Chapter 34: Nucleic Acid Structure & Function

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Okay, let's unpack this.

Today we are going deep into really the chemical basis of heredity.

We're talking about the structure and function of nucleic acids pulling everything directly from Harper's Illustrated Biochemistry.

And our mission here isn't just to, you know, list off a bunch of facts.

It's to build a real framework, a chemical one that connects the structure of DNA and RNA right to human health and disease.

For anyone heading into the health sciences, this is just, it's the foundation.

And you have to keep this flow in mind.

DNA makes RNA and RNA directs and importantly regulates how proteins are made.

That's the whole system.

Precisely.

And it's so critical to get past just the names of the molecules.

You need to understand this because genes don't just exist in a vacuum, their replication, their function.

It's all controlled by gene products that are constantly interacting with these complex signal pathways in the cell.

And that complexity, that's really the key to understanding the genetic basis of disease.

If we're going to start at the beginning, we have to go back to that key discovery, the proof that DNA carries our genetic blueprint.

I'm talking about the experiments in 1944, Avery, McLeod, and McCarty.

They showed that this

transforming factor - Which they figured out was just purified DNA.

Exactly.

They showed that this purified DNA from one type of bacterium could actually, you know, genetically change another type.

And it's amazing how that core idea is still so relevant.

I mean, taking some genetic material and using it to change a cell, that's basically what we do today with modern genetic engineering, just a lot more sophisticated.

It really is.

The fundamental rules were established very early on.

Okay.

So let's move from the history into the blueprint itself, DNA.

What is this molecule actually built from?

Well, at its core, DNA is a polymer.

It's just a very, very long chain made of repeating units.

And those units are?

There are four specific deoxynucleotides.

You have deoxycannolate, which we call A, or G.

C for deoxycycannolate and T for thymidolate, the four letters.

The only four letters in the alphabet for the entire instruction manual of life.

And they're all linked together in a single strand.

By what kind of connection?

What holds that chain together?

They're held together by these incredibly strong bonds called 3O -phosphidistor bonds.

And these bonds are what create the strand's directionality or its polarity.

That polarity is so important, isn't it?

It's not optional.

It means one end of the 5 -year end is the 3 -year end.

It's like a one -way street for all the cellular machinery.

Absolutely.

And that strict directionality, plus the need for perfect high -fidelity copying, is what led Watson, Crick, and Wilkins to propose the double helix.

They're building on Rosalind Franklin's X -ray data and, of course, Chargaff's rules.

Yes, Chargaff's rules.

The simple, elegant observation that the amount of A in any DNA sample always equals the amount of T.

And G always equals C.

That symmetry was the clue.

It dictated the entire pairing system.

It dictated the Watson -Crick base pairing.

Yeah.

A only pairs with T using two hydrogen bonds, and G only pairs with C using three hydrogen bonds.

That complementarity is what makes replication possible.

And the two strands, they're anti -parallel.

Meaning they run in opposite directions, like a highway.

One goes five -row off to three off, the other goes three of five -row.

Exactly.

And the whole structure is usually a right -handed spiral.

Okay, so the hydrogen bonds explain the A -T -G -Z rule, but hydrogen bonds are, well, they're relatively mech.

How is the DNA molecule so incredibly stable inside a cell?

That's a fantastic question.

And the answer is really clever.

The bases themselves, the A, T, C, and G, they're flat aromatic rings.

And inside the helix, they stack right on top of each other, almost perfectly.

This creates what we call base stacking forces.

So van der Waals forces and electrostatic interactions.

Yes.

And these vertical stacking forces are immensely strong and aggregate.

So it's not just the side -to -side hydrogen bonds holding it together.

It's also this powerful vertical stacking.

Precisely.

And that's also why GC pairs add more stability.

Because they have three hydrogen bonds instead of two.

Three hydrogen bonds and stronger stacking forces.

So DNA that's rich in GC pairs is much, much more resistant to being pulled apart, or what we call denaturation.

So why isn't the whole genome just GC pairs, then?

For maximum stability?

Well, because sometimes you need the DNA to open up easily.

Think about where replication starts.

Those areas, the origins of replication, are almost always A -T rich.

Because they're easier to pull apart.

That makes perfect sense.

Okay, so let's get some dimensions on this structure.

The common form, BDNA, has a diameter of about two nanometers, and it makes one complete helical turn every 3 .4 nanometers, which contains, what is it, 10 base pairs.

10 base pairs per turn, that's right.

But the surface isn't a smooth cylinder.

You have these grooves.

The major groove and the minor groove, do they have a function?

Oh, a huge function.

They are the recognition sites.

The grooves expose the edges of the base pairs in a way that allows regulatory proteins like transcription factors to come in and read the sequence.

Without having to unzip the whole molecule.

Exactly.

