Chapter 17: Microbial DNA Technology & Genetic Engineering

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Welcome to the Deep Dive.

Today we're jumping right into the molecular toolkit, you know, the absolute essentials for manipulating life at the DNA level.

That's right.

We're looking at microbial DNA technologies drawing heavily from Prescott's microbiology.

Think scissors, glue, printers,

all the tools discovered in microbes that underpin modern biotech.

It's the foundation for so much.

Our mission today is really to unpack these tools so you can, well, understand how the big breakthroughs actually happen.

Exactly.

From cloning a single gene all the way to editing entire genomes.

And we'll start with something pretty cool.

Super strong spider silk.

Ah, yes, the spider silk hook.

It's amazing material, isn't it?

It really is.

I mean, think about it.

Lightweight, super elastic, stronger than steel by weight.

You hear about potential uses like bulletproof vests, artificial ligaments, fancy sutures.

And it all comes down to one protein,

spidjoin.

That's the one.

But here's the catch.

You need tons of it for industrial use.

And, well, farming spiders.

Let's just say it doesn't scale well.

Not exactly practical, no.

So scientists got clever.

They thought, okay, let's take the gene for spidjoin, the instructions, and put it into something easy to grow, like E.

coli or yeast.

Even goats, actually.

Goats producing spider silk protein in their milk?

Wild.

Yeah, it is.

But there was another twist.

Just using the natural gene didn't always work perfectly.

The recombinant fibers sometimes ended up weaker.

Right.

So they had to get creative with the engineering.

They did.

They basically designed a better blueprint.

They took the best bits, like the end terminal and middle parts, from one spider species,

E.

prostenops australis, I think, and combined them with the C terminal end from a totally different spider, Arrhenius ventricosus.

They essentially built a hybrid gene.

A chimeric gene.

And they stitched this together using PCR.

We'll definitely get to PCR later.

Yeah, built with PCR.

And the result, get this.

From just one liter of recombinant E.

coli culture, enough spidroin to spin a whole kilometer of fiber.

Wow.

That really shows the power of these tools we're about to discuss.

Absolutely.

It all starts with the basic idea of genetic engineering changing the genetic code.

And the key thing underpinning it all is the universality of DNA.

Meaning the code works the same everywhere, right?

Spider gene and bacteria still makes spider protein.

Exactly.

The gene is a gene, more or less.

You can cut DNA from one organism, paste it somewhere else, and it generally works.

Which brings us to the tools,

starting with the scissors.

The molecular scissors.

We need something to cut the DNA precisely.

These are the restriction enzymes, or restriction endonucleases, discovered back in the late 60s by Arbor and Smith.

They found them in bacteria.

And they're naturally a defense mechanism for the bacteria.

Right.

They chop up foreign DNA, like from viruses.

But for us, they're amazing because they only cut at very specific DNA sequences, usually short ones, like four to eight base pairs long.

Super specific, like the enzyme E.

cori, it only cuts at GATTC, correct?

That's the one.

Five prime GATC3 prime.

And crucially, how it cuts is important.

It makes staggered cuts, right?

Not straight across.

Exactly.

It leaves these little single -stranded overhangs.

For E.

cori, it's a five prime

ATT3 prime overhang.

We call these sticky ends.

Sticky ends.

And they're sticky because they're complementary.

They want to pair up.

Precisely.

If you cut two different DNA molecules with the same enzyme, they'll have matching sticky ends that can anneal or stick together.

Super useful for joining DNA pieces.

Though not all enzymes make sticky ends, right?

Some make blunt ends.

Correct.

Like aloe.

Blunt ends can be ligated, too.

But sticky ends are generally much more efficient for cloning.

Okay.

So we've cut our DNA.

How do we check the pieces?

We usually run them on a gel, agarose gel electrophoresis.

DNA is negatively charged, so you put it in gel, apply electricity, and it moves towards the positive pole.

And smaller pieces move faster.

Yep.

Separates them by size, so you can see if your enzyme cut properly and if your DNA fragment is the size you expected.

Okay.

Cut checked.

Now we need to join our gene, our insert, into the vector.

Time for the molecular glue.

That's DNA ligus.

It forms the covalent bonds, seals the gaps in the DNA backbone.

Jackson, Simons, and Berg first did this back in 72, creating the first recombinant DNA.

But wait, you mentioned cloning a eukaryotic gene, like the spider gene, into bacteria.

Your eukaryotic genes have introns, right?

Those non -coding bits.

Ah, yes.

The intron problem.

Bacteria don't have the machinery to splice out introns from eukaryotic pre -mRNA, so if you just put the raw gene in, you get junk protein.

So how do we solve that?

We need a version of the gene that already has the introns removed.

The solution came from studying retroviruses.

Temin and Baltimore discovered reverse transcriptase, RT, in 1970.

