Short Wave - The secret history of DNA: Pus, fish sperm, life as we know it

Episode Date: November 11, 2021

It's been 150 years since the first article was published about the molecule key to life as we know it — DNA. With help from researcher Pravrutha Raman, Short Wave producer Berly McCoy explains how DNA is stored in our cells and why the iconic double helix shape isn't what you'd see if you peeked inside your cells right now. Read more about the discovery of DNA: https://bit.ly/3wNe7hnCurious about all the other biology that defines us? Email the show at [e-mail] — we're all ears ... and eyes and toes and ... a lot of things. Thanks, DNA!See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hi, Shortwavers. Maria Godoy here with producer Burley McCoy. Hi, Maria. All right, Burley, what do you have for the pod today? I want to talk about the beautiful, extraordinary substance that is responsible for all of life as we know it. Nucleon. Uh, say what?

You don't know about nuclean? Not so much. It's got beauty. It's got mystique and it's got a really long history. Really? Yeah, the first article about it was published back in 1871, which was 150 years ago this year. It was found by a Swiss scientist named Friedrich Meischer in Germany. And back then, we didn't know much about cells, so he was literally just trying to figure out what was on the insides of cells.

What did he do? How do you find this? The only logical way. With pus, he got from off of used surgical bandages. Oh, my God, Burley, that's pretty gross. The things we do for science. Indeed. And so Friedrich is doing his experiments, and he keeps getting a mysterious clump of goo. That's definitely not proteins.

So he eventually realizes it's a whole new thing, an undiscovered molecule. Ooh, that sounds exciting. And did he find it in the nucleus? Is that why it's called nucleon? Yes, but these days, nucleon's got a new name. And I think if I tell you about his later experiments, you might be able to be able to the guess its new name. See, later, Meishir started experimenting with fish sperm. My God, first bus now fish sperm. Are you kidding me? I know, I know. So that's when he started guessing

Nucleon had something to do with fertilization and inheritance. Oh my God, is it DNA? Did I win? Ding, ding, ding. Yeah. Woo-hoo! Yes, ding, ding, ding! It turns out sperm have a lot of DNA in them, right? Which makes sense, since it's kind of their purpose in life, literally. I see what you did there. Today on the show, we're talking about the myth, the molecule, the legend, DNA. And how that nice, long, double helical shape you might have learned about is not what you would see if you could peek inside yourselves right now. I'm Maria Godoy and I'm Burley McCoy. You're listening to Shortwave, the Daily Science podcast from NPR. Okay, Burley, so we're talking all about DNA, deoxy ribonucleic acid, a freaking mouthful to say.

It definitely is. And as molecules go, it's a pretty big one. Well, it's got a big job. It made you and me, you know. It does make up you and me. DNA is the blueprint that our cells use to make all of the proteins that we need to function, from digestion enzymes to keratin proteins that make up our hair and nails. I talked to Prav Ruth Raman about all this. She's a researcher at the Fred Hutchinson Cancer Research Center, and she says that even though we are really complex, our DNA is important. impressively simple. It's just four chemical letters. It's just different combinations of ATGC. That is essentially what makes up DNA, which is kind of cool. It's just four nucleotides, and that's all it takes

to make up all of the DNA. So if you remember learning about DNA structure, all those nucleotides are basically like rungs on a ladder, where the sides of the ladder are, you know, two long strands of alternating sugar and phosphate molecules. Then those two strands wrap around each other to make the iconic double helix shape. Okay, but Burley, just a little bit ago, you told me that if I could take the magic school bus and go into my cell nucleus right now, I would not see a long strand of double helices of DNA. So what gives? I know, I know, minds are being blown, but there's a good reason for it.

If you took all of the DNA from just one of yourselves and stretched it out, it would be more than six feet tall. Burley, I'm only 5'4 on a good day. Like, how am I getting six feet of DNA inside me? I know. And not even just your body in an individual cell. Whoa. That's a lot of DNA in an itty-bitty living space.

