Chapter 19: Nucleic Acids

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In 1869, a young Swiss doctor named Friedrich Miescher was digging through, well, the bloody infected pus of discarded surgical bandages.

Which is just an incredibly disgusting image.

Oh, absolutely gross.

He was originally trying to study the proteins inside white blood cells, but as he washed that pus and treated the cells with acid, he found something else.

Something that wasn't a protein.

Right.

It was this weird gooey precipitate that was just absolutely packed with massive amounts of phosphorous, and he called it nuclein.

I mean, he had absolutely no idea at the time, but that gooey substance he pulled out of medical waste was actually DNA.

He had literally just isolated the blue trend for all human life.

It's just an incredible origin story, and it brings us perfectly to our mission for today.

Welcome to a very special deep dive brought to you by the Last Minute Lecture Team.

We are so excited for this one.

We really are.

If you are a college -level biochemistry student staring down an exam, or if you're just insanely curious about the molecules that make your own life possible, you are in the perfect place.

We're looking at chapter 19 of Principles of Biochemistry, the fifth edition.

And we are focusing entirely on nucleic acids today.

Exactly.

We are turning today's session into your own personal one -on -one tutoring session.

We're going to build this molecule from the ground up.

Starting with a raw chemical structure, right?

Yeah, figuring out how that structure dictates its function, watching the molecular mechanisms in action, and finally seeing how your cells regulate the whole thing.

So to really understand how that gooey substance holds the code of life, we have to zoom way, way in.

Down past the cell, past the nucleus, down to the fundamental building blocks, the nucleotides.

Okay, let's unpack this.

If we pull a single nucleotide out of the chain and look at it, what are we actually looking at?

Well, you're looking at a molecule with three distinct parts.

First, right in the center, you have a five -carbon sugar.

Okay, a central sugar.

Right.

And attached to one side of that sugar is at least one phosphate group.

Then attached to the other side is a nitrogen -containing ring structure, which we usually just call the base.

And that sugar in the middle isn't always the same, right?

I mean, RNA and DNA have different names for a reason.

Yeah, the difference all comes down to a single oxygen atom.

In RNA, that central sugar is called ribose.

It has a hydroxyl group, so an oxygen and a hydrogen attached to its second carbon.

But in DNA?

In DNA, it's called deoxyribose.

Deoxy literally means missing an oxygen.

On that exact same second carbon, it just has a lone hydrogen atom.

It's wild that a single missing oxygen atom across this massive molecule is the defining difference between RNA and DNA.

But what about the bases?

Those are the famous letters of the genetic code, right?

They are, yeah.

And they come in two architectural styles.

Yeah.

Pyramidines and purines.

The pyramidines are two of the smaller ones.

Like how small?

If you picture them in your mind, they're just a single simple ring of carbon and nitrogen atoms.

The major pyramidines are cytosine, thiamine, and uracil.

And the purines?

The purines are larger, much bulkier.

They feature a fused double ring system.

It's basically a single pyramidine ring fused side by side with another five -sided ring.

The purines are adenine and guanine.

A good trick I used to remember this is the bigger, longer word pyramidine goes with the smaller single ring structure.

And the shorter word purine goes with the larger, bulkier double ring structure.

That's a great mnemonic.

It really helps keep them straight.

Now, before we start snapping these pieces together, there is a terminology trap that catches a lot of students.

The difference between a nucleoside and a nucleotide?

Oh, it's a crucial distinction.

Think of a nucleoside as the incomplete version.

It's just sugar and the base joined together.

So no phosphate.

Exactly.

No phosphate.

It only becomes a fully -fledged nucleotide once you attach that phosphate group to the sugar.

Got it.

So if we zoom in on the 3D structure of one of these nucleotides, say a guanine nucleotide, how do these three pieces physically sit together in space?

Are they just lying flat like a piece of paper?

Not at all.

The sugar ring is actually puckered, meaning it bends a little bit out of a flat plane.

