DNAの構造と解析 | 第10章 遺伝学の概念（12年生）
Welcome to the deep dive. Today we’re jumping straight into uh what you could call the historical battleground of genetics. Yeah. The search for the molecule of heredity. It sounds simple now, but it was a huge puzzle. Exactly. We’re unpacking the foundational science, the key experiments, and you know, the molecular architecture that finally proved DNA was the stuff of genes and showed us its famous shape. It’s really important for you to realize that um for the longest time the chemical nature of the gene was just this massive question mark in biology, right? I mean, right up until around 1944, most scientists were actually betting on protein, not DNA. They kind of dismissed DNA. So, our mission today is to track how a few really groundbreaking experiments managed to flip that whole scientific consensus on its head and how they figured out the secret code, essentially the structure itself. Okay, so before we even look at the evidence for what it is, maybe we should define what the genetic material actually needs to do. What’s its job description? Good point. Our sources lay out four uh absolutely crucial characteristics, things this molecule must have. And these requirements were like the big hurdles for the early researchers, weren’t they? Definitely. First up, replication. It absolutely has to be able to copy itself perfectly before every single cell division so that each new cell gets the complete blueprint identical precisely mitosis. Meiosis underlies all of that. Second is storage of information. And we’re not talking small amounts of data here. No, it has to be like a massive library, right? Holding all the instructions to build and run well everything. Exactly. Think about how say your liver cell uses totally different instructions compared to a skin cell. Melanin genes versus hemoglobin genes for instance. They’re all there but accessed differently. Okay, scared info. Then what? Third, that information has to be usable. There needs to be expression of information. Ah, the central dogma. You got it. DNA gets transcribed into an RNA message which then gets translated into the proteins, the polyeptides that actually do things in the cell. Right. And the fourth maybe counterintuitive one is variation by mutation. There has to be a way for it to change. Room for mistakes essentially kind of. If the genetic material never changed, evolution just couldn’t happen. Any chemical alteration needs to be inheritable. Providing the raw material for natural selection to act on. Okay. Four key jobs. Replication, storage, expression, variation. Got it. So with those in mind, let’s rewind to the early 20th century. Why protein? Why was that the favorite? Well, you said it earlier. Proteins were known to be incredibly diverse, right? Loads of them doing all sorts of complex jobs. Exactly. They seemed chemically sophisticated enough. DNA, on the other hand, Oh, it was first isolated way back in 1869 by Misher as nuclean. I think that’s the one. It just seemed chemically simple, too simple, maybe. And that idea got a huge boost from a really critical mistake, didn’t it? Lavine’s tetronucleotide hypothesis around 1910. Oh, absolutely. That was a major setback looking back. Lavine proposed the DNA was just this incredibly boring repetitive structure. Like the four nucleotide bases, AGCT, just repeated in the same order over and over and over. Yeah. AgCT, something like that. If that were true, DNA couldn’t possibly hold complex information. It lacked, you know, chemical diversity. So why did people buy into that for so long? It seems almost willfully simple now. Well, it fit the available data at the time or seem to. and it conveniently pushed DNA aside, directing all the attention back towards the much more complex seeming proteins. It really slowed things down. So, it took something pretty dramatic to shift that thinking. It did. It took a really clever highstakes experiment. Yeah. And that story starts in 1927 with Frederick Griffith. Okay. Griffith, he was working with bacteria, right? Diplocus pneumonia. That’s right. It’s now called streptocockus pneumonia. But yeah, he had two strains. He was looking at one nasty, one not. Basically, there was the virulent ihaste strain. A is for smoothie. Had a slimy protective capsule. It killed mice. Then there was the a virulent II strain. R for rough. No capsule. Totally harmless. Okay, straightforward enough. But then he did something weird. He did. He took the deadly eye cells and killed them with heat. Then he mixed these dead eye cells with living, harmless IR cells. So dead