Hi. It's Mr. Andersen and welcome to biology Essentials video number 27. This is on DNA and RNA. I want to start with a picture of a peanut plant. Right here we have the same peanut plant in both of these. On the left side it's been decimated by the larvae from a corn stock borer; the one on the right, however, you can see the borer sitting right here, but it's not eating the peanut. And the reason why is this one over here has been engineered. They've actually added a gene from a bacteria called Bacillus thuringiensis. And it produces a toxin that the larvae doesn't like so it takes a couple bites quits eating it.

So there are two things I wanted to show you with this picture. Number one is this idea that no matter what you are a virus, a bacteria, eukaryotes like a plant or an animal, you have the same genetic material, and that's called DNA. The other thing that's interesting is that humans can tamper with this, we can actually transfer DNA from one organism to another, we can transform that organism. And that whole field is called genetic engineering and it's exploding right now. So in this podcast, I'm going to try to accomplish five things.

First of all, we're gonna talk about the history of DNA, how these three experiments the Avery, McLeod McCarty, the Hershey Chase, and finally the Watson Crick, Wilkins and Franklin experiment showed us what DNA looks like where it is and how it works. Next, I'll talk about how DNA is organized, organized in chromosomes, both prokaryotic and eukaryotic. We'll talk about the structure of DNA and RNA, mostly how they're different, and then how DNA makes copies of itself. We'll then discuss the central dogma how DNA is transferred through transcription into RNA, which is then translated into proteins, which then makes you and then finally, we're going to talk about this Brave Frontier of genetic engineering and how we can do things like transform bacteria to make important things, especially for diabetics, like human insulin. And so that's a lot to do. So we better get started.

Let's start way back in history with the Frederick Griffith experiment. This was in 1928, he was a medical doctor. And so what he was looking at was bacteria. And they would do serological testing. So they're trying to figure out what bacteria causes disease. And they were using a mouse as a lab experiment. So right here, they're using Streptococcus pneumoniae, they're taking one type of that it's called Roth, because when you grow it in plates, it has a rough appearance, they would inject that into the mouse and the mouse would be happy. They then inject a different type of that streptococcus, a virulent type, this one is smooth, they'd injected into the mouse, and then it would die. And so he hasn't learned anything at this point. He then took this evil smooth strain of streptococcus. He heat killed it. So he did a dot. And he found when he inject that heated into the into the mouse, the mouse was good to go. So we haven't learned anything yet. What he then found in this, it'd be that discrepant event is that when he took the rough strain, which normally doesn't hurt the mouse at all, he then mixed it with the heat killed, smooth strain, which normally doesn't hurt the mouse at all, the mouse died. And so what did he learn from that? Well, he learned a lot. And the big thing he learned is that there was a transforming factor, something was being transferred from these dead, smooth strain to these live rough strains. It was transforming them into a variant type of bacteria. He didn't know what it was. But we took the next 30 years to figure out that it was DNA and we figured out the structure of that.

So the first step came through the Avery McCarty McCloud experiments. And this is in the 30s and 40s. And what they did is looked at Frederick Griffiths experiment, and they tried to figure out what was this transforming factor, what was being transferred from these heat killed, smooth strain over to these rough strain. And so they broke down the bacteria, they then isolated the major molecules inside that. And so what they had was RNA, they also had proteins. And then the last thing that they found was DNA. And we knew what DNA was, we'd known it for, you know, 50 years before then. And so what they then used was enzymes that broke down each of these, and then they see if you could transform the bacteria again, so they add a ribonucleases and broke down the RNA and it still was able to transform, they added a couple of enzymes, Trypsin and chymotrypsin. To break down proteins, it was still able to transform, and then they added de deoxyribonuclease A's and which breaks down DNA and then they couldn't transform. And so what did Avery McCarty McCloud figure out? DNA was this transforming factor. Now most of their work was largely ignored. And the reason why is most scientists thought DNA was not complex enough to be the stuff of life. It only has four different letters, and we'll talk about that in just a second. And so that couldn't be the stuff of life. And so a lot of their work was actually ignored. But in retrospect, we look back and they show that they were the ones who figured out it was DNA, where it was the definitive answer well, most of the argument came from is it DNA?

Or is it proteins that are actually being transferred? And proteins are very complex. And so most of the people were thinking that it's proteins that was the genetic material, not DNA. And so the Hershey chase experiment, sometimes called the blender experiment, used bacteriophages. And a bacteriophage is simply a virus that infects bacteria. It looks kind of like a lunar lander, it lands on the bacteria injects its hereditary material in, and then it has, it hijacks that bacteria to make more of the bacteriophage. And so at Hershey Chase did it's a really elegant experiment is they use two different atoms, they used in one experiment, sulfur, and in this case, the sulfur is labeled a red, but they use the red dye to dye the bacteria phages. In this experiment, they then infect the bacteria blend it all up, they precipitate it out and see what color came out. Now, why was it important they use sulfur, it's because sulfur is found in proteins, but it's not found in DNA. They then used a different dye to dye phosphorus. Phosphorus is found in DNA, but it's not found in proteins. And so what they were able to show is that the only one that was doing the transforming was this green dye. That means that it was the phosphorus. And that means that it wasn't proteins that were transferring the information that it was DNA. And so the Hershey chase experiment was definitive proof that DNA was the hereditary material. And so this is in the 50s. And now the race is on to figure out not only mostly to figure out what's the structure of DNA, how's it all work? These are interesting people. Apparently Hershey and Chase, they work together, their lab was totally silent, and they just very worked very effectively together. Sadly, Martha Chase goes crazy later in life, but a really cool experiment.