And the bulky nitrogen base doesn't lie flat against the sugar either.

So how does it sit?

Instead, it twists itself so it sits almost completely perpendicular to the sugar ring.

Biochemists call this the anti -confirmation.

Why does it twist away like that?

Is it just random?

No, it's to avoid what we call steric clash.

That is just a fancy way of saying atoms physically bumping into each other because they take up space.

Oh, right.

They physically crowd each other.

Yeah.

If the bulky base rotated inward over the sugar, their electron clouds would repel each other.

So twisting away into that perpendicular anti -confirmation is the most stable, relaxed posture for the molecule.

That makes perfect physical sense.

Now, earlier you mentioned that the phosphate attaches to a specific carbon on the sugar.

How do biochemists actually keep track of all these carbon atoms?

They use a numbering system, but because there are carbon atoms in both the base and the sugar, they have to differentiate them.

The base gets the regular numbers, like one, two, three, four.

And the sugar.

To separate the sugar carbons, they add a little prime symbol next to the number.

So the sugar carbons are one prime, two prime, three prime, four prime, and five prime.

It's almost like the nucleotide is a little city.

The nitrogenous base is the bustling downtown core, so it gets the prime real estate, the regular addresses.

I like that analogy.

Thanks.

And the sugar ring is like the suburbs, so to avoid mail getting lost, those addresses get the prime designation.

So when we talk about the three prime and five prime ends of DNA, we're specifically talking about locations out in the sugar suburbs.

That is a brilliant way to visualize it.

And that downtown area, the base, has a chemical quirk we need to mention.

The atoms within the base can actually undergo a sort of chemical shape shifting called tautomerism.

Tautomerism?

What does that actually look like?

This just means the hydrogen atoms can spontaneously hop back and forth between different positions on the ring.

Wait, does that sheet shifting cause problems for the genetic code?

It would, if it happened randomly all the time.

But under the physiological conditions inside your own cells, like your body temperature, your pH, the bases overwhelmingly lock into very specific stable shapes.

What are those shapes called?

They're known as their amino and lactam forms.

That specific arrangement of hydrogen atoms is absolutely critical because it perfectly sets up the molecule's ability to form hydrogen bonds.

Which brings us to the actual construction of the DNA strand.

How do we link these individual city blocks together to form a sprawling microscopic metropolis?

You build a chain by connecting the suburbs.

The phosphate group sitting on the five prime carbon of one sugar reaches out and forms a strong covalent bond with the three prime carbon of the next sugar in line.

Okay, so it links five prime to three prime.

Exactly.

We call this a three single nucleotide connects in that exact same way.

The whole chain ends up having a distinct direction.

It's like a one -way street.

Yes, exactly.

One end of the chain will always have a free five prime phosphate sticking out and the completely opposite end will have a free three prime hydroxyl group.

And not only is it a one -way street, but when Watson and Crick proposed their famous BDNA model in 1953,

they realized that a DNA molecule isn't just one street.

Right, it's a double helix.

Yeah, it's two one -way streets running parallel but in completely opposite directions.

We call this anti -parallel.

One strand runs five prime to three prime and the strand across from it runs three prime to five prime.

And the bases are pointing inward toward each other, pairing up across the middle.

Adenine always pairs with thymine and guanine always pairs with cytosine.

Those are Chargaff's rules.

Which gives us that classic twisted ladder shape.

The sugar phosphate backbones are the sturdy side rails and those paired bases in the middle form the rungs of the ladder.

Okay, I have to push back here.

We always hear about the twisted ladder, but what is actually holding that massive twisting structure together in the watery environment of a cell?

You're thinking of the hydrogen bonds, right?

Well, yeah.

I know the bases in the middle share hydrogen bonds.

Two bonds for A and T, three bonds for G and C.

But if I remember my basic chemistry, hydrogen bonds are relatively weak.

That doesn't seem strong enough to hold millions of base pairs together.

