And we are going to talk about several of these mechanisms right now starting with transformation.0103

Transformation is a process through which bacteria can take up small pieces of DNA from their surroundings, from their environment.0109

This process occurs out in nature. It is also used in a laboratory.0118

And in fact, one of the recommended AP biology laboratories is transformation, and you will do that in your class most likely using the heat shock method.0123

That is also covered as a review at the end of this course.0132

Again, small pieces of DNA are taken up from the environment.0138

When we are working in the lab, and we have a bacteria we are working on; and we want it to produce something. We want it to produce a protein.0141

We want to manufacture a vaccine or a medication, or we want to study a particular protein; and we need a lot of it. We get the bacteria to make it for us.0150

A way that we can do that is to put the gene encoding that protein into the bacteria, and we can get it in there using transformation.0161

When we want to put some DNA into the bacteria, we need to use what is called a vector.0170

It is a piece of DNA that is going to carry the genes, a small piece of DNA.0177

And it is a small piece of DNA that is used to carry particular genes or regions into the bacteria.0185

An example would be a plasmid. A plasmid can be used as a vector to get DNA inside a bacterial cell.0193

A second method that creates genetic diversity in bacteria and allows for the exchange of genetic material is called transduction.0204

In transformation, the bacteria was simply taking up DNA from its surroundings.0213

In transduction, a phage acts as an intermediary through which DNA is transmitted from one bacteria to another.0218

This is not something the virus likely does on purpose, but, rather, it is usually the result of a mistake.0231

Recall that when phage replicate, they do so by infecting a bacterial cell, degrading host DNA, making viral DNA,0237

packaging up that DNA and with the lytic cycle, lysing the bacterial cell, leaving the cell, infecting another bacteria.0249

Now, let's say we have some bacterial DNA here, and we have a phage. We will just make it a really simple phage with phage DNA.0259

And as usual, per the lytic cycle, the phage is going to inject its nucleic acid into the bacteria and degrade the bacterial DNA.0277

Then, using host cell machinery, we get all this duplication, replication of phage DNA and also production of capsid proteins.0296

Correct self-assembly would involve and usually does involve capsid being assembled and the viral DNA being packaged inside the capsid.0315

But think about what could happen.0330

What could happen is that bacterial DNA could accidentally be packaged inside the capsid along with or in lieu of the viral DNA.0332

Then, when the lytic cycle continues, and the bacterial cell is lysed, out come the new viruses, only this one is carrying bacterial DNA.0343

Let's say we have another bacterial cell over here. It has its own genome, and now, this virus goes to infect it.0368

It is going to inject its DNA, which is now inside this new bacterial cell.0382

But since the viral DNA is not there or may not be there correctly or although may not be there, this phage is not going go through its whole life cycle.0394

This little piece of DNA could even end up integrated into the bacterial genome through recombination.0405

There could be exchange of homologous segments of DNA, and then, we could end up with this recombinant cell.0412

It has got its own original genome plus a little bit of DNA from...we will call this bacterial cell A. This is bacterial cell B.0420

Now, bacterial cell B is carrying a little bit of DNA that originated with A, so this allows for recombination between cells.0433

And with transformation as well, DNA that is taken up can recombine and be integrated in the genome0442

Or a plasmid can also just stay separate, stays outside the nucleoid region, but sometimes this crossing over occurs; and we end up with recombination.0449

Phage are used in the lab, and we use them to transduce to help us get DNA into bacterial cells; and in this way, we can study different genes and proteins.0461

Overall, with biotechnology, we are able to develop immunizations. Medications such as insulin can be produced using biotechnology.0476

Finally, bacteria, although they do reproduce asexually, can undergo what is considered to be a primitive form of sexual reproduction.0487

And this is known as conjugation.0496

During conjugation, DNA is transferred from one bacterial cell to another.0498

In order for a bacterial cell to donate its DNA in this manner, it needs to be carrying a set of genes called the F-factor.0504