bad guys plus live good guys should be fine. You’d think so. Neither component alone could kill the mice. But when he injected the mixture, the mice died. And even more surprising, when he looked at the blood of the dead mice, he found living, virilent eye cells, the deadly kind. So the harmless ones had somehow changed, turned into the deadly ones. Exactly. Griffith didn’t know what caused it, but he called it the transforming principle. Something physical must have passed from the dead eye cells to the living Iir cells. permanently transforming them and crucially that change was heritable passed down. So whatever this principle was that was the genetic material but he didn’t know if it was protein, DNA, RNA. Nope. That question hung in the air for well almost 17 years until 1944. Avery Mloud and Mccardi, this is the big one. This is arguably one of the most pivotal experiments in biology. They set out to chemically identify Griffith’s transforming principle. And it was a massive effort, wasn’t it? Like huge amounts of bacteria needed. unbelievable scale for the time. They started with 50 to 75 liters of bacterial culture just to get enough of this transforming substance to work with. Wow. So, how did they isolate it? Their approach was just beautiful in its precision. They carefully purified the active filtrate, the stuff that could transform the R cells. Then they systematically destroyed different types of molecules within that filtrate using specific enzymes. Okay. So, targeted attacks like use an enzyme that chews up protein. Exactly. They used proteasises and guess what? The transforming ability was still there. Protein wasn’t it. Okay, so not protein. What about RNA? They use RNAs which destroys RNA still worked. Transformation continued. RNA wasn’t it either. So the suspense builds. Then they added DNA. The enzyme that specifically digests DNA. The transforming activity completely vanished. Gone. Wow. So that was it. Direct proof. As direct as you could get at the time. DNA was the transforming principle. Therefore, DNA had to be the genetic material, at least in these bacteria. Incredible. But, you know, scientific skepticism dies hard. Was there more confirmation? Oh, yes. To really hammer the point home, we get the Hershey Chase experiment in 1952. Elegant, simple, decisive. This one used viruses, right? Bacteria phages. That’s right. Bacteria phase T2, which infects E.coli. The beauty of phasages is their incredibly simple structures. basically just a protein coat surrounding a DNA core. Roughly 50% protein, 50% DNA. So the question became, which part actually gets inside the bacteria to hijack its machinery and make new viruses? The protein coat or DNA core? Precisely. And they used a brilliant trick to find out radioactive labeling. Ah, the molecular trackers. Exactly. DNA contains phosphorus but almost no sulfur. Protein contains sulfur but no phosphorus. So they grew one batch of phages with radioactive phosphorus to label the DNA. Okay. Glowing DNA. And another batch with radioactive sulfur to label the protein coat. Glowing protein coats. Got it. Then they let each batch of labeled phages infect separate cultures of E.coli. After infection started, they used, believe it or not, a kitchen blender. A blender. Seriously? Yep. To basically knock the empty fage coats off the outside of the bacteria. Then they separated the bacteria from the fage ghost using a centrifuge and looked to see where the radioactivity ended up. And the results were crystal clear. Most of the TTT the DNA label was found inside the bacterial cells. While the protein almost all of the protein label remained outside with the empty phage coats. Conclusion. DNA carries the instructions. DNA is the genetic material in T2 fage. Case closed. Pretty much nailed it shut for viruses and bacteria. And this evidence started piling up for ukarotes too. Although the initial proof there was a bit more indirect. How so? What were the lines of evidence in ukarotes? Well, first there was the distribution. Where do you find DNA ukarotic cells? Primarily in the nucleus, on the chromosomes, also in mitochondria and chloroplasts, right? Where genetic stuff happens. Exactly. Protein, meanwhile, is kind of everywhere in the cell. Okay. So, location strongly suggested DNA. Okay. Location. What else? Second, a really neat quantitative correlation. If you measure the amount of DNA in different cells, deployed sematic cells, your regular body cells consistently have twice the DNA to hoid gametes. The sperm and egg cells a perfect 2 to1 ratio across the board. Humans, chickens, trout, the