What's fascinating here is that you're absolutely right to question that.

Hydrogen bonds are crucial for specificity.

They act like a lock and key to make sure adenine only pairs with thymine.

But they aren't the main glue.

No, they are not the primary glue holding the helix together.

The real hidden strength comes from something called stacking interactions.

How do those work?

Picture those paired bases in They are flat planar rings.

When they pair up, they stack on top of each other rung after rung, almost like a tightly packed roll of coins.

Oh, okay.

I can visualize that.

Because they are packed so tightly, all the water gets pushed out to the outer edges where the highly charged sugar phosphate backbone is.

So the inside of the ladder is completely dry.

Yes, it creates a hydrophobic or water -fearing interior.

And inside that dry core, the electron clouds of those stacked flat bases interact with each other through van der Waals forces.

But aren't van der Waals forces even weaker than hydrogen bonds?

Individually, yes.

A van der Waals interaction is incredibly weak.

But when you add up millions of them perfectly stacked on top of each other like a roll of coins, the cumulative structural integrity is massive.

Wow.

It's actually these stacking forces that force the entire ladder to naturally twist into a helix.

That twisting creates those distinct grooves we always see in models of DNA, right?

The major and minor grooves.

Yes.

Because the two backbones aren't spaced perfectly evenly, you get a wide major groove and a narrow minor groove spiraling up the outside.

What do those grooves actually do?

Think of those grooves as windows into the core of the DNA.

They allow specialized proteins in your cells to reach in, feel the edges of the bases, and read the genetic sequence without ever having to pry the two strands apart.

But let's say a scientist actually does want to pry the two strands apart in a lab.

You mentioned those stacking forces are strong.

How do we break them?

You apply heat.

If you slowly heat up a test tube full of DNA, the thermal energy eventually overpowers those stacking forces and hydrogen bonds.

The two strands separate or denature.

We call this melting the DNA.

Can you physically see it melt?

It's not like an ice cube melting, right?

No, not at all.

But you can

actually absorb significantly more UV light than double -stranded DNA.

Oh, that's a cool trick.

Right.

So if you plot this on a graph, the absorbance stays totally flat as the temperature rises.

But then it hits a breaking point.

Suddenly the line on the graph shoots straight up in a steep S -shaped curve.

Why does it snap open all at once like that?

Because the stacking interactions are highly cooperative.

Once the heat pops open a tiny microscopic bubble in the middle of the DNA, the stacked roll of coins is compromised and the rest of the strand unzips almost instantaneously.

And that breaking point has a name, right?

Yes.

The specific temperature where half the DNA is unzipped is called its melting point.

Now, I noticed a really counterintuitive piece of data in the textbook.

If you have a DNA strand made entirely of GNC -based pairs, it requires a much higher temperature to melt than a strand made of A and T -based pairs.

And most people assume they know why.

Right.

The gut reaction is to assume that's just because GNCs share three hydrogen bonds instead of two.

It is the most common misconception in biochemistry.

While the extra hydrogen bond helps a little, the real reason poly -GC DNA melts at a higher temperature is because granine and cytosine base pairs have significantly stronger stacking interactions than adenine and thymine.

Oh, so they just stack better.

Exactly.

Their flat rings just stack together more tightly, creating stronger van der Waals forces.

That makes so much sense.

So that's the classic relaxed BDNA double helix.

But if these stacking forces are holding the ladder together so fiercely, what happens when a living cell actually needs to bend or twist this massive structure to fit inside a tiny microscopic space?

Does the ladder just snap?

It doesn't snap, but it does comport.

A relaxed BDNA helix naturally wants to have exactly 10 .4 base pairs for every full turn of the twist.

1 .4, right?

But if you take a circular piece of DNA, like the chromosome, inside a bacterium and you forcibly underwind it or overwind it, you create structural tension.

Think of an old school coiled telephone cord.

If you grab the handset and keep twisting it in one direction, eventually the cord stops just turning and starts knotting up on itself, looping into dense little coils to relieve the physical tension.