The F is for the fertility factor, and cells carrying this F-factor are capable of producing a type of pilus called a sex pilus0515

and thereby, donating some genetic material to another bacterial cell.0539

The F-factor can exist. These genes can exist on the F-plasmid, or they can actually be integrated into the bacterial genome.0545

We have here the example where there is the F-plasmid, and I am going to show over here an example where we have the F-factor in red.0557

The F-factors is integrated into the bacterial genome.0571

The terminology is a bit different.0575

When you have a cell that contains an F-plasmid, so the F-factor genes are present in the cell, but they are on the plasmid, then, we call this cell an F⁺ cell.0578

Cells lacking the F-factor are F^- cells.0593

They cannot produce a sex pilus. They cannot donate DNA.0598

However, they can receive DNA.0602

Cells that contain the F-factor but have it integrated into the genome, in that bacterial chromosome, are called Hfr cells for high frequency of recombination,0604

which makes sense because cells carrying the F-factor are capable of recombining their DNA at a high frequency.0628

Looking at what happens here, we have got the regular bacterial circular genome. We have the F-plasmid,0638

which was actually much smaller than the genome, but it is larger here for clarity.0647

We have an F^- cell.0650

This F⁺ cells capable of making a pilus, and then, this forms what is called a mating bridge.0651

The plasmid and the genome are single, or excuse me, double-stranded DNA, so these are double-stranded DNA.0661

And what happens is from the F-plasmid, one strand of the DNA can be donated to the recipient cell, to the F^- cell.0668

The donor cell is left with this single strand.0685

The recipient cell is a single strand, and from that, each of them can manufacture, can synthesize, the complementary strand.0689

This cell is now F⁺ because it has acquired this F-factor via the plasmid.0696

High-frequency Hfr cells - high frequency of recombination cells - can also donate some genetic material to another cell.0707

They have the genes to make the sex pilus, and similarly to what happens with the plasmid, a strand of DNA will be donated to another cell.0717

But usually, the entire genome is not transferred.0727

These F-factor genes are, and then, usually it breaks off before transferring the entire strand.0730

And then, through recombination, these F-factor genes can be integrated into the genome of the recipient cell.0736

One type of plasmid that is important is what is called an R plasmid, so we are talking about plasmids here donating genetic material.0745

Well, R plasmids are important because they carry genes for antibiotic resistance, so think of the R as standing for resistance.0754

Let's say you have an ear infection, and that might be treated with an antibiotic such as amoxicillin.0765

However, it is possible that some bacteria due to mutation or acquiring an R plasmid are resistant to the amoxicillin.0771

What will happen, then, is that the antibiotic, amoxicillin, will kill off the bacteria that are not resistant.0780

And the ones that are left, the ones that are selected for, will be these resistant ones.0786

Because of the broad use of antibiotics, resistance has become an increasing problem because these bacteria are selected for.0791

And then, they can also pass on that ability to be resistant to other bacteria.0798

We keep having to be one step ahead and make more and more powerful antibiotics to out run this selection for resistance.0803

In fact, some of these R plasmids can actually carry genes for resistance to multiple antibiotics, and the mechanisms vary.0812

The mechanism of resistance can include pumping out the antibiotic, cleaving the antibiotic once it is in the cell, preventing the antibiotic from entering.0819

This is actually very important as far as applications in medicine.0830

Now, much of what we know about gene regulation, we learned from studying bacteria, and one concept of gene regulation is that of the operon.0836

Gene regulation is extremely important. We talked an earlier lectures about transcription and translation, so you know how a protein is made.0849

You know how a gene can be expressed to make a protein.0857

But it would be very chaotic if the cell was just making all the proteins that it carried genes for all the time.0860

Things that did not need to be used at the moment would be made. It would be a waste of cell resources.0867

Certain elements could...it would just be crazy.0874

The correct protein needs to be made at the correct time, and there are multiple mechanisms of regulation.0876