data lined up perfectly. This strongly suggested DNA was the stuff being hald and passed on during reproduction. Makes sense. And there was a third indirect line. Yeah, mutagenesis. Scientists knew that UV light could cause mutations. They measured which wavelength of UV light was most effective at causing mutations. The peak effectiveness was at 260 nmters. Guess what else absorbs light most strongly at 260 nm? Nucleic acids, DNA, and RNA. Bingo. Protein absorbs best around 280 nm and UV light at that wavelength wasn’t nearly as mutogenic. Another strong pointer towards nucleic acids being the target. Okay, so indirect evidence was strong, but what about direct proof in ukareotes like the Avery or Hershey Chase experiments? That really came with the advent of recombinant DNA technology. Modern genetic engineering basically provided the ultimate proof. How? Well, think about it. We can take a specific gene, say the human gene for insulin, which is a defined sequence of DNA. Okay. Snip it out and insert it into a completely different organism like a bacterium. The bacterium starts making human insulin. Exactly. That DNA sequence directly confers a heritable trait, the ability to make insulin, and dictates the synthesis of a specific product. That’s about as direct as proof gets that DNA carries the functional blueprint. Wow, that really is powerful. Of course, biology loves exceptions. Are there any cases where DNA isn’t the main genetic material? Absolutely. The classic exceptions are certain viruses. RNA viruses like tobacco mosaic virus actually use RNA as their genetic blueprint. Okay. And then you have the really interesting ones retroviruses like HIV. Ah they work backwards right kind of. They have an RNA genome but they carry a special enzyme called reverse transcriptise. It’s an RNA dependent DNA polymerase. So it reads the RNA template and makes a DNA copy. Exactly. That DNA copy then gets integrated into the host cell’s own DNA genome. So RNA stores the info initially, but it gets converted to DNA for replication and expression within the host. Fascinating detour. Okay, so we’ve established DNA is it? Let’s get into the nitty-gritty, its structure. What’s it actually made of? All right, the building blocks are called nucleotides, sometimes called monucleotides. Each one has three parts, okay? Three components. A nitrogen containing base, a fivecarbon sugar, that’s a pentos, and a phosphate group. Let’s break those down. The bases two main families the purines which have a double ring structure that’s adinine A and guanine G A and G double rings got it and the pyramidines which have a single ring cytosine C thymine T and uricil C TU single rings and the DNRNA differences here too right right DNA uses A G C and T RNA uses AGC and swaps in uricil u instead of thymine T okay next part the sugar it’s a pentos Five carbon sugar in RNA it’s ribos in DNA it’s deoxyibbos and the difference is key isn’t it deoxy critical difference deoxxyribbos is missing an oxygen atom specifically a hydroxal group at the number two carbon position the C2 position ribos has it deoxxos doesn’t and that makes DNA more stable much more stable chemically less reactive which is good for long-term information storage right and the third piece is the phosphate group yeah a phosphate group attached to the C5 carbon of the It’s what makes nucleic acids acidic and gives them their negative charge. So base plus sugar equals nucleotide. Add a phosphate nucleotide. You got it. Adine plus deoxupose equals deoxyenosine. Add phosphate acid deoxidelic acid or datp if it has three phosphates like the energy currency. Okay. So how do these building blocks link up to make a chain? They link via what’s called a phospidastaster bond. The phosphate group of one nucleotide attaches to the C3 carbon of the sugar on the next nucleotide linking the fif carbon of one sugar via phosphate to the three carbon of the next. Exactly. This creates a long chain, a polyucleotide with a sugar phosphate backbone and the bases sticking out. Short chains are oligoucleotides and the sequence of bases is where the information is stored. The potential for variation must be huge, astronomical. Remember Lavine’s simple tetronucleotide idea? Totally wrong. With four bases, even a relatively short chain of, say, a thousand nucleotides can be arranged in four to 1,000 different ways. That’s a number bigger than the number of atoms in the universe. Massive storage capacity. Mindboggling. Okay, so this potential was there, but figuring out the 3D structure, the double helix, that was the next giant leap. Watson