Chapter 8: Nucleotides and Nucleic Acids: Structure, Chemistry, and Biological Functions

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

Okay think about this.

What is the actual blueprint of life?

The molecule carrying all the instructions for everything.

From a tiny cell right up to, well you, we're going to unpack that today.

This is our deep dive into nucleotides and nucleic acids, drawing straight from chapter 8 of Leninger's Principles of Biochemistry.

Yeah, our mission here is really to cut through the complexity.

We want to unravel the chemistry, the structures, the functions, focusing on those core molecular mechanisms, the pathways, kinetics, thermodynamics, all that good stuff.

Think of this deep dive as, well, your shortcut to getting properly informed on something absolutely fundamental to life.

And it will uncover some surprising facts along the way, make some interesting historical connections,

and hopefully give you a really clear picture of why these tiny molecules are just so critical and how understanding them completely changed science.

Seriously, this is where it gets fascinating.

Let's start with the basics, the building blocks.

What is a nucleotide?

Right.

So fundamentally, it's got three parts, a nitrogenous base, a pentose sugar, and then at least one phosphate group.

And if you take away that phosphate.

Then you've just got a nucleotide base and sugar, simple as that.

Okay, those bases, there's a bit of variety there.

Definitely.

You've got your purines, that's adenine A and guanine G, they have that double ring structure.

And the pyrimidines.

Single ring,

cytosine C, thymine T, and uracil.

Each one's derived from a parent compound,

and they have specific numbering systems which become important later.

There are even some less common minor bases you find sometimes.

Now here's something absolutely crucial for you to grasp, the sugar.

This is what defines whether we're talking RNA or DNA.

Exactly.

It's all about that two prime carbon on the pentose ring.

In RNA, the sugar is diribose, and it has a hydroxyl group, an OH, right there at the two prime position.

But in DNA?

It's two prime deoxy diribose, meaning that oxygen atom is gone, just a hydrogen there.

That subtle difference, the presence or absence of that one oxygen is the defining feature, not whether it has U or T.

Okay, so you have these three components.

How do they actually link together to form these long chains, the nucleic acids?

Good question.

First, the base connects to the sugar's one prime carbon.

That's through what's called an N -beta -glycosyl bond.

Got it.

Base to sugar.

Then the phosphate.

The phosphate group attaches via an ester bond to the five prime carbon of that same sugar.

That gives you a single nucleotide unit.

But the chain?

Ah, the chain is built by the phosphatidicuster linkage.

This is key.

It connects the five prime phosphate of one nucleotide to the three prime hydroxyl group on the sugar of the next nucleotide.

Forming that sugar -phosphate backbone.

Precisely.

It's this strong covalent backbone, this alternating pattern of phosphate and sugar, that holds the whole polymer together.

And that linkage creates directionality, doesn't it?

The five prime and three prime ends.

Absolutely.

Every strand has a distinct five prime end, usually with a free phosphate group and a three prime end, typically with a free hydroxyl group.

That's why sequences are always, always written five prime to three prime.

It reflects the chemical reality.

Right.

It's not just random convention.

And we call short chains oligonucleotides.

Yeah, maybe 50 bases or fewer.

Anything longer, we generally just call a polynucleotide.

Chemically speaking, what are these bases like?

Well, they're aromatic rings, which is important.

They're also weakly basic and pretty hydrophobic.

They don't like water much, especially around neutral pH.

They can also flip between different structural forms called tautomers.

But at physiological pH, one particular form is overwhelmingly preferred, which is vital for how they interact.

And that aromatic nature gives them a useful property, right?

Yeah.

UV absorption.

Exactly.

They absorb UV light very strongly, around 260 nanometers.

It's incredibly useful in the lab for detecting and quantifying nucleic acids, like a standard way to measure DNA concentration.

OK, so we have the covalent backbone.

But what holds the whole 3D structure, especially the double helix, together?

Two main types of interactions beyond the backbone.

First, and this is huge hydrophobic stacking interactions.

Those flat aromatic bases stack on top of each other, almost like coins, minimizing their contact with water.

Driven by van der Waals forces.

D -pull -di -pull.