They can identify specific sequences just by docking to the grooves.

It's how the cell controls everything.

All right, let's talk more about that unzipping or melting.

When we talk about denaturing DNA in the lab, say, by heating it up, the strands separate.

They do.

And as they separate, you see a couple of physical changes.

The solution becomes less viscous, and you see this increase in optical absorbance.

That's the hyperchromicity of denaturation, right?

It is.

Single strands just absorb more UV light than double strands.

It's a really useful diagnostic.

And the temperature right at the midpoint of that transition, where half the DNA is single stranded, that's the melting temperature, the TM.

And as you'd expect, the TM is higher for DNA with more GC content.

But the environment matters, too.

For instance, if you increase the salt concentration, the monovalentations, by tenfold, you can increase the TM by over 16 degrees Celsius.

Why does salt do that?

Because the positive ions from the salt, like sodium, they shield the negatively charged phosphate backbone of the DNA.

Ah, so they reduce the repulsion between the two strands, making them harder to pull apart.

You've got it.

And researchers use this.

We add salt to stabilize DNA, or we might add something like formamide, an organic solvent, to do the opposite.

It destabilizes the hydrogen bonds and lowers the TM.

Which is really useful when you don't want to damage the DNA with high heat.

Exactly.

And the reverse process, re -annealing or hybridization, is just as important.

The fact that complementary strands will find each other and zip back up is the basis for, well, for so many critical lab techniques.

Like southern blotting, for detecting specific DNA.

And northern blotting, which uses an RNA probe to detect a specific RNA molecule.

It all relies on hybridization.

So we have this massive, you know, six foot long molecule.

How does it all fit inside a tiny cell nucleus?

It can't just be floating around in that relaxed double helix form.

No, it can't.

In many systems, like bacteria or mitochondria, where the DNA is a closed circle, it exists in what we call a supercoiled form.

And biological systems seem to prefer negative supercoiling.

What does that mean?

Negative supercoiling is when you twist the DNA in the opposite direction of the right -handed B -form helix.

It makes the DNA underwound.

And why would the cell want to do that?

Because it stores energy.

That underwinding puts a strain on the molecule, and that strain makes it easier to separate the strands locally when it's time for replication or transcription to start.

So it's like preloading a spring.

That's a perfect analogy.

And the enzymes that manage all of this twisting and untwisting are the topoisomerases.

And they use ATP to do their work.

They do.

They are absolutely essential, especially for dividing cells.

And that makes them a fantastic target for cancer chemotherapy.

Right.

Because if you inhibit the topoisomerases, the DNA gets hopelessly tangled during replication and the cancer cell dies.

A clear clinical link.

So just to summarize DNA's main job,

it's a template.

A template for two things.

First, replication.

The strands separate, and each one serves as a template to make a new complementary strand.

That's semi -conservative replication.

And second, transcription, where one strand, the template strand, is used to synthesize an RNA molecule.

Right.

And the other DNA strand, the coding strand, will have the same sequence as the RNA, just with T instead of U.

And that brings us perfectly to RNA.

It's also a polymer, also linked by those same three RN phosphatister bonds.

But it has a few major chemical differences from DNA that completely change its function.

Okay.

What's the first one?

The sugar.

RNA uses ribose, which has an extrahydroxyl group at the two prime position.

DNA, of course, uses deoxyribose.

And the base is different.

Instead of thymine T, RNA uses uracil U.

Correct.

And third, RNA is almost always a single strand, though it can and does fold back on itself to form complex structures.

And since it's single stranded, it doesn't follow Chargaff's rules.

G doesn't have to equal C.

Exactly.

And that little two prime hydroxyl group on the ribose sugar, it makes RNA alkali labile.

Meaning it breaks down easily in an alkaline solution while DNA is stable.

Another useful property for lab work.

It really is.

All right.

Let's get into the different classes of RNA, starting with the big one, messenger RNA or mRNA.

These are the templates for making proteins.

They carry the genetic message from the DNA to the ribosome.

And they're incredibly diverse.

They vary hugely in size and in how many copies are in the cell at any given time.

And in eukaryotes, they need some serious modifications to work, right?

The cap and tail.

Oh, absolutely.

At the five prime end, you have the seven -methylgrinocene triphosphate cap.

Why is the cap so important?

Two main reasons.

One, it protects the mRNA from being chewed up by enzymes called five prime exoribonucleases.

It adds stability.

It adds stability.

And crucially, it's the recognition signal for the ribosome.

The translation machinery looks for the cap to know where to start.

No cap, generally no protein.

And then at the other end, the three prime end.

There you have the poly A tail.

It's a long string of adenine residues, maybe up to 250 of them, added after transcription.

And it does a similar job.

A very similar job.

It protects the three prime end from degradation and also helps facilitate translation.

Okay.

