In our last lecture, we learned about the structure of DNA and how one DNA molecule can get copied to make two DNA molecules, and how each of these new DNA molecules can end up moving into a daughter cell so that you can have one cell dividing into two and each having a copy of all of the DNA. I like to think of DNA as a blueprint. So if you think about a house, you can have a blueprint for a house but you can't live in the house.
And what DNA is, is it's simply a blueprint. But you can't make a cell out of just DNA. What DNA has is the information for building a cell.
And cells are made of nucleic acids and carbohydrates and lipids and proteins primarily. And proteins are certainly important in terms of being structures in the cell. There are many... parts of cells that are proteins, but they also are important as enzymes. They build the lipids, they build the carbohydrates, they build the nucleic acids that the cell needs.
So a question that scientists had was how does the information in DNA then get turned into a protein? How, you know, a builder would read the blueprint. How is DNA read by the cell to turn the information into proteins that can both be structures in the cell? as well as enzymes that build these other molecules that the cell needs. And we refer to that process as transcription and translation, and that's what we're going to talk about today.
This GIF from the amoeba sister shows a ribosome translating messenger RNA into protein. So this ribosome is reading the messenger RNA, is reading the information that's in the messenger RNA and using it to determine what order to put the amino acids in as it builds a protein. protein. And of course, this is just an animation. You know, ribosomes normally don't have mustaches.
Typically, they're clean shaven. One of the terms you're going to hear us mention several times in the next few lectures is the term genotype. And genotype refers to all of the genes that an organism has.
So if it's a bacteria, the genotype would refer to all of the different pieces of DNA, and of course it's all one big piece, but we think about it in segments called genes, which code for proteins, and then there are regions in between the genes that are non-coding regions. So all of that information, that entire sequence of DNA, is the genotype, and of course the molecule itself we refer to as the bacterial chromosome. In your case, your genotype are the genes you inherit from mom and dad.
So for example, if you have a gene for type O blood for mom and a gene for type A blood for dad. Your genotype would be A O. You'd have both genes, even if your blood type was actually A. And your blood type would be a phenotype, and we'll talk more about the difference in another lecture. So this is showing an E.
coli bacterium. It has died, and so the genome is spread out onto a microscope slide. This is a transmission electron microscopy image, and then this is a live E.
coli bacterium where the nucleic acid is tucked inside the cell fully enclosed in the nucleoid. This is a little bit of a blurry picture. What I tried to do is find an image that gave you a sense of how we map genes. So this is a bacterial genome and what it's showing you is this is a map essentially of all of the genes and it's kind of pointing out in different locations what some of these genes do.
So here's a gene you'll recognize. This is stuproxide dismutase. So you can guess, I believe this is an enterobacter that infects plants.
But you can guess that this is probably an organism that can tolerate oxygen because it makes that particular enzyme. Here's a gene that's involved in building the flagella. And another one here.
Here's a gene that's involved in chemotaxis. And here's a gene that's involved in adhesion to surfaces. So here's a gene for catalase.
So now we're thinking this is probably an... an aerobic organism that can use aerobic respiration because it has both superoxide dismutase and catalase. So we can actually sequence the entire gene of bacteria and then we can identify the locations of various genes and what they do. And of course, many bacteria, we have their entire genomes sequenced and we've identified all of the genes that are there. And that's actually one of the ways we tell the difference between, for example, your garden variety E.
coli in your gut. helps you digest food versus E. coli 0157H7 that actually causes hemolytic uremic syndrome and does kidney damage.
And the difference is that hemolytic uremic associated E. coli, it's going to have genes that cause damage. They encode a toxin that's going to cause damage to the kidneys. So we can see these differences between the bacteria at the gene level. All right, so what's the question that we're tackling today?
How does the DNA direct the synthesis of proteins in the bacterial cell? So let's talk about that. We've been calling this process the central dogma of biology since I was a high school student, so for quite a while.
And the way I've always thought of it is DNA makes RNA makes protein. So maybe that will help you remember the process. We know that DNA can actually copy itself. Of course, it uses enzymes, right?
So when I say it copies itself, we really are referring to the idea that enzymes are used to make a copy. But the DNA itself provides the information to DNA polymerase, right? It's going to provide the information to make two strands, two new strands of DNA from one parent strand. Another thing that DNA can do is it can, the information that's in DNA can be turned into a molecule of RNA.
And this is done using an enzyme called RNA polymerase. And that process is called transcription. So when the information in DNA is used to make an RNA molecule, that process is called transcription, so DNA going to RNA.
And then from an RNA molecule, a specific type of RNA molecule called messenger RNA, cells are able to use the information in the messenger RNA to tell the cell how to build a protein. And this process is called translation. So when we go from RNA to protein, that's referred to as translation.
So this is the central dogma, DNA makes RNA makes protein. This is how we take our blueprint in DNA and turn it into the actual molecules that make up a cell or build other molecules like lipids and carbohydrates that are part of a cell. This is a quick picture of transcription and then I'm going to show you an animation that I hope will make it a lot easier to understand, but let's just do a quick overview before you move on.
Here's our molecule of DNA and I want to show you how it's wound as a double helix on this end. and wound into a double helix on this end. When a gene gets transcribed, when it gets turned into an RNA molecule, what's going to happen is this double-stranded DNA will come apart just at the site where the gene is. So we're not going to take the whole molecule apart. We're only going to open up the part of DNA that has the gene.
And what's going to happen is one of these strands is going to be used to synthesize a messenger RNA molecule. And so the strand that's used to make the RNA is called the template strand. And the strand that gets produced is going to be the same as the coding strand, which is the complementary strand. So if you use a sequence of T-A-G, excuse me, T-A-C-T, to make a complementary strand of RNA, you'll get A-U-G, A-U-C-U, from this sequence of DNA. And if you look at this, it's going to match this one.
exactly with the only difference being the thymines in DNA will become uracils in RNA. So that's transcription. And we're going to talk more about template and coding strands in a few minutes. Now, what happens after this? Well, this is the rest of the process.
This transcript, which is the RNA molecule that was made, is going to fall off the DNA. It's going to, in the case of a eukaryotic cell, it'll exit the nucleus because this process would happen in the nucleus. Now in prokaryotes, there is no nucleus, so it all happens in the cytoplasm.
This molecule of RNA will then go to a ribosome, and the ribosome will read this messenger RNA, which tells the ribosome, in this case, to begin building a protein, and the first three amino acids are methionine, isoleucine, and serine, and that's what these three codons, these first three codons, are saying. In fact, UAA, I believe, is a stock. codon so that's why there's only three in this particular case so let's watch an overview of both transcription and translation so this is a beautiful animation that shows the whole process and then we're going to go back a step and talk first about transcription and I'll show you an animation just on transcription and then we'll talk about translating an RNA to becoming a protein