Right now, we are going to cover some that occur in the bacterial cell.0882

And then, in the next lecture, we are going to talk about some mechanisms of regulation in eukaryotic cells.0886

What an operon is, is it is a set of genes that are regulated together.0892

And this makes a lot of sense because let's say a bacterial cell needs to make an amino acid;0897

and that is actually just an example here of an operon is this operon called the Trp operon.0905

Well, what Trp stands for is the amino acid tryptophan, which bacteria need to make. They need to have that to survive.0910

There is a whole pathway to make tryptophan, and different enzymes are required for each step of the pathway.0921

It makes sense that if you need one enzyme, one of these enzymes, you need all of them.0926

Here, these five genes here, encode for enzymes needed for the synthesis of tryptophan.0931

What an operon does is it allows all of the genes within the operon to be regulated together.0939

They all can be turned off because they are needed, off when they are not needed and on when they are needed.0946

This type of regulation, regulating a set of genes together, is known as coordinate control.0953

And it allows the cell to regulate products that are made very efficiently.0960

We are going to talk about two operons that are well-known and well-studied called the trp operon and the lac operon.0966

For right now, I am just using this trp operon as an example to show you the different elements present in an operon.0975

The genes that actually encode enzymes are known as the structural genes.0986

In this case, these are enzymes to make tryptophan, but they could be enzymes to break down something or to make a different product.0998

Here we have structural genes. They make the product that the operon is trying to control.1007

These are the structural genes. These code for enzymes.1016

As always, transcription is initiated at a promoter. What is interesting here is that there is a single promoter for the genes on an operon.1028

We have a single promoter for the set of structural genes. Therefore, these structural genes comprised a single transcription unit.1039

One long mRNA could be made, and then, when it comes time to translate, these five different proteins, these five different polypeptides,1054

there is a start and stop codon around each one so that they can be made into separate peptides, but they are transcribed together.1063

You are familiar with genes and promoter. Now, we have a couple of new things.1070

Here, I have an example of a regulatory gene. This particular one is called TrpR.1075

Regulatory genes produce regulatory proteins that can bind to the operator.1084

Some of these are repressors. They bind to the operator, and they turn the operon off.1098

Others can be activators or inducers. These turn the operon on.1114

When an operon is regulated by regulatory protein that turns the operon off, we call this negative control.1128

When a regulatory protein can bind an operator, and it is a type of protein that would induce or activate it, we call this positive control, so activator.1140

Or I am going to call it also the other name inducers- either one.1159

OK, this regulatory protein is made, and it can bind to the operator.1163

And if it is a repressor, the binding of this regulatory protein repressor to the operator will stop RNA polymerase from being able to transcribe these genes.1170

An activator or inducer could bind to the operator and induce transcription.1183

This is a way of controlling the entire process together, an entire set of genes to be transcribed together, and it is a very efficient way of achieving regulation.1195

We are going to start out by focusing on the lac operon, and what lac stands for is lactose.1210

You may have heard of lactose. It is a sugar that is found in milk.1216

E. coli preferentially use glucose.1224

If lactose is around though, they will use it especially, when glucose levels are low.1228

Ideally, if there is glucose, bacteria will use glucose. If there is not much glucose, there is not any glucose, but there is lactose, they can use lactose.1234

First, just looking at the setup of this operon. Here, we have the three structural genes.1243

In order to utilize lactose as a nutrient, E. coli needs to produce these three genes,1252

or express these three genes for enzymes needed to break down lactose.1263

You do not need to memorize the names of them, but just to tell you, the three enzymes that are utilized to breakdown lactose, one is beta-galactosidase.1267

Remember that lactose is a disaccharide, and beta-galactosidase breaks it down into monosaccharides glucose and galactose.1280

Two other enzymes needed are the proteins permease - it is galactose permease - and thiogalactoside transacetylase.1291

These are enzymes needed to utilize lactose as a nutrient, and they are encoded on these structural genes that are part of the lac operon.1304