and Crick 1953, right? But they didn’t do it in a vacuum. They relied heavily on two crucial pieces of prior data. What were they? First Irwin Charargaff’s work on base composition. He analyzed DNA from many different species and found some consistent rules. Ah Chargaff’s rules. Yep. He found that the amount of adinine A was always pretty much equal to the amount of thymine T and the amount of guanine G always equal the amount of cytosine C. So A pairs with T, G pairs with C. Somehow it strongly suggested that it also meant that the total purines A plus G equals the total pyramidines C plus T. This directly contradicted Lavine’s idea of equal amounts of all four bases in a simple repeat. Okay, that’s huge. What was the second piece? The X-ray defraction images produced by Rosyn Franklin and Maurice Wilkins. Ah, especially Franklin’s famous photo. What did that show? It provided physical measurements. The pattern clearly indicated a helical structure. It showed the bases were stacked like coins about $3 four text drums apart. And crucially, it revealed the helix had a consistent diameter of about $20 text drums neck. So, helical specific dimensions. Watson and Crick put these puzzle pieces together. They did. They built physical models trying to fit everything together and they came up with the iconic double helix model. Let’s walk through the key features. Okay. First, it’s two long polyucleotide chains coiled around a central axis. A helix. A right-handed double helix specifically. Like a spiral staircase turning clockwise as you go up. And the two chains run in opposite directions. Critically important, they are antiparallel. One strand runs in the $5 to three bill direction. The other runs three laws into $5. Think of it like opposite lanes on a highway. This antiparallel arrangement is essential for replication and function. Okay. What about the dimensions from Franklin’s data? They fit perfectly. The overall diameter is $20 Tex angstros. One complete turn of the helix is $34 tech sanctions long and contains 10 base pairs later refined to about 10.4 but 10 was the initial model and the bases are on the inside. Yes, the flat base pairs are stacked on top of each other in the core of the helix $3 and vortex angstroms apart perpendicular to the axis. The sugar phosphate backbones form the outside rails of the staircase and this structure creates grooves on the outside. Right? Because of the way the backbones twist, there’s an alternating pattern of a wider major groove and a narrower minor groove. These grooves are important because proteins can access the edges of the base pairs there to read the DNA sequence. Now, the most significant feature how the bases pair up. This explains charg. Absolutely. This is the core of complimentarity. Adinine A on one strand always pairs specifically with thymine T on the opposite strand way with T and guanine G always pairs specifically with cytosine G with C purine with pyramidine always. Yes, a purine double ring always pairs with a pyramidine single ring. This keeps the $20 Tex angstrom diameter constant all the way down the helix. AT pairs and GC pairs have the same width and they’re held together by hydrogen bonds. Correct. A pairs with T using two hydrogen bonds. G pairs with C using three hydrogen bonds. Ah, so GC pairs are slightly stronger than ATRs. That’s right. Those three bonds make the GC connection more stable, requiring more energy to break. And the overall stability of the helix, it’s not just the hydrogen bonds, is it? No. While the hydrogen bonds are key for specificity, the sheer number of them adds up. But another major factor is base stacking. The flat water-hating hydrophobic bases stacked tightly in the core away from the surrounding water. This is very energetically favorable. Well, the sugar phosphate backbone is water loving, hydrophilic, and faces outwards. Makes sense. So, you have hydrogen bonds plus base stacking contributing to a very stable yet readily separable structure. We should probably mention quickly that this standard helix is called BDNA, but there are other forms. Yeah, under certain conditions like dehydration, DNA can adopt an ADNA form which is more compact. And there’s also ZDNA, a weird left-handed helix which might pop up transiently in specific regions, possibly involved in regulation, but its role in vivo is still debated. BDNA is the main form in cells. Okay, that couples DNA structure beautifully. Let’s quickly contrast it with RNA structure. Key differences again, RNA uses the sugar ribos instead of deoxyibbos and it uses uricil u instead of thymine