Both.

This stacking is a major, major stabilizing force for the 3D structure.

Second, you have the hydrogen bonds.

Ah, the famous Watson -Crick base pairing.

The very same.

Adenine forms two hydrogen bonds, specifically with thymine A pairs with T.

And guanine forms three hydrogen bonds, specifically with cytosine G pairs with C.

And that specific pairing is the secret to copying genetic info, right?

Absolutely.

It's the foundation of heredity.

Which brings us neatly to DNA.

The icon of life.

Getting to that double helix structure was quite a journey, wasn't it?

Like a detective story.

It really was.

Started way back in 1869, Friedrich Miescher isolating this stuff he called nucloin from pus cells.

But knowing what it did took much longer.

Then came the proof that DNA was the actual genetic material.

Right.

Key experiments in the mid -20th century.

Avery, Macleod, and McCarty in the 40s showed that DNA from nasty streptococcus bacteria could transform harmless waste.

And making them nasty, too.

Exactly.

And then Hershey and Shace in 1952 used viruses and radioactive labels.

They showed definitively it was the viral DNA, not the protein coat, that got inside bacteria and directed them to make new viruses.

Game changer.

And around the same time, Erwin Chargef was finding those weird ratios.

Crucial clues.

Chargef noticed that in DNA from any species, the amount of A always equaled T and G always Which means, you know, the total purines, A plus G, equals total pyrimidines, T plus C.

Chargef's rules.

But why?

The structure had to explain it.

And the X -ray data started pointing towards a helix.

Yes.

Roslyn Franklin and Maurice Wilkins, working in the early 50s.

Their X -ray diffraction patterns of DNA fibers were critical.

They showed DNA was helical and had regular repeating patterns specifically, dimensions of 3 .4 angstroms and 34 angstroms.

That gave Watson and Crick vital constraints.

And then, 1953, Watson and Crick pull it all together.

The double helix.

Bam.

The model explained everything beautifully.

Two strands, twisting around each other, running in opposite directions anti -parallel.

With the sugar phosphate backbones on the outside, facing the water.

Right.

Hydrophilic.

And the hydrophobic bases stacked neatly on the inside perpendicular to the axis, like stairs in a spiral staircase.

This structure naturally creates those major and minor grooves you always see in diagrams.

And the pairing.

A with T, G with C.

Fit perfectly with Chargaff's rules.

A, T, G, C.

But here's a key point for, you know, a deeper understanding.

While the hydrogen bonds dictate the specific pairing, they ensure A only pairs with T, and G with C, they aren't the main source of the helix's stability.

Really?

I always thought it was the H bonds holding it all together.

They contribute, but the primary stabilizing force is actually those base stacking interactions we talked about earlier.

The collective effect of all those van der Waals and dipole forces between the stacked pairs.

Plus, you need metal occasions like magnesium around to shield the negative charges on the phosphate backbone.

Okay, so stacking is king for stability.

And that's why GC -rich DNA is tougher to melt.

Exactly.

GC pairs have three H bonds, yes, but more importantly, they stack more favorably, contributing more to that overall stacking energy than AT pairs.

So higher GC content means a more stable helix.

And the structure itself immediately suggested how DNA could be copied.

That was the genius of it, the complementarity.

If you know the sequence of one strand, you automatically know the sequence of the other.

Just unzip the two strands, and each one can act as a perfect template for building a new partner strand.

Replication mechanism.

Right there in the structure.

Mind -blowing, really.

But DNA isn't always just that standard B -form helix, is it?

It can bend and twist.

Oh yeah, it's flexible.

BDNA is the classic, most stable form in the watery environment of the cell, about 10 .5 base pairs per helical turn.

But under dehydrating conditions, it can shift into ADNA.

Which is different now.

ADNA is wider, shorter, with 11 base pairs per turn, and the bases are tilted relative to the axis.

And then there's ZDNA.

The weird one.

Yeah, the radical one.

It's a left -handed helix, which is bizarre.

The backbone zigzags, hence the Z.

It has 12 base pairs per turn, and tends to form in stretches of alternating purines and pyrimidines, especially CG repeats.