There is a single promoter for the operon. There is an operator, and there is also a regulatory gene known as lacI.1315

Let's say nothing was bound to this operator. This is just sitting here, this DNA.1323

Well, it could be transcribed. RNA polymerase could bind to the promoter.1331

It could transcribe these genes and produce these three enzymes. However, the lac operon is actually...its base state is off.1335

Lac operon- I am going to say 'usually off".1346

It usually cannot be transcribed. Something has to happen to allow transcription.1353

Now, why would that be? What happens is that this lacI, this regulatory gene, is transcribed, and it produces a repressor.1358

Transcription of lacI produces a repressor, so the lac operon repressor.1371

This lac repressor binds the operator and turns the operon off.1382

When the lac repressor is bound to the operator, RNA polymerase cannot bind to the promoter and transcribe these structural genes.1398

Operators are usually located within or near their promoter.1410

Because this is a repressor, it is binding the operator and turning transcription off. This is an example of negative regulation, negative gene regulation.1415

The lac operon also is controlled by positive gene regulation, but negative gene regulation is the aspect we are studying at the moment.1427

Think about when the equalized cell would need these genes to be made. Normally, this is off.1442

E. coli, I would say, has glucose available. It is going along fine, but let's say there is not enough glucose; and there is lactose.1449

That is when the lac operon needs to be on, and you can see why it makes perfect sense not to waste resources making enzymes1458

that are not really needed when glucose is around or when there is no lactose around to breakdown.1467

If there is no lactose around, you do not need these genes to be transcribed and translated.1472

What we want, what the cell wants, is for these genes to be expressed in the presence of lactose. Here is what happens.1478

Lactose, when it is present, a small amount of that exists or is converted to its isomer.1489

We have lactose, and we have an isomer of it called allolactose.1500

I am going to draw allolactose like this. This is allolactose.1506

When lactose is present, there will be some allolactose present, as well.1512

This lac repressor, as mentioned, is made in its active form actually, and it binds the operator; and now, this operon is off.1517

There is actually a site that allolactose can bind to this repressor.1533

When it binds, it induces that conformational change in the lac repressor that converts it to an inactive form.1544

This is an allosteric regulator. We talked about this earlier with enzymes.1553

Allolactose binds the lac repressor and induces a conformational change. It puts it in its inactive form, so I am just going to say "and puts it in inactive form".1558

If the repressor is in its inactive form, it can no longer bind the operator, so it releases the operator. It falls off the operator.1581

If it falls off the operator, there is nothing bound to the operator. There is nothing stopping transcription.1591

RNA polymerase binds the promoter. These three enzymes are transcribed.1597

They are translated, and the lactose that is present can be broken down, so you see how well this system works.1602

The time when you would need these genes expressed is when lactose is present.1609

Therefore, it is an isomer of lactose that allows for the expression of this gene.1614

We described the lac operon as being inducible. This is something you should remember, so this is an inducible system.1620

This is a little bit confusing in terms of terminology because I said this is negative gene regulation.1633

And it is because the regulatory gene that binds to the operator is turning it off.1639

However, it can be induced. We can stop that negative effect through the presence of a chemical signal.1646

A chemical signal will allow these genes to be expressed.1656

Again, it is negative gene regulation because it is a repressor that binds and turns it off, but the system is inducible. It is possible to induce it to go back on.1660

If something starts out off, and we can turn it on, what we say is we are inducing it.1669

It is normally off, but we are turning it on; so it is being induced.1673

OK, the Trp operon, this is also an example of negative gene regulation, but this operon is repressible.1683

We talked about the lac operon which is inducible. This is a repressible operon.1692

Let's just review. Inducible, which is the lac operon, is usually off, can be turned on by a chemical signal. Repressible is the opposite.1705

It is usually on, can be turned off by a chemical signal.1721

When we talked about operons in general, I was using the Trp operon as an example, so again, we have the regulatory gene. Here, it is TrpR.1733