So how are we cramming that much information, something like that long, into a tiny individual cell? Maria, very precisely. Because we still need to be able to use it all the time. Packing our DNA is actually a super organized, complicated process. And it's what Perritha studies, how DNA is packed down into the nucleus of our cells, which is only about 10 micrometers across. That's somewhere on the order of a hundred times smaller than a grain of sand. And all of my six feet of DNA is packed into that? Like, what does that even look like? I asked Perruta the same thing. So the double helix

is actually packaged much more tightly in a cell. So there are proteins that we have. And we have around which this DNA is packaged. And it was very beautifully called beads on a string. And every bead is made up of these histone proteins. And then the string that's wrapped around the beads or between the beads is the DNA double helix. It's been years since I've been in bio-class. So remind me, these beads, these histone proteins, what are they? Yeah. Essentially, their whole job is to help organize the DNA. And really, as proteins go, histones are ancient. Prabritha told me we basically have the same histone proteins as yeast, which we parted ways with on the evolutionary tree more than a billion years ago. Whoa. So if evolution hasn't changed

them in all this time, I'm guessing that must mean that they're really, really important, right? They definitely are. These proteins are absolutely essential. You can't remove them. If you lose histones, all cells are dead. Whoa. So basically these histone proteins serve as kind of a spool for thread, which in this case is the DNA. And because our DNA has a negative charge and these histone proteins hold a positive charge, they attract each other, which helps compact the DNA down.

And those beads can then be really close to each other or further away from each other, which helps with more tight or more loose packaging. And then the beaded strings have even more levels of organization that Provrutha says scientists are still trying to understand. That's really kind of clever and sort of amazing that it can all fit in there. Yeah. Okay, but here's my question. If DNA is a blueprint and our cells need to read that blueprint to make us,

then how do our cells read our DNA if it's all packed away like that? This is where it gets really interesting, Maria. Pravruthu told me that the different kinds of packaging change how accessible the DNA is to our cells. So like really tightly packaged DNA isn't accessible. And that's where our star protein, the histone, comes in. The parts of the DNA that are accessible or not can be changed using these histones. And essentially every time you want to access a piece of DNA that is bound by these beads, you can either sort of evict the beads or remove the beads or move the beads so that the DNA becomes more accessible.

So our cells are busy with a million jobs all of the time. But those jobs change. Like if a cell is about to divide, it'll need to read different parts of the DNA than if it isn't. Depending on what a cell needs, it'll send different signals, which tell the histones how tight or loose to pack the DNA, which then in turn control which sections of the DNA can be read. So these histone proteins that the DNA is wrapped around, they're like really powerful because they actually control which parts of the DNA can be read. they're like really powerful because they actually control which parts of the DNA can be read, right? Yeah. That's really cool.

That's kind of effect like a lot of what's going on in a cell. Totally. So if you think about sort of through development or if you look at different cells, you want to make different proteins in these different cells. Your muscle cells are going to need to read a different part of the blueprints than the cells in your tongue or the cells in your lungs. And that's controlled by histone proteins. So the DNA in our nucleus is actually a mix of super tight and super loosely packed DNA, depending on what the cell needs at the time. Okay, Burley, so I get what DNA looks like in a cell. It's very organized beads on a string, and I get why it's organized that way. But I have a more poetic question for you.

Okay. How doth this tiny string makes something as complex as you and me, Burleigh? Okay, it's pretty astounding, right? Especially since the DNA code is really simple. So to describe how our DNA makes us, Previtha starts from the beginning. So we all start out as that one cell embryo, but then you go on to make skin cells and blood cells and heart cells.

And so to make each of those cells, you have to access different parts of the DNA to make the protein that you want so that the cell can then become sort of what it's finally meant to be. And those cells make up tissues, and those tissues make up organs. it's a pretty amazing process. To start with something that's simple,

but to be able to make something really complex at the end of it, that all functions together as one cohesive unit. That's crazy to me. It's also kind of wondrous, though, if you think about it. The marvels of the human body. Yeah. Thank you, DNA. And thank you, Burley, for bringing us this story.

You're welcome. This episode was produced by Eva Tesfi, edited by Rebecca. Ramirez and fact check by Margaret Serino. The audio engineer was Stu Rushfield. I'm Maria Godoy. Thanks for listening to Shortwave, the Daily Science podcast from NPR. Adios for now.