Reverse transcriptase, it goes backwards, from RNA to DNA.

Exactly.

You take the process messenger RNA, mRNA, from the eukaryotic cell, the version that's already had introns spliced out, and use RT to make a DNA copy.

And that DNA copy is called cDNA, complementary DNA.

Right.

The cDNA is intron -free, it's stable, and that's what you clone into the bacteria for It's a really clever workaround.

Okay, brilliant.

So we have our intron -free cDNA, now we need the delivery truck, the cloning vector,

usually a plasmid.

What makes a good vector?

Three essential things.

First, an origin of replication, the ori.

This lets the vector replicate independently in the host cell.

And the ori also controls how many copies there are.

The copy number.

Yes.

Some vectors, like PUC19, are high copy number, meaning hundreds of copies per cell.

Good if you want lots of your product.

Okay, ori is number one.

What's second?

A selectable marker.

Typically an antibiotic resistance gene, like Ampir for ampicillin resistance.

Ah, so you grow the bacteria on ampicillin plates.

And only the ones that successfully took up the plasmid, the transformants, will survive.

It lets you select for success.

Makes sense.

And number three.

The multi -cloning site, or MCS.

This is a stretch of DNA engineered to contain lots of unique restriction enzyme cut sites packed closely together.

Unique sites, so you can cut the vector open at just one specific spot to insert your gene.

Exactly.

It gives you flexibility in choosing which enzyme to use for cloning your insert.

Now, sometimes you need a vector that works in different organisms, like E.

coli and yeast.

Right.

For that, you use a shuttle vector.

It basically has two different oris.

One that works in bacteria and one that works in yeast.

UB24 is a common example.

Allows you to move your construct between hosts easily.

Okay, so we've mixed our cDNA insert and our vector, added ligus.

Then we need to get this recombinant plasmid into the bacteria.

You mentioned electroporation.

Yeah, or chemical transformation.

Electroporation uses a high voltage pulse to temporarily create pores in the cell membrane.

Lets the DNA sneak in.

We plate them on ampicillin, and only the transformants grow.

But some of those might just have the original plasmid that closed back up without our insert, right?

That's a very common issue.

Just selecting for antibiotic resistance isn't enough.

You need to screen the colonies to find the ones that actually contain your insert.

And this is where that clever blue -white screening comes in.

Yes, it's brilliant.

The MCS is usually placed right in the middle of the gene called lacZ.

Which codes for an enzyme, beta -galactosidase.

Correct.

If you successfully insert your foreign DNA into the MCS, you disrupt the lacZ gene.

It gets broken.

So no functional enzyme is made.

Right.

Then you plate the cells on a special medium containing a chemical called X -gal.

If the lacZ gene is intact, meaning no insert, the enzyme is made.

It cleaves X -gal and the colony turns blue.

But if lacZ is disrupted by our insert… No functional enzyme, X -gal isn't cleaved, and the colony stays white.

So the white colonies are the ones you want.

They have the recombinant plasmid with your insert.

That is a really elegant biochemical trick.

Blue means empty, white means success.

It saves a ton of work.

But what if you're not cloning a single gene?

What if you need to clone really huge pieces of DNA for like mapping a genome?

Right.

Plasmids have size limits.

What do we use then?

We move up in scale.

First, there are cosmets.

They're kind of hybrids, part phage, part plasmid.

They can handle bigger inserts, maybe up to 25 ,000 base pairs.

Still not huge for genome work, though.

True.

For the really big stuff, like building the libraries for the human genome project, or that synthetic mycoplasma genome, you need artificial chromosomes.

BACs, bacterial artificial chromosomes, and YACs, yeast artificial chromosomes.

Exactly.

YACs can take massive inserts, up to a million base pairs, maybe even more.

BACs are a bit smaller, maybe up to 300 ,000 base pairs, but they tend to be more stable and easier to work with.

Essential tools for large -scale genomics.

Okay.

Cut, paste, got it into a vector, selected the right clones.

But we still might only have a tiny bit of this specific DNA molecule.

We need lots of it to study.

Now we get to the amplifier.

Polymerase chain reaction, PCR, Kary Mullis's invention.

It totally changed molecular biology.

Because it lets you make billions of copies of a specific DNA sequence in vitro in a test tube.

Billions.

Yeah.

Exponentially.

You start with potentially just one molecule and end up with enough to see on a gel or clone or sequence all in a few hours.

It's incredibly powerful.

And the key ingredients.

You need your template DNA.

Right.

Plus two short DNA strands called oligonucleotide primers.

These flank the specific region you want to copy.

They define the start and end points.

Then you need the DNA building blocks, the DNTPs.

Yep.

And the real star, a heat stable DNA polymerase, usually TAC polymerase.