t. A still pairs with u and RNA and it’s usually single stranded generally. Yes. But that single strand can and often does fold back on itself to form complex 3D structures. These folds can include short doublestranded helical regions where complimentary bases pair up A with U, G with C. These structures are crucial for RNA’s functions. Right? Because RNA isn’t just a messenger. It does a lot of jobs. Let’s touch on the main types. Okay. The three major players in protein synthesis. First, ribosomal RNA. RNA. This is the most abundant type. Liifar makes up about 80% of all the RNA in a typical cell. It’s a major structural and catalytic component of ribosomes, the actual workbenches where proteins are built. And they come in different sizes. You mentioned Spedberg units earlier, right? The S’s value relates to how fast they settlement in a centrifuge which depends on size and shape. Procaryotic ribosomes have components like 16S and 23S RNA while ukarotes have things like 18S and 28S RRNA just different sizes. Okay. RNA builds the factory. What carries the instructions? That’s messenger RNA, mRNA. It’s transcribed from a DNA gene and carries the genetic code, the sequence information from the nucleus out to the ribosome. The blueprint copy. Exactly. Its size varies a lot depending on the size of the protein it codes for. It’s the template for translation. And the third type, transfer RNA, tRNA. These are the smallest RNA molecules. Their job is crucial. They act like adapters. Adaptors. How? Each tRNA molecule recognizes a specific three-base code word, a codon on the mRNA, and it carries the corresponding amino acid to the ribosome to be added to the growing polyeptide chain. They physically transfer the amino acids. Got it. RNA, mRNA, tRNA. The core team for building proteins based on the DNA plan. Precisely. And understanding these structures, especially the complimentarity of DNA, open the door to powerful analytical techniques like being able to melt DNA. Exactly. If you heat a DNA solution, the energy eventually overcomes the hydrogen bonds holding the two strands together. The helix unwinds and separates into single strands. This is called dennaturation or melting. And you can monitor this? Yes. Because singlestranded DNA absorbs more UV light at 260 mm than doublest stranded DNA does. As the DNA melts, the UV absorption goes up. This is called the hyperchromic shift. Okay? And the melting temperature, the tendol, the tandelars is the temperature at which half the DNA is denatured. It’s a measure of the helix stability. And remember GC versus AT bonds. GC has three H bonds. AT has two, right? So DNA with a higher percentage of GC pairs has a higher tier of dollars. It requires more heat to melt because there are more triple bonds to break. This tier dollar value can actually tell you about the base composition of the DNA. Cool. And can you reverse it? Can you put the strands back together? You can. If you slowly cool the DNA DNA, complimentary strands will find each other and reform the double helix. This is called reaturation or molecular hybridization. And you can hybridize DNA with RNA too. Absolutely. If you mix denatured DNA with complimentary RNA strands, they’ll form a DNA RNA hybrid helix. This is incredibly useful for finding specific DNA sequences or measuring gene expression like N5ish. Exactly. Fluorescent incitu hybridization FIH uses fluorescently labeled DNA or RNA probes, short specific sequences that hybridize to their complimentary targets directly on chromosomes within cells. So you can literally see where a specific gene is located on a chromosome or count chromosome copies. Precisely. It’s a powerful diagnostic and research tool all based on that fundamental complimentarity. And another workhorse technique is electropharesis. Oh, absolutely. Fundamental. Gel electropharesis is used constantly to separate DNA and RNA fragments. How does that work? Based on size. Primarily size. Yes. Nucleic acids have a strong negative charge because of all the phosphate groups. So if you put them in a gel matrix like aguros or polyacryumide and apply an electric field, they’ll migrate towards the positive electrode. The gel acts like a civ. Smaller fragments can wiggle through the pores more easily and move faster. Larger fragments get tangled up more and move slower. So it separates them into bands based on their length. Exactly. Since the charge to mass ratio is essentially constant for all DNA fragments, size is the primary factor determining migration speed. It lets you analyze fragment sizes, purify specific pieces, and much