That phenomenon is called supercoiling.

If DNA is underwound, it folds into negative supercoils to try and restore that perfect happy state of 10 .4 base pairs per turn.

And your cells actually do this intentionally, right?

They do.

They use specialized enzymes called topoisomerases.

Think of them as molecular scissors and tape.

They cut the DNA strand, physically twist it to add or remove coils, and then perfectly seal it back up.

But why would the cell intentionally put its own DNA under that kind of structural stress?

Because negative supercoiling acts like a wound -up spring.

It stores physical energy.

By keeping the DNA under just a little bit of torsional strain, it becomes much easier for the cell to quickly pop open those weaker AT -rich regions when it needs to replicate the genome or read a gene.

That transitions us beautifully to the molecule that actually does the reading and the messenger work, RNA.

If DNA is the heavily guarded supercoiled archive vault of the cell, RNA is the active messenger running around the cytoplasm actually getting things done.

And it does a lot of different jobs.

Most of the RNA in your body, about 80%, is ribosomal RNA, or rRNA, which acts as the physical scaffolding to build protein -making factories.

That's a huge chunk.

Yeah.

Then another 15 % is transfer RNA, tRNA, which ferries the individual amino acids to those

The messenger RNA, mRNA, which carries the actual copy of the genetic code, only makes up about 3%.

Wow.

Only 3%.

Right.

And the rest are small regulatory RNAs.

We established earlier that RNA is single -stranded.

But if you look at a structural model of RNA in the text, it is definitely not just a floppy piece of string floating around.

Far from it.

Because RNA is single -stranded, it has the freedom to fold back on itself.

Picture a strand of RNA bending in half.

If the bases on one side of the bend happen to complement the bases on the other side, they will pair up and zip together.

Creating a 3D shape.

Exactly.

A stable 3D shape called a stem loop or a hairpin.

RNA is constantly folding into these complex, incredible architectures to perform its various jobs.

It's amazing how much dynamic structure can come from a single strand.

But let's scale way, way up for a second and look at the sheer physics of storing all this material.

This is one of my favorite parts.

It's mind -blowing.

If you took the DNA from just the largest chromosome in a single human cell in your body right now and stretched it out, it would be about 8 centimeters long.

But during cell division, your cell condenses that 8 centimeters down into a microscopic package just 10 micrometers long.

That is an 8 ,000 -fold packing ratio.

How is that physically possible?

It requires an incredibly elegant packaging system called chromatin.

If you gently extract chromatin from a cell and zoom in with an electron microscope,

it literally looks like a necklace of tiny beads on a string.

What are the beads?

The beads are called nucleosomes.

Each nucleosome is a tiny disk made out of eight proteins called histones.

You have two copies each of histone H2A, H2B, H3, and H4.

And the string is the DNA?

Yes.

The DNA strand acts like the string wrapping tightly around this protein disk almost two full times, about 146 base pairs.

Then there's a short stretch of linker DNA leading to the next bead, which is pinned into place by a final protein called histone H1.

But how do those histone proteins hold onto the DNA so tightly?

It's not like they have hands.

It all comes down to electrostatics.

Those histone proteins are absolutely packed with two specific amino acids, lysine and arginine, and both of those amino acids carry a very strong positive chemical charge.

Oh, and the phosphate backbone of the DNA is highly negatively charged.

So it's like rubbing a balloon on your hair to create static electricity.

The positively charged histones and the fiercely negative DNA backbone just chemically snap together.

Yes.

But think about what that means for the cell.

If the DNA is snapped tightly around a histone, the cell can't read those genes.

They are physically locked away.

So how does it get around that?

To solve this, the cell uses a family of enzymes called histone acetyltransferases, or HATs.

So what do HATs actually do to loosen the grip?

If we connect this to the bigger picture, they chemically attach an acetyl group to those positively charged lysines on the histones.

This simple modification neutralizes the positive charge.