TAC, from Thermos Aquaticus, that hot spring bacterium.

Why does it need to be heat stable?

Because the PCR process involves repeated cycles of heating and cooling.

There are three main steps per cycle.

First, denaturation.

High heat.

Around 95 degrees Celsius to melt the DNA strands apart.

Second, annealing.

You cool it down, maybe 50 to 60 degrees, so the primers can bind to their complementary sequences on the template DNA.

And third, extension.

Raise the temperature slightly, maybe 72 degrees.

The optimal temperature for TAC polymerase to synthesize the new DNA strands starting from the primer.

You just repeat that cycle, denature, anneal, extend 25, 30, sometimes 40 times.

Each cycle doubles the amount of your target sequence.

Exponential growth.

So you quickly get huge amplification.

What are the main ways we use PCR?

Well, there's basic endpoint PCR.

You run the cycles, then analyze the result on a gel.

It's mostly qualitative, is my target sequence there, yes or no.

Good for diagnostics, forensics, just checking if a clone worked.

But sometimes you need to know how much was there to begin with.

Exactly.

For quantitative measurements, we use real time PCR, or QPCR.

It uses fluorescent dyes or probes to measure the amount of DNA being produced in real time during each cycle.

And because you measure it during the exponential phase, you can work backwards to figure out the starting quantity of template DNA or RNA very accurately.

Precisely.

QPCR is essential for things like measuring gene expression levels.

Now, PCR also offers a way around relying on finding convenient restriction sites, doesn't it?

You mentioned designing primers.

Yes.

This is a key advantage.

The 5' end of the PCR primer doesn't actually have to match the template DNA perfectly.

Yeah, you can add extra sequence onto the 5' end like a restriction site you want, or a sequence needed for another technique.

As long as the 3' end binds specifically, the polymerase will still extend, and that extra sequence gets incorporated into all the new copies.

That sounds incredibly useful.

It leads to techniques like seamless cloning.

Absolutely.

Like Gibson assembly.

It's a method that lets you stitch multiple DNA fragments together in one reaction without needing any restriction enzymes at all.

No restriction enzymes?

How does that work?

You design your PCR primers so that the ends of the fragments you want to join have short overlapping sequences, maybe 20 -40 base pairs of homology.

Okay, so the pieces have matching ends.

Then you mix them with a cocktail of enzymes.

First, a T5 exonuclease chews back the 5' end slightly, exposing single -stranded 3' overhangs.

Ah, creating sticky ends, but custom ones based on the overlap.

Exactly.

These complementary overhangs then anneal.

A DNA polymerase fills in any gaps, and DNA ligus seals the nicks.

All in one pot.

Wow.

So you can assemble multiple pieces, like a whole operon or parts of a synthetic genome in one go.

That's powerful.

It's incredibly efficient for complex constructs.

A big step beyond traditional cloning.

Okay, so we've cloned our gene, maybe using Gibson, maybe traditional methods.

Now we want the cell to actually make the protein.

Right, we need to express the gene.

For that, we often use specialized expression vectors.

They're designed not just for cloning, but for high -level protein production.

And they usually have strong promoters to drive lots of transcription, often inducible ones.

Yes, inducible promoters are key.

Things borrowed from the lac operon, for instance.

You want to be able to turn on the expression of your foreign or heterologous gene only when you're ready.

Why the control?

Why not just let it run constantly?

Because producing massive amounts of a foreign protein can be stressful, even toxic to the host cell.

It can slow growth or even kill the cells.

So you grow the cells up first, then induce expression just before harvesting.

Makes sense.

Okay, we've induced expression.

The cells are full of our protein, like spid drawing.

Now we need to get it out and purify it away from all the other bacterial gunk.

Purification is crucial.

Yeah.

And one of the most popular ways to do it is his tagging.

That involves adding a little tag to the protein?

Yep.

You engineer the gene to add a sequence coding for six histidine amino acids, the six by his tag, usually under the beginning or end of your protein.

And histidine has a special property.

It has a high affinity for certain metal ions, particularly nickel or cobalt.

So you least your cells and run the whole messy mixture over a column containing resin beads coated with nickel.

And only the his tagged protein sticks.

Pretty much.

Most other cellular proteins wash right through.

Your tacked protein binds tightly.

Then how do you get it off the column?

You watch the column with a buffer containing a high concentration of imidazole.

Imidazole looks similar to the histidine side chain and competes for binding to the nickel, effectively knocking your purified protein off the column.

It's a very effective one -step purification method.

Very clever.

Beyond just purifying, sometimes we want to see where the protein is or when its gene is turned on.

Inside a living cell.

That's where fluorescent proteins come in, especially green fluorescent protein, GFP from the jellyfish Echoria victoria.

It revolutionized cell biology.

You can fuse it to your gene or promoter.