The promoter is a bit upstream actually of the rest of the operon.1742

We have the promoter, single promoter for the five structural genes.1747

We have an operator, and the five structural genes encode enzymes needed for the synthesis of the amino acid tryptophan.1751

With nothing bound to this operator, RNA polymerase can bind to the promoter. It goes along.1762

It is going to transcribe these proteins. They will be translated.1768

These enzymes will be made, and they will catalyze the synthesis of tryptophan. However, we have this TrpR gene.1773

It gets transcribed, and it encodes a repressor. This is a repressor for the Trp gene, for the Trp operon.1786

If the repressor binds the operator, the operon is turned off.1798

The promoter cannot be bound by RNA polymerase. These genes cannot be transcribed.1804

Up until this point, pretty similar to what we talked about with the lac operon.1810

However, it is not the same, and the reason is the Trp repressor is manufactured in its inactive form- made in inactive form.1817

The lac repressor is manufactured in active form. It is manufactured.1834

It binds the operator. It shuts lac operon off.1839

This Trp repressor is made - let's show it like this - in a way that is not going to fit into here.1842

It blocked off somehow, let’s say, OK, so inactive when it is manufactured.1852

Therefore, the base state for the Trp operon is to go ahead, RNA polymerase binds, transcribe, translate, we end up with the structural genes expressed.1858

Tryptophan is being made, which is fine. E. coli needs that to be made.1871

Now, when would I not want the cell to express these genes? When would I not need these enzymes?1874

When would I want to turn the operon off?1880

It is normally on. When do I want to turn it off?1882

Well, the cell would want it to be off when there is enough tryptophan. Therefore, the signal to turn this operon off is the presence of tryptophan.1885

Tryptophan acts as what is called a corepressor. It binds to the repressor protein, and I will make this as a circle.1899

Let's say this is tryptophan, and when tryptophan binds - so this is a repressor, this is a corepressor - it allows this repressor to go into its active form.1914

When tryptophan builds up - there is going to be a lot of tryptophan around - it will go ahead and bind to this Trp repressor.1925

When it does, binds repressor, and it induces a conformational change and puts it in its active form.1932

Remember with the lac operon, allolactose bound the repressor and put it in its inactive form. This is the opposite.1949

This repressor is made in its inactive form, binding of tryptophan to the repressor. Now, this will get eliminated.1956

This would get eliminated etc., and the repressor will bind to the operator.1963

Transcription will be halted. It will turn off, and these genes will no longer be expressed, which makes sense.1971

When the cell has plenty of tryptophan, it does not want to waste resources making more.1979

Therefore, this is an example of a repressible operon, an operon that is usually on, that can be turned off.1986

Frequently, anabolic pathways like this, pathways in which something is being made or synthesized, are usually repressible.1993

Because what we want to do is we want to make enough of that item.2002

We want to make it and then, turn it off when we have enough, so it makes sense that it would be repressible.2009

And this type of regulation where either a product or the intermediate of a pathway is used to shut off that pathway is common in biology.2013

We talked about that at the level of the enzyme that enzymes can be shut on or off. They can be activated or inactivated.2025

When we are talking about the regulation of enzyme, enzymes can be regulated. They are already made.2033

It is a pre-made protein that can be activated or inactivated.2038

This is an earlier step. We can turn on or off, whether or not the enzyme is even made in the first place.2041

Then, once it is made, it can also be regulated often by positive or negative feedback.2048

Anabolic pathways are usually repressible, so that would be a negative feedback pathway.2052

Catabolic pathways, when we are breaking down something as with the breakdown of lactose, and the lac operon are usually inducible.2057

In other words, we just have them turned off. We have the operon turned off.2065

We do not need it unless something is present.2070

This is the opposite. When there is something present, we want to turn it off.2073

Now, back to the lac operon.2080

Both types of regulation I just showed with the first lac operon and with the Trp operon2084

made a regulatory protein that binds the operator and turns the operon off.2091

Those are negative gene regulation.2101

In positive gene regulation, what we have is something binding to the DNA that increases transcription.2103