So the magnet turns off?

Exactly.

Suddenly the static electricity is gone, the magnetic light grip turns off, the DNA physically unwinds from the bead, and the cell can swoop in and read the gene.

When it's finished, a different enzyme removes the acetyl group, the positive charge returns, and the DNA violently snaps shut again.

It's a dynamic breathing archive.

It opens up, it snaps shut.

But what if we don't just want to read the archive?

What if a cell or a scientist wants to actually cut the nucleic acid?

That's a whole different ballgame.

First, let's look at RNA versus DNA stability.

We talked about how DNA is missing an oxygen atom on its 2' carbon.

Why does that tiny difference matter when it comes to cutting the molecule?

Because that extra oxygen atom on RNA is a massive chemical liability.

If you put RNA into a basic alkaline solution, that 2' hydroxyl group loses a hydrogen proton.

When it loses that proton, the oxygen atom becomes what chemists call a nucleophile.

Let's break that term down.

What is a nucleophile in this context?

A nucleophile is an atom that is suddenly highly reactive and absolutely desperate to share its extra electrons.

It becomes so desperate that it actually turns around and attacks the nearest target, which happens to be the phosphate group on its own chemical backbone.

It forms a messy intermediate structure and literally cleaves its own backbone in half.

It is chemical self -sabotage.

Within a few hours in an alkaline solution, an entire RNA strand will shred itself into

But DNA doesn't have that 2' oxygen.

It physically can't become a nucleophile.

It can't attack itself, so it just sits there, completely stable.

Which is exactly the kind of indestructible chemistry you want for a genetic archive.

Exactly.

But while RNA shreds itself in basic solutions, cells can also destroy RNA intentionally using specialized enzymes called ribonucleases.

A classic example is bovine pancreatic RNA -SA.

This enzyme is basically a microscopic workstation with three robotic arms in its active site.

Those arms are three amino acids, histidine -12, histidine -119, and lysine -41.

Walk me through the choreography here.

How do these three arms physically cut the RNA?

First, the enzyme binds the RNA so the target phosphate is perfectly positioned.

Then the first arm, histidine -12, acts as a base.

It reaches out and steals a proton from the RNA's 2' oxygen.

Ah, so just like we saw in the alkaline solution, this forces the oxygen to become a desperate nucleophile and attack its own phosphate.

So the enzyme basically tricks the RNA into attacking itself.

That's brilliant.

Yes.

Meanwhile, the second arm, histidine -119, acts as an acid.

It donates a proton to the other half of the RNA strand, helping it officially break away.

And what about the third arm, lysine -41?

Finally, that third arm steps in.

It carries a positive charge, and it uses that charge to stabilize the very chaotic, highly negative chemical transition that's happening in the middle.

The whole synchronized dance happens in a fraction of a second.

That is an absolute masterclass in chemical precision.

Okay, so that's cutting RNA, but what about cutting our indestructible DNA?

This brings us to one of the most powerful tools in modern science.

Restriction endonucleases.

Where do these DNA scissors even come from?

They are an ancient bacterial immune system.

When a virus attacks a bacterium and injects viral DNA, the bacterium uses these restriction enzymes to hunt down the foreign DNA and chop it to pieces before it can take over the cell.

If you zoom in on one of these restriction enzymes, let's take a famous one called echo -i.

It almost looks like a molecular claw or a pair of tweezers.

It has two identical halves that physically hug the DNA double helix.

It slides along the major groove, feeling the bases, until it finds one highly specific sequence of letters.

Echo -i is looking for the exact sequence, G -A -A -T -T -C.

And what makes that sequence special is that it's a palindrome.

Like a word that reads the same forwards and backwards?

Basically.

Because the two DNA strands run anti -parallel, if the top strand reads G -A -T -T -C from 5' to 3', the bottom strand directly underneath it also reads G -A -A -T -T -C in the same 5' to 3' direction.

Wait a second.

