Information in Cells - Chapter 1

- Hello, I'm Lindsay Turnbull and I teach biology at the University of Oxford. In this video, I want to tell you how cells process information. It's a crucial concept from chapter one of my new book, "Biology: The Whole Story".

(birds chirping) (frogs croaking)

All Life on Earth is made from cells. Indeed, most cells live alone and they're too small for us to really notice. But what we can't help noticing is all the larger beings that roam the Earth, and they too are made from cells. You and I, for example, are each made from around 37 trillion cells, which is a pretty mind-boggling number. And if we are going to function properly, then every single one of those cells needs to know exactly what it's supposed to be doing or we'll kind of fall apart.

So how do cells know what it is they're supposed to be doing? Well, they have to make all the right tools and machinery and spare parts that they need. And in order to do that, they have to have the right information. So if we're going to understand

this process of information flow in cells, then we're going to have to look inside them. And there are two real problems with doing that. The first is that cells are really tiny, but the second is that cells are fiendishly complicated. But the good news is that I can show you a diagram that's really very simple because it's just going to contain these core components that all cells have. Let's start by just putting on the outer membrane.

That's important for all cells, because it means that they can keep all their components together and they won't just drift off. And now let's put these crucial things inside. So the very first thing that we're going to add is a genome, and that is a massive book of instructions. So all the information that the cell could possibly need is contained within this genome. The second thing are messages that are going to fly out of the genome that just carry individual instructions.

The third thing are the cell's key workers, and they're called ribosomes. And their job is to trap these messages and read them, and they use the information contained in the message to build all of the tools, machinery, and the spare parts that the cell needs. So this, this set of things here is really central to the functioning of any cell on Earth. It doesn't matter whether that cell lives alone or whether it's part of a larger body.

Let's take a closer look now at each of those four crucial components inside the cell. So the first thing we looked at was the genome. Now, the genome is made from a molecule called DNA. And like many molecules in cells, a long strand of DNA is formed by sticking together smaller molecules, in this case the DNA letters. Now, the DNA letters come in four kinds, the letters A, C, G, and T, and they can be stuck together in any order. And then we get this very long strand.

So just how many letters do we need to build an organism? Well, if you want to build a bacterium, you're going to need around a million letters. This worm, probably gonna need a hundred times more. And if you wanna make a shark or a pigeon or a horse, then you're probably gonna need a billion letters and more. So we need that molecule to be enormously long, and that means it's got to be very stable. And that's why you get this classic double helix shape.

So only one of the strands carries the information and the second strand is really there to support the first strand and make sure that the molecule is stable. The second thing we looked at was those little messages that were flying out of the genome just carrying a single instruction. And they're made from a different molecule, although it's very similar, it's called RNA, and it's also letters but it's got a slightly different alphabet.

So it uses the letters A, C, G, and U. The third thing we looked at was those ribosomes. Now they're very large molecular machines. Now we've drawn them as sort of fat guys in braces, but of course that isn't really what they look like, and they're actually made from a type of RNA and protein. The fourth thing we looked at was actually all that machinery and all the tools that the cell was making. And they're not made from DNA or RNA, they're made from protein.

Now, proteins are also made by sticking together lots of smaller molecules into a long chain. But those smaller molecules in this case are called amino acids. And once they've been stuck together into a chain, then the chain starts to fold and twist and bend itself into an exquisite shape. And the shape of the molecule then allows it to perform whatever function or job that it needs to around the cell.

Okay, so proteins we said are the molecule of choice for making all of the machinery and the tools and spare parts that the cell needs. So how is it possible for just one molecule to make such an extraordinary array of shapes? You know, cells make some incredible things, not just crude tools. They make locks with perfectly fitting keys.

They can make channels that they can put into the membrane which only certain molecules will be allowed to enter, and they can even make an amazing molecular turbine to generate all the energy that they need. So how can this one molecule protein make all of those shapes? Well, as we said, once the chain of amino acids is put together by the ribosome, it can twist and turn into an amazing shape.