Its exact job in the cell is still a bit murky, but it's likely involved in regulating genes somehow.

And DNA can form other unusual shapes too, right?

Hairpins and things.

Absolutely.

If you have a sequence called a palindrome, where the sequence reads the same forwards on one strand as it does backwards on the complementary strand, like JDTC paired with

The DNA can form structures like hairpins if it's single -stranded, or even cruciforms like cross shapes in double -stranded DNA.

Different from just a mirror repeat.

Yeah, mirror repeats don't have that complementary structure needed to fill back on themselves.

You can also get triplex DNA with three strands interacting, often using different hydrogen bonding patterns like Hoogsteen pairs.

And G -rich sequences can form G quadruplexes involving four guanine bases interacting in a square.

Are these just lab curiosities?

Not at all.

These unusual structures often serve as recognition sites for specific proteins, playing important roles in things like DNA replication, recombination, and gene regulation.

They have real cellular functions.

Okay, what about RNA structure?

It's usually single -stranded, but it's not just floppy, right?

Definitely not.

Single -stranded RNA still tends to form right -headed helices, again largely driven by base stacking.

And because it's single -stranded, it can fold back on itself in incredibly complex ways.

How so?

If you have sequences within the same RNA molecule that are complementary, they can pair up, forming structures like hairpin loops, internal loops where there are mismatched bases,

and bulges.

This intricate folding creates specific 3D shapes crucial for RNA's function.

And if RNA is double -stranded?

It usually adopts an A -form helix similar to ADNA.

And RNA has a bit more flexibility in pairing.

You often find G pairing with U, which is much rarer in DNA.

And the functional range of RNA is just, wow, it's way more than just a messenger.

Right, we have mRNA carrying the protein code.

Yep, messenger RNA.

Can be monocistronic, coding for one protein, or in bacteria, often polycistronic, coding for several.

But then you have tRNA, transfer RNA that brings the amino acid.

And RNA, ribosomal RNA, which is actually part of the ribosome machine itself.

And a structural and catalytic part.

Some RNAs act as enzymes' ribozymes.

Plus, there's a whole universe of non -coding RNAs, NCRNAs, with regulatory roles, structural roles.

RNA is incredibly versatile.

It's an information carrier, a structural molecule, and a catalyst.

Amazing stuff.

OK, let's switch gears to DNA stability.

It has to be stable to store information, but it's also vulnerable.

It is.

It's remarkably stable chemically compared to, say, RNA, thanks to that missing 2' hydroxyl.

But it's not immune to damage.

And even slow rates of damage add up over a lifetime, contributing to things like cancer and aging.

So how do we measure its stability?

You mentioned melting.

Right.

Denaturation or melting.

If you heat DNA or expose it to extreme pH, the hydrogen bonds break, the base stacking gets disrupted, and those two strands unwind and separate.

And we can watch this happen using UV light, the hyperchromic effect.

Exactly.

Remember how stacked bases absorb less UV at 260 millimeters?

That's the hyperchromic effect in double -stranded DNA.

When the strands separate, the bases unstack and their UV absorbance goes up.

That increase is the hyperchromic effect.

So you can plot absorbance versus temperature.

And find the melting point, the team.

That's the temperature where half the DNA is denatured, half is still double -stranded.

It's a direct measure of helix stability.

And this, Kimi, depends on the GC content.

Directly.

More GC pairs means stronger stacking, means a higher team.

You need more heat to melt it.

That thermophilic bacterium living in a hot spring bet its DNA has a much higher GC content than ours.

Makes sense.

And you mentioned RNA duplexes are even more stable.

Yeah.

Surprisingly, double -stranded RNA generally has a higher team than DNA of the same sequence.

RNA -DNA hybrids are intermediate.

But despite this inherent stability, DNA is constantly getting damaged inside the cell.

Constantly.

Through non -enzymatic chemical reactions, one common one is deamination losing an amino group.

Cytosine deaminating to uracil is a big one.

And you said this explains why DNA uses thymine.

It's a major evolutionary reason.

Think about it.