more. Okay, so let’s wrap this up. We’ve covered a lot of ground from that early confusion about protein versus DNA. Through those absolute critical experiments, Griffith, Avery, Mloud, McCarti, Hershey, Chase that definitively proved DNA was the molecule of heredity. to the structure itself. Targap’s rules, Franklin’s X-rays, and the Watson Creek double helix model with its antiparallel strands and specific base pairing. That structure immediately screamed function, didn’t it? Replication storage. It really did. And that foundational understanding, that molecular architecture is literally the bedrock of all modern genetics, genomics, biotechnology, you name it. Yeah. You can’t understand gene editing or genetic testing or anything else without grasping this core structure. Absolutely. So maybe a final thought for you, our listener, to chew on. We mentioned BDNA, the standard right-handed helix, and the less common ZDNA, which is a left-handed helix. Think about that mirror image quality. If ZDNA does exist and function in our cells, perhaps transiently, what completely different roles might that opposite twist, that different shape with its tilted bases play? Could it act as a temporary switch? Maybe flagging certain genes for activation or silencing in a way BDNA can’t? Something to ponder. It highlights how even subtle changes in that fundamental structure could have really significant biological consequences. Thanks for joining us on this deep dive.
This chapter comprehensively details the foundational experimental evidence that established deoxyribonucleic acid (DNA) as the universal genetic material in almost all living organisms, a molecule required to perform the essential functions of replication, information storage, expression, and permitting variation through mutation. Before this realization, proteins were generally favored as the hereditary substance due to their structural complexity, contrasting with the initially flawed tetranucleotide hypothesis proposed by Levene for DNA. The shift in understanding began with Frederick Griffith’s 1927 discovery of bacterial transformation, demonstrating that a heritable agent, the “transforming principle,” derived from heat-killed virulent Diplococcus pneumoniae (IIIS) could convert live avirulent cells (IIR) into infectious IIIS strains. The chemical identity of this agent was pinpointed in 1944 by Avery, MacLeod, and McCarty, whose rigorous testing proved that only the enzyme DNase destroyed the transformative capacity, confirming DNA as the genetic factor responsible for heredity. Further decisive confirmation came from the Hershey-Chase experiment in 1952, which utilized radioisotopes to selectively label the protein coat (35 S) and the DNA core (32 P) of T2 bacteriophage; they proved that only the 32 P-labeled DNA entered the host bacterial cell to direct viral reproduction. In eukaryotes, DNA’s role was supported by indirect observations, such as its correlation with chromosomal location and the stoichiometric relationship between cell ploidy and DNA content, and later directly confirmed by modern recombinant DNA technology. Although DNA is paramount, RNA acts as the genetic material in certain viruses, including retroviruses, which rely on the enzyme reverse transcriptase to synthesize a DNA intermediate. Chemically, nucleic acids are constructed from nucleotides, composed of a phosphate, a pentose sugar, and a nitrogenous base (purines A/G or pyrimidines C/T/U). DNA is distinguished by using deoxyribose and thymine, while RNA uses ribose and uracil. Drawing upon Chargaff’s rules (A=T, G=C) and critical X-ray data from Rosalind Franklin, James Watson and Francis Crick modeled the definitive structure of DNA as a right-handed double helix. This structure features two antiparallel polynucleotide strands linked by specific complementary base pairing: Adenine forms two hydrogen bonds with Thymine, and Guanine forms three hydrogen bonds with Cytosine. While B-DNA is the biologically relevant form, alternative conformations like the dehydrated A-DNA and the left-handed Z-DNA also exist. RNA typically exists as a single strand and functions in genetic expression through molecules like mRNA, tRNA, and rRNA. Finally, key analytical methods include the hyperchromic shift, where increased UV absorption upon heating indicates DNA denaturation and is used to determine the melting temperature (T m ), a value related to G-C content, alongside techniques like molecular hybridization (e.g., FISH) and electrophoresis for separating molecules by size.
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