And because there are 20 different amino acids to choose from, that's why each chain can really be quite different. And those different amino acids have very different properties. So some of them attract each other and some of them repel each other. And that's why the chain can fold up and bend and twist in the way that it does. Now, what's absolutely crucial, of course, is that the amino acids are joined together in exactly the right sequence.

If that doesn't happen, then the thing isn't going to function properly. So that's the job of the ribosome, is to make sure that it reads the message carefully and it puts the amino acids together in exactly the right sequence. And the way it does that is rather interesting. So when the message arrives, it's going to read the letters three letters at a time, and each of the three letters corresponds to a single amino acid.

So by reading the three letters, it can decide which amino acids should I add next. For example, the three letters GGA tell the ribosome that the next amino acid to add to this chain is glycine. And what's truly incredible is that it wouldn't matter what cell you went into anywhere on Earth, if you gave it the letters GGA, the ribosomes in that cell would know to add the amino acid called glycine.

And that's one of the reasons why we're very, very confident that all life on Earth is related and descended from a single common ancestor.

So hopefully we can see now why this information is so crucial to the functioning of cells. They have to get the right information from the genome via the message so that the ribosomes can actually make the right protein, the one that's gonna work properly. So let's imagine that ribosomes can make some kind of hammer. In reality, cells make much more sophisticated things than hammers, but it's something we can think about. So the message comes out and they build the hammer.

But in some cells, it might be that there's some slight change to that message. Perhaps one of the letters in the genome has been changed. And that does happen, and it's a process called mutation. So sometimes the letters can change from one into the other, an A might become a C, for example. Or sometimes a few letters get added or they're missing. And that means that the message that the ribosomes get is slightly different.

And they'll still go ahead and translate it, but this time they might put the wrong amino acid in. And so the hammer might look a bit different. It might look like this now. And that hammer might not work quite as well as the normal hammer that they should be making. And actually most of us have got a few mutations like that. So there'll be a few proteins in your body and in my body that are a little bit suboptimal.

But for some people, those changes are much more serious, and that can lead to something called a genetic disorder. And examples of those are things like cystic fibrosis, which affects a protein that's very important in the functioning of lungs, or sickle cell disease, which affects the protein called haemoglobin that transports oxygen around the body. So genetic disorders can be mild and they can be very serious, and they're all down to changes in those DNA letters in the genome.

And that's a little bit of the problem with this information transfer process. It only flows one way. The ribosomes can't try out different proteins. They can't send feedback to the genome and say, well, this hammer's not quite right, thanks very much. Information flows one way through cells from the genome to the RNA messages, to the ribosomes, and that's it.

So we might think, well, that seems a bit terrible. Why do we have genetic disorders? Why do we have mutations? But of course, mutations are absolutely crucial for evolution. Organisms look different because their genomes are different. So if an organism is going to change through time and evolve, then its genome has to change. And the only way its genome can change is through this process of mutation.

Now, most mutations, as we've seen, are going to be harmful because you are a very well-oiled machine, and some random change is unlikely to make you function any better. But that's not to say that it can't happen. Sometimes, for example, there might be a change in the genome in the instruction for building a hammer that makes a hammer like this which is much better than the original hammer. And if that happens, then that individual is going to be more successful perhaps than those around it.

So what we'll see in the next episode is exactly how evolution works.

So I hope you enjoyed that video, and if you did, then please do share it with friends and colleagues who you think might also enjoy it. There's also the book itself, of course, and there's a link below if you'd like to buy it and have your own copy. There's a lot more depth and detail in the book on this topic and on others.

So for example, in chapter one, there's a whole load of stuff about the origins of life and how understanding information flow in cells can help us understand the origins of life better. We're also planning to make more in this series. So look out for the next videos. We're hoping to make one for each chapter. And the next one coming up is all about evolution. (birds chirping)