As I mentioned, the time when the cell wants to use lactose is one, when lactose is present and two, when glucose is low.2115

When the presence of lactose specifically allolactose, its isomer, turns lac operon on.2124

A second type of regulation of the lac operon is to increase the rate of transcription, so the lac operon is on.2135

If glucose is low, the rate of transcription of the lac operon is increased through a positive gene regulation.2143

Before, what we were talking about is just turning the operon on, turning it off.2169

If lactose is around, the operon is on, but the RNA polymerase actually does not have a great affinity for the lac operon promoter.2174

It just does not bind that well.2185

If you really want to get plenty of production of the enzymes to breakdown lactose, you need to increase this rate of transcription.2187

You need to increase the affinity of RNA polymerase for the promoter, and here is how that works.2195

When glucose levels are decreased, the level of cyclic AMP increases.2201

So, lactose is around, let's say. Therefore, we have allolactose bound here.2215

This is going to be in its inactive form. The lac repressor is not going to bind with the operator.2221

These genes are being transcribed but not at a very high rate.2228

There is a regulatory protein called the catabolite activator protein, and I will draw that like this.2233

Cyclic AMP can come along and bind CAP, and when it does, it activates it; and CAP can, then, bind to the promoter region.2249

When glucose is low, that is when the cell really needs to switch over to using lactose. When glucose is low, cyclic AMP increases.2268

Cyclic AMP binds CAP to activate it. The activated CAP can bind to the promoter, and this increases the affinity of RNA polymerase to the promoter.2276

Not only now is lac operon on. Lactose is around.2295

Repressor is not bound to the operator. Lactose can be broken down, but we need a better rate of production of enzymes if glucose is not available.2300

So, the decrease level of glucose, we get an increased rate of transcription.2310

Now, let's say you put some glucose in the media. Now, the glucose is around for the bacteria to utilize.2315

What is going to happen is when glucose level is increased, the cyclic AMP level will decrease.2322

There will be less cyclic AMP around to bind CAP. CAP will not bind to the promoter.2327

And then, the level of transcription will decrease as the cells, which is overutilizing glucose.2332

You see here, we have an example with the lac operon, where both negative gene regulation and positive gene regulation come into play.2338

Our first example, example one: in the1920s, Frederick Griffith performed a series of experiments on Diplococcus pneumoniae,2348

which is a type of bacteria that causes pneumonia.2356

He discovered that there were two types of bacteria, S, which is smooth-type bacteria, so two types of bacteria.2360

We have smooth-type bacteria, and this has a capsule, so, smooth-type bacteria, which has a capsule and R or rough-type bacteria, which does not.2371

Mice injected with S-type bacteria became ill and died, so to keep track on what is going on, S-type kill a mice.2377

Those injected with R-type bacteria lived- R-type, the mice live.2391

Griffith also injected mice with heat-killed S-type bacteria. These mice did not develop pneumonias, and they therefore, did not die.2405

Now, we have heat-killed, so dead S-type bacteria- the mice lived.2416

Live S-type infects the mice. They developed pneumonia.2425

They die.2429

R-type does not make the mice ill. Dead S-type does not make the mice ill.2430

The only thing that is killing these mice is the live S-type bacterial infection.2437

When he killed the S-type bacteria, where he injected along with live R-type bacteria, the mice developed pneumonia.2443

Now, what he did is he combined heat-killed S plus live R-type, and the mice died- kills mice.2452

The bacteria recovered from these mice had capsules.2472

When he recovered some bacteria from those dead mice, grew them out, he found that they had capsules.2475

How could the process of transformation account for these findings?2484

These works, experiments, performed and these were early experiments that discovered the process of transformation discussed earlier in this lecture.2488

Recall that transformation is the process through which bacteria can take up DNA from their surroundings.2497

Dead S-type, which have a capsule on them, will not infect mice. It will not cause pneumonia.2508