Now we go to the ones that you're probably familiar with the names that you're familiar with. And that's probably Watson and Crick, James Watson, Francis Crick are mostly given credit for discovering the structure of DNA, but there were three other probably even more people that were that played in this discovery of the structure of DNA. One of those is Maurice Wilkins, Maurice Wilkins was really good at X ray crystallography. So that is taking pictures of crystallized material. It's kind of like shining light through a chandelier and then figuring out what the structure of the chandelier is. He was working with Rosalind Franklin, they didn't get along that well. Maurice Wilkins is an interesting guy died just a few years ago. They didn't work well together, but they had the best data out there. This is a picture of some of the this would be the X ray crystallography of DNA. So they were looking at DNA and trying to figure out its structure. If you know anything about crystallography, you'd know that this is a helix. Or it suggests the structure of a helix.

Actually, James Watson sat in on one of Rosalind Franklin's secret meetings and took notes on it, and it actually helped them to figure out the structure a lot. Next, we've got Erwin Chargaff, Erwin Chargaff, was looking at different organisms and studying the amount of A's T's, C's and G's. And so A, G, C and T are the three four different bases that are found in DNA. And he found something unique. If you look at, for example, octopus, the amount of A is 33.2. And the amount of T is exactly the same, about the same. And if you look at the amount of G 17.6, and C, 17.1, that's about the same as well. In other words, the amount of A and the amount of T is always the same, and the amount of G and the amount of C is always the same, we sometimes call this Chargaff's rule. So as you look all the way down here, like in humans, we have 29.3% A and 30% T. Likewise, we have 20% G and C, and so he didn't know what that meant. But Crick and Watson knew that they knew the structure of a of a helix coming from the work of Franklin Wilkins. And so they use models to figure out the structure of DNA. Why do we always have the A and the T equal and the G and the C equal? Well, if you look at the structure of DNA, you have a backbone. This is actually a model of this is the model that Watson and Crick we're working on. So you've got a backbone that looks like this. But then on the inside, you have your bases and if you have an A on this side, a T will be on the other side. And if you have a C on this side of G will be on the other side. And so the amount of A's and the amount of T's are always equal because they bond to each other. And so this is this double helix. So Watson and Crick are given the credit for that. They actually share the Nobel Prize with Maurice Wilkins. Rosalind Franklin doesn't get the Nobel Prize Sadly, she had died before then of cancer. And it was probably as a result of the X rays that she was using it in her lab. And you can't get a Nobel prize if you die.

Okay, so let's now go to the structure, structure of DNA. DNA doesn't just sit loose inside the nucleus, it's organized into something called a chromosome. And so in us, in eukaryotic cells, we have this characteristic shape of a chromosome. If you actually look at how the DNA is organized, the DNA is wrapped around these proteins are called histone proteins. And those are swirled around other proteins and other proteins. And eventually you get to the structure of a chromosome that looks like this. Now, the reason it's characteristically looks like an X is that when we take a picture of our chromosomes, this would be a picture of our chromosomes. They're usually in metaphase. And so they usually have this characteristics, XX structure, what does that mean? That means that the left side is a mirror copy of the right side. And so in a lot of my diagrams, you'll see me drawing a chromosome, just silicon like this with the centromere in the middle. And that's because that's what a chromosome usually looks like. It's a linear stretch. And so in eukaryotic cells, we have this long stretch of DNA wrapped around proteins. And that's where the genetic material is found. And it's really, really, really long compared to the size of the actual cell itself. If we look at prokaryotic chromosomes, it's different. In a prokaryotic chromosome. The chromosome is simply loose here, it's not in a nucleus at all. And it's also a loop. And so in us, we have a linear chromosome. In other words, it's a length with a definite end on either side. But in prokaryotic cells, they've got just a loop. Now the loop is wrapped around itself. So it can fit in what's called the nucleoid region of the bacteria, but it's a loop nonetheless. They also have extra little tiny loops called plasmids, and those have DNA in them as well. And they carry genetic information. And these can actually be swapped between bacteria. So it's like an extra set of genes. Another important difference between us and bacteria is that a lot of our chromosomes is what's called junk DNA. In other words, it's DNA that's not actual genes. It's between genes. And if you look at the DNA of a prokaryotic cell, each of those little stretches is going to be one gene after Gene after Gene after Gene. Now we're starting to figure out that it's not really junk DNA, it actually has an important function. We'll talk about that in a different podcast.


Last modified: Tuesday, October 18, 2022, 10:27 AM