If a bacterium has thousands of these molecular claws floating around, waiting to chop up any G -A -T -T -C sequence they find, wouldn't the bacterium accidentally chop up its own DNA?

That is the genius of the system.

The bacterium produces a companion enzyme called a methylase.

The methylase acts like a bodyguard.

How does it protect the DNA?

It scans the bacterium's entire genome, finds every single G -A -T -T -C sequence, and glues a bulky methyl group onto one of the adenines.

That metal group acts as a physical shield.

Oh.

Because steric clash again?

Exactly.

When the Ikari claw comes along, the bulky shield causes steric clash.

It physically blocks the claw from gripping the DNA.

So the host's DNA is totally safe, while the undershielded viral DNA gets shredded.

So what does this all mean for us?

If this is just a bacterial immune system, why did discovering it completely revolutionize modern human science?

Because it gave scientists the ultimate tool to manipulate the code of life.

When Ikari grips that palindromic sequence and cuts it, it doesn't make a flat, straight cut.

It makes a staggered, jagged cut.

Which leaves overhangs, right?

Yeah.

It leaves short, single -stranded overhangs on the ends of the DNA, which scientists affectionately call sticky ends.

They're sticky because those overhanging bases are desperately trying to find their complementary partners to hydrogen bond with again.

Precisely.

And we can use these enzymes for incredible things.

We can map DNA.

If you take a piece of mysterious DNA and cut it with different restriction enzymes, you get a bunch of fragments of varying lengths.

And then you can sort them out.

Right.

You can drop those fragments into a slab of agarose gel and run an electrical current through it.

The DNA is negatively charged, so it pulls through the gel.

The smallest fragments move the fastest, and the large ones get tangled and move slowly.

This creates a distinct barcode -like pattern of bands.

Which is the foundation of DNA fingerprinting.

Because every single human has slight, unique variations in their genetic code, the locations where those enzymes cut will be slightly different for you than they are for me.

If a forensic scientist compares the barcode from crime scene evidence to the barcode of a suspect, they can definitively prove if it's a match.

And beyond forensics, those sticky ends allow us to play molecular architect and build recombinant DNA.

Like making medicine.

Exactly.

Imagine you want to make human insulin.

You can cut the human insulin gene with Ecorirar.

Then you cut a bacterial plasmid, a circular ring of DNA with that same Ecorirar enzyme.

So they have matching sticky ends.

Yes.

Because both pieces were cut by the same scissors, their sticky ends are perfectly complementary.

They will naturally stick together.

You seal the backbone with an enzyme called DNA -le -gay, and suddenly you have a bacterium that can pump out human insulin.

It is truly staggering to think about the scale of what we've covered today.

We started this session talking about a young scientist pulling gross phosphorus -rich chemicals out of pus on discarded bandages.

We deconstructed those chemicals down to a sugar, a phosphate, and a base.

We really went from the micro to the macro.

We did.

We saw how van der Waals forces and stacking interactions twist them into a tight ladder, how charged histones wind that ladder into dense supercoiled beads, and finally, how complex molecular claws can read and cut those strands with absolute atomic precision.

It is a remarkable journey through the molecules that build us.

And as we wrap up this tutoring session, I want to leave you with one final thought to mull over.

Think about the beautiful paradox of DNA.

The paradox.

Yeah.

On a fundamental chemical level,

DNA relies entirely on extreme, unbreakable stability.

Its deliberate lack of that two -prime oxygen atom to preserve the blueprint of your life against degradation over eons.

But on a structural level, it relies entirely on weak, easily broken forces like hydrogen bonds and stacking interactions.

It is simultaneously an eternal, indestructible vault and an easily accessible library that can be unzipped by your cells in a millisecond.

It is an absolute masterpiece of biological engineering.

Thank you so much for joining us for this intensive deep dive.

Keep studying, keep asking questions.

And on behalf of the Last Minute Lecture Team, thank you for listening.

We'll see you next time.