If DNA used uracil like RNA does, when a cytosine deaminated to uracil, the cell repair systems wouldn't know if that uracil was supposed to be there or if it was a damaged cytosine.

But because DNA uses thymine, which is just methylated uracil, any uracil found in DNA is immediately recognized as damage as a deaminated cytosine and fixed back to sea.

It prevents a slow but steady conversion of GC pairs to AT pairs over time.

Clever.

Very clever.

What else happens?

Depurination is another big one.

The bond connecting a purine base, A or G, to the sugar can just spontaneously break, hydrolyze, leaving a gap, an AP site, a purinic site, happens thousands of times per cell per day.

Thousands.

Wow.

And radiation.

Yup.

UV light, especially from the sun, can cause adjacent pyrimidine bases, particularly thymines, to link together, forming pyrimidine dimers.

These distort the helix.

Ionizing radiation, like eps rays or gamma rays, is harsher.

It can break bonds, open rings, even snap the DNA strand.

And chemicals.

Lots of chemical culprits.

Deminating agents like nitrous acid, alkylating agents that add methyl or ethyl groups to bases like dimethyl sulfate, creating O6 -methylguanine, which mispares.

And then there's oxidative damage.

From just breathing.

Pretty much.

Reactive oxygen species, like hydroxyl radicals, are byproducts of using oxygen in metabolism.

They attack DNA bases and the sugar backbone.

It's a constant battle.

It sounds like DNA should be falling apart constantly.

How does it survive?

Because, and this is unique among biological macromolecules, DNA has dedicated extensive repair systems.

Cells invest a huge amount of energy in constantly patrolling the DNA, finding damage, and fixing it.

If there are multiple, over -leaping pathways to handle different kinds of lesions, it's absolutely essential.

Are there also intentional DNA modifications in methylation?

Yes.

Enzymatic methylation is different from damage.

Adding methyl groups to specific adenine or cytosine bases is a regulated process.

In bacteria, it helps distinguish their own DNA from invading viral DNA.

In eukaryotes like us, DNA methylation, often at CPG sequences, is a key epigenetic mark.

It doesn't change the sequence, but it changes how genes are read and expressed.

Hugely important for development in cell identity.

Okay, so we've covered structure, stability, damage, repair, but nucleotides do more than just build DNA and RNA, right?

Oh, absolutely.

Their rules are incredibly diverse.

Think about energy.

ATP.

ATP adenosine triphosphate, the universal energy currency of the cell.

Breaking those high -energy phosphon hydride bonds between the phosphate groups releases

about 30 kilojoules per mole per bond that powers almost everything the cell does.

And other nucleotides like GTP, CTP, UTP play similar interview roles in specific pathways.

And they pop up in other important molecules too.

They do.

Many essential enzyme cofactors think coenzyme A, NAD plus C, FAD have an adenosine component.

Why adenosine so often?

Good question.

One theory is evolutionary.

ATP was likely abundant early on, so it got incorporated into various biochemical tools.

Plus, many enzymes that bind these cofactors share a common structural fold designed to grab onto that nucleotide part.

Evolutionary efficiency, maybe.

They also act as signals.

Big time.

Cyclic AMP, CMP, and cyclic GMP, C -GMP are classic second messengers.

Hormones bind to cell surface receptors, triggering enzymes inside to make sac OP or C -GMP, which then relay the signal onward, changing cell activity.

Bacteria have things like PPGPP for stress responses.

And ATP itself can be a signal.

Yes.

ATP and ADP can be released from cells, like neurons, and act as signaling molecules by binding to specific receptors on other cells.

This triggers all sorts of responses, from pain sensation to blood clotting.

These tiny molecules are central to how cells talk to each other.

It's amazing how versatile they are.

Now let's talk about the technologies.

Our understanding of this stuff has led to some revolutionary tools.

Has it ever?

Being able to synthesize DNA chemically was a massive first step.

The phosphoramidite method, developed by people like Carana, Letzinger, Carruthers, lets labs make custom DNA strands, all eonucleotides, quickly and automatically.

Essential for primers, probes, synthetic genes, and then came PCR.

Polymerase chain reaction.

Carey -Mullis, 1983.