There are two main ways to do that, right?

Transcriptional and translational fusions.

Exactly.

And they tell you different things.

A transcriptional fusion is when you replace your gene of interest with the GFP gene, but keep the original promoter.

So GFP is made whenever the original gene would have been turned on.

Right.

The glowing green tells you about gene regulation when and under what conditions that promoter is active.

Okay.

And the other way,

translational fusion.

In a translational fusion, you attach the GFP coding sequence directly to the end of your gene's coding sequence.

So the cell makes a single combined protein, your protein with GFP stuck onto it.

A chimeric protein.

And because GFP is attached wherever your protein goes in the cell, the green glow follows.

Exactly.

This tells you about protein localization.

Where does the protein live and work inside the cell?

It lets you visualize it directly.

Amazing tools for watching cellular processes unfold.

Okay.

We've covered cutting, pasting, copying, expressing, purifying, visualizing.

What about making precise changes directly in the genome itself?

Now we get to the cutting edge.

Cas9 nucleus and CRISPR based genome editing.

This technology, originally a bacterial immune system against viruses, has given us unprecedented power to edit DNA.

How does Cas9 find its target?

Is it like a restriction enzyme?

Similar in that it cuts DNA, but the recognition is totally different and much more specific.

Restriction enzymes use protein DNA interactions to recognize short, maybe four, eight base parasites.

And Cas9.

Cas9 is a ribonucleoprotein.

It's a protein guided by an RNA molecule, the guide RNA,

the specificity comes from about 20 bases of this gRNA hybridizing perfectly with the target DNA sequence.

20 bases of RNA DNA pairing.

That's way more specific than four, eight base pairs.

Massively more specific.

It means you can, in theory, target Cas9 to virtually any unique site in a huge genome, just by designing the right gRNA sequence.

So you design the gRNA to match your target, introduce Cas9 and the gRNA into the cell and Cas9 cuts the DNA right there, making a double strand break.

What happens next?

The cell hates double strand breaks and rushes to repair them.

It mainly uses two pathways.

The first is quick and dirty, non -homologous end,

joining NHEJ.

NHEJ often makes mistakes, right?

Inserts or deletes a few bases.

Yeah, it just jams the ends back together.

Those small insertions or deletions in the venelles often cause a frame shift mutation, which usually knocks out the gene.

It's a good way to disable a gene.

But if you want to make a precise change, not just break the gene, like correct a mutation.

And then you need the second pathway, homologous recombination, HR.

For this to work, you provide a donor DNA template along with the Cas9 and gRNA.

And this donor template has the sequence you want to insert flanked by sequences that match the DNA on either side of the Cas9 cut.

Exactly.

The cell uses the donor DNA as a template to repair the break accurately,

seamlessly incorporating your desired into the genome.

It's much more precise than NHEJ.

Incredible precision.

And there's even a version of Cas9 that doesn't cut, dCas9.

Right, dCas9.

The nucleus function is mutated, so it binds to the target site specified by the gRNA, but it doesn't cut the DNA.

Take a programmable DNA binding protein, a homing device.

Precisely.

You can then fuse other functional domains to dCas9.

Attach a repressor domain, and it blocks transcription to the target site.

Attach an enzyme.

Yes.

People have fused dCas9 to enzymes that modify DNA bases directly, like changing a C to a T without any cutting.

These are all base editors.

Or enzymes that modify histones to change epigenetic states.

The possibilities are huge.

It's really moved beyond just cutting DNA to, well, programmable molecular machines acting directly on the genome.

It's an astonishing toolkit.

We started talking about making spider bacteria using restriction enzymes, lagos, vectors,

maybe some PCR.

Then we saw how to purify proteins with his tags, visualize them with GFP.

And now we end with Cas9 and dCas9, giving us the ability to rewrite or regulate almost any gene with incredible precision, all guided by a simple RNA molecule.

So to quickly recap for everyone listening, we've covered the molecular scissors, restriction enzymes, the glue, ligase, the RNA to DNA bridge, reverse transcriptase, the delivery vehicles, cloning vectors, artificial chromosomes, the photocopier, PCR, tools for protein production and analysis, expression vectors, his tags, GFP.

And finally, the precision editor, Cas9.

Understanding these microbial -derived tools is absolutely fundamental.

Whether your interest is in medicine, agriculture, basic research, or biotechnology, these concepts are the bedrock.

Which brings us to a final thought.

We talked about dCas9 being a programmable platform, able to recruit almost any enzyme to a specific DNA site via an RNA guide.

If we can essentially program enzymatic activity, base editing, epigenetic modification, activation, repression to any location in the genome, does that start to reduce the genome to something like software we can debug and rewrite at will?

What does truly programmable biology look like and what are its limits, ethical or biological?

Something to think about.