It will not kill them, but if you put the dead S-type with the live R-type, what could happen via transformation?2516

Well, the R-type would be exposed to the DNA from these dead bacteria, and they could take up some of that DNA through transformation.2522

Some of that DNA could contain the genes to make a capsule, so R-type took up DNA from S-type.2537

The genes that they took up, the DNA that they took up, allow them to make capsules and make the mice ill.2555

Therefore, transformation allowed these R-type bacteria to become more virulent.2571

And if this is correct, what you would expect is that the bacteria recovered from the mice that now had capsules, if you plated those out, grew those out2578

and looked at the offspring of those bacteria, you would expect the offspring to have capsules, as well, and in fact, that is what Griffith found.2591

The virulence from the S-type bacteria could be conferred upon the formerly not virulent R-type through transformation.2600

Match each of the terms related to an operon with their description: structural genes, promoter, operator and regulatory genes.2611

Structural genes code for enzymes or structural proteins. It sounds pretty good.2623

A segment of DNA located in or near the promoter that regulates transcription.2631

Segment of DNA where transcription is initiated or code for regulatory proteins.2639

Well, structural genes code for enzymes such as enzymes needed to make tryptophan or to break down lactose.2646

So the correct answer for one is A- the correct description.2653

Two, promoter: A segment of DNA located in or near the promoter, and it regulates transcription.2657

That would not be correct. We are talking about the promoter itself, not something near the promoter or something else that is in it.2665

Segment of DNA where transcription is initiated or code for regulatory proteins.2674

C is correct. Promoter is a segment of DNA where transcription is initiated.2680

Three- the operator: a segment of DNA located in or near the promoter that regulates transcription.2685

That is correct. Remember that the operator can be bound by a repressor or an inducer that allow this operon to be turned on or off, so that is B.2693

And then, finally, regulatory genes code for regulatory proteins like the lac repressor or the Trp repressor.2705

Why is the lac operon described as an inducible, while the Trp operon is described as repressible?2717

Well, recall that the lac operon is usually off.2724

Because the lac repressor is manufactured in its active form, it goes ahead. It binds the operator, and it keeps the lac operon off normally.2734

However, the presence of a particular chemical signal, allolactose allows the repressor or allows the operon to be turned on by inactivating the repressor.2744

Lac operon is usually off. Presence of a chemical allows it to be turned on.2757

If it is normally off and something turns it on, that is inducible, so that is the definition of inducible.2777

Whereas, repressible the Trp operon is usually on.2784

It is sitting around. It is on because its repressor protein is just made in an inactive form.2791

The presence of tryptophan will activate the repressor so that it can bind to the operator and be turned off.2799

The presence of, in this case, tryptophan turns operon off.2807

If something is on and the chemical signal turns it off, it is repressible. If something is off and the chemical signal turns it on, it is inducible.2819

The lac operon is depicted below.2832

Describe the series of events that occurs when the level of lactose in the environment2835

increases the result in the production of the enzymes needed to metabolize lactose.2841

Normally, the operator is bound by this repressor because this repressor is in its active form, and the operon is off.2850

When lactose is around, some of it will exist as its isomer allolactose. Allolactose, you will recall, binds the lac repressor.2869

So, this is the lac repressor. Allolactose can bind it.2894

Let's say this is allolactose, and it comes along and binds. Binding converts the lac repressor to its inactive form.2899

Allolactose is an allosteric regulator. It binds to the lac repressor and induces a conformational change that puts it in its inactive form.2924

Lac repressor cannot bind the operator anymore. It cannot bind the operator anymore once it is in its inactive form.2935

So, we end up with the lac repressor bound, and it puts it in its inactive form.2949

When it is bound to allolactose, it will no longer, then, be bound to the operator.2961

RNA polymerase binds the promoter, and the genes for lactose break down. Utilization are transcribed.2966

That concludes this section of Educator.com on molecular genetics.2978

Thanks for visiting.2983