Just brilliant in its simplicity.

You need tiny amounts of DNA, two synthetic DNA primers flanking the region you want to copy.

A heat -stable DNA polymerase like TAC polymerase from hot spring bacteria, and nucleotide building blocks.

And you just cycle the temperature?

Yep.

Heat high to denature the DNA, separate the strands, cool down so the primers anneal to their target spots.

Warm up slightly so the polymerase extends the primers, copying the DNA.

Repeat.

And it amplifies exponentially.

Exactly.

Each cycle roughly doubles the target DNA.

30 cycles gives you like a billion -fold amplification.

From almost nothing, you get enough DNA to see and analyze.

The uses are just endless, forensics.

Absolutely.

DNA fingerprinting, looking at those short tandem repeats, STRs, that vary hugely between people.

CODIS database relies on it.

Identifying remains, like the Russian Tsar's family.

Agent DNA.

Pulling DNA out of mammoth bones.

Medical diagnostics, detecting infections, genetic diseases, PCRs everywhere.

And then sequencing.

Actually reading the As, Ts, Cs, and Gs.

Sanger sequencing was the classic method for a long time.

Frederick Sanger, another giant.

His Didyoxy method from 1977 was the workhorse for decades, including the Human Genome Project.

It uses special chain -terminating nucleotides, DDNTPs.

They stop the copying process.

Right, because they lack the 3 -prime hydroxyl needed for the next nucleotide to attach.

You run four reactions, each with a small amount of one type of DDNTP, DDA, DDT, DDT, This generates fragments ending at every possible position for that base.

Then you separate them by size.

On a gel, or later, using automated machines with fluorescent tags on the DDNTPs.

Read the colors, read the sequence, brilliant.

But now we have next -generation sequencing, which is just mind -bogglingly fast.

Totally different scale.

Instead of sequencing one fragment at a time, NGS methods sequence millions or even billions of fragments simultaneously.

How does that even work?

The general idea is you break the DNA into tons of small pieces, attach them to a solid surface like a flow cell or a bead,

and then sequence them in place.

Many methods use fluorescently labeled nucleotides that are added one base at a time, imaged, and then prepared for the next addition.

Like Illumina sequencing.

That's a dominant one.

It uses reversible terminators, nucleotides that have a fluorescent tag and a blocking group.

One base gets added, the machine images the color, then the tag and block are chemically removed, allowing them to cycle.

Super high throughput, very accurate for short reads.

Are there other approaches?

Yeah.

Pacific Biosciences' SMRT sequencing is different.

It watches a single DNA polymerase molecule working in real time, incorporating fluorescent nucleotides into a single, long DNA strand, often tens of thousands of bases long.

Great for tricky genomes with lots of repeats.

And then you have all these millions of short reads.

How do you make sense of them?

That's where bioinformatics comes in.

Powerful computers and clever algorithms align the overlapping reads to reconstruct the original sequence.

You need sufficient sequencing depth or coverage, meaning each base in the genome is read multiple times, ideally dozens or hundreds to ensure accuracy and fill gaps.

The overlapping reads are assembled into longer contexts.

It's incredible.

The ability to sequence so quickly and cheaply is changing everything.

Personalized medicine.

Forensics, agriculture, evolutionary biology.

The impact is just immense.

So what a journey.

From the simple definition of a nucleotide base, sugar, phosphate, to the elegance of the double helix, the constant threat of damage, and the amazing repair systems.

And then all the other jobs nucleotides do, energy, signaling, cofactors, plus the technologies like PCR and NGS that this knowledge unlocked.

It really underpins so much of modern biology.

It really does.

We've covered a lot of ground from Chapter 8 of Leninger.

So here's something to think about as we wrap up.

We've seen how understanding these molecules revolutionized science.

As sequencing gets even cheaper, faster, and we get better at manipulating DNA and RNA.

What's next?

What breakthroughs do you see coming down the pipeline?

How is this going to keep changing our world?

Definitely food for thought.

We hope this deep dive has fueled your own curiosity to keep exploring this amazing molecular world.

Thank you, as always, for being part of the Last Minute Lecture family.