So that's mRNA.

What about the adapter molecules, the transfer RNA or tRNA?

The tRNAs are just, they're molecular marvels.

They're the actual translators.

They're what convert the four -letter language of nucleic acids into the 20 amino acid language of proteins.

And they have that very distinct folded clover leaf structure.

They do.

And they all share a specific feature at one end, the acceptor arm, which always ends in the sequence CCA.

And that's where the amino acid gets attached.

That is the business end, yes.

A specific enzyme attaches the correct amino acid to the correct tRNA.

It's an incredibly precise system.

They make up about 20 % of the cell's RNA.

Finally, we have ribosomal RNA, rRNA.

The most abundant type making up about 70 % of all RNA in the cell.

They're the structural and importantly, the catalytic components of the ribosome, the protein synthesis factory itself.

The catalytic component.

I thought the ribosome was made of protein and rRNA.

The proteins did the work, right?

That's what everyone used to think.

But we now know that it's actually the large RNA molecule that performs the key chemical reaction, the peptidyl transferase activity that links amino acids together.

So the RNA is an enzyme.

It's a ribozyme.

Yes.

One of the most important discoveries in molecular biology.

Wow.

Okay.

So moving into the more regulatory side of things, we have small nuclear RNAs, SNRNA.

Yes.

Or SNRPs, when complex with proteins.

They are critical players in RNA processing, especially the splicing process.

Splicing being the removal of introns from the pre -mRNA?

Exactly.

This N and RNAs are what carry out that precise cutting and pasting job.

And this is where the field has just exploded in the last couple of decades.

The discovery of all these non -protein coding RNAs or NCRNAs.

A total revolution.

We're now realizing the genome is transcribed far more extensively than we ever thought.

You have the small NCRNAs like microRNAs, muranes, and silencing RNAs.

Then what do they do?

They are powerful gene silencers.

They typically find a target messenger RNA through complementary base pairing and then trigger its degradation or block its translation.

So they're like a dimmer switch for gene expression.

A very specific dimmer switch.

And because of their specificity, they're incredibly exciting as potential drugs.

You can design NCRNA to target and knock down almost any protein you want.

And then there are the big ones, the long non -coding RNAs, LNCRNAs.

Yes.

Thousands of nucleotides long.

They seem to be involved in all sorts of regulation, including organizing the large scale structure of chromatin in the nucleus.

It's a hugely active area of research.

So to wrap up the cast of characters, we need the enzymes that cut nucleic acids, the nucleases.

Right.

You can classify them in a few ways.

You have ribonucleases that cut RNA and deoxyribonucleases that cut DNA.

Then you have endonucleases versus exonucleases.

Exactly.

Endonucleases cut in the middle of a strand while exonucleases chew away from the ends.

And there are two types of sequence specific enzymes that have totally changed biology.

First, restriction enzymes.

The original genetic engineering toolkit.

They recognize a specific short sequence of DNA and cut both strands right there.

And now the game changer, the CRISPR -Cas system.

The ultimate tool.

It's a protein RNA complex where you can program a guide RNA to take the Cas nucleus to literally any DNA sequence you want to cut.

It's revolutionized genetics.

One last nucleus to mention.

The proofreading one.

Ah, yes.

The 3' to 5' exonuclease activity that's part of the DNA replication machinery in bacteria.

If the polymerase adds the wrong base, this exonuclease can immediately back up, snip it out, and let the polymerase try again.

It's the cell's delete key.

An essential function for maintaining fidelity.

Okay, let's try to recap the absolute core takeaways here.

DNA is the double -stranded anti -parallel blueprint.

Its stability comes from base stacking and hydrogen bonds, A with T, G with C.

It serves as the template for its own replication and for transcription.

And RNA is the dynamic single -stranded worker molecule.

It uses U instead of T, has a ribose sugar, and comes in many flavors.

You have mRNA, the message, TRNA, the translator, and rRNA, the factory machinery that is also a ribosome.

And then you have that whole other layer of regulatory RNAs,

CERNase, LNC RNAs, that are constantly fine -tuning gene expression.

And this isn't just, you know, abstract biochemistry.

Understanding this informs everything.

From using hybridization and diagnostic tests like northern blots, to designing cancer drugs that target two poissama races, or even developing brand new therapies based on CERNase.

It's the absolute starting point for modern medicine.

And I'll leave you with a final thought to mull over.

The data suggests that over 90 % of our genome is transcribed into RNA.

But most of that is non -protein -coding RNA.

So if the main point of DNA for so long was thought to be just making protein, what does it mean that the vast majority of the output is actually regulatory RNA?

It suggests that the control system itself is vastly more complex, more elaborate, and maybe even more important than the final products it's designed to regulate.

A truly complex and still mysterious control system.

Thank you for joining us for this deep dive.