Over the past year, the SciWhys column has explored a number of different topics, from our immune system to plants, from viruses to DNA. But why is an understanding of topics such as these so important? In short, using science to understand our world can help to improve our lives. In my last post and in this one, I want to illustrate this point with an example of how progress in science is providing hope for the future for one family, and many others like them.
By Jonathan Crowe
In my last post, I introduced you to Carys, a young girl living with the effects of Rett syndrome. Thanks to scientific research, we now understand quite a lot about why Rett syndrome occurs – what is happening among the molecules within our cells to mean that some cells don’t behave as they should. Simply knowing about something is one thing, though; making constructive use of this knowledge is another thing entirely. During this post I hope to show you how our understanding of what causes Rett syndrome is being translated into the potential for its treatment – a cure for Carys, and the other young girls like her.
In my previous post I mentioned how Rett syndrome is caused by a faulty gene called MECP2 that affects the proper function of brain cells. However, the syndrome doesn’t actually kill the cells (unlike neurodegenerative diseases that do cause cells to die). Instead, the cells affected by Rett syndrome just function improperly. This leads us to an intriguing question: if the faulty gene that causes the syndrome could be ‘fixed’ somehow, would the cells start to behave properly? In other words, could the debilitating symptoms associated with Rett syndrome be relieved?
Obviously, researchers can’t simply play around with humans and their genes to answer questions such as these. Instead, researchers have studied Rett syndrome by using “mouse models.” But what does this mean? In short, mice and humans have biological similarities that allow the mouse to act as a proxy – a model – for a human. How can this be? Well, even though the huge variety of creatures that populate the earth look very different to a casual observer, they’re not all that different when considered at the level of their genomes. In fact, around 85% of the human and mouse genomes are the same.
Now, if the biological information – the information stored in these genomes – is similar, the outcome of using this information will also be similar. If we start out with two similar recipes, the foods we prepare from them will also be very similar. Likewise, if two creatures have similar genes, their bodies will work in broadly similar ways, using similar proteins and other molecules. (It is the bits of the mouse and human genomes that aren’t the same that make mice and humans different.)
In essence, the mouse Mecp2 gene is to all intents and purposes the same as the human MECP2 gene, and has the same function in both mice and humans. Equally, if this gene malfunctions, the consequences are the same in both mouse and human: a mouse with a mutation in its Mecp2 gene exhibits symptoms that are very like a human with a mutation in the same gene – that is, someone with Rett syndrome. In short, mice with a Mecp2 gene mutation are a model for humans with the same mutation.
With all this in mind, if we can learn how to overcome the effects of the Mecp2 mutation in the mouse, we might gain valuable insights into how we can overcome the equivalent effects in humans.
And this is wh
Over the past year, the SciWhys column has explored a number of different topics, from our immune system to plants, from viruses to DNA. But why is an understanding of topics such as these so important? In short, using science to understand our world can help to improve our lives. In this post and the next, I want to illustrate this point with an example of how progress in science is providing hope for the future for one family, and many others like them.
By Jonathan Crowe
Carys is an angelic-looking two-year old, with a truly winning smile. At first sight, then, she seems no different from any other child her age. Yet Carys’ smile belies a heart-rending reality: Carys has Rett syndrome, a disorder of the nervous system that is as widespread in the population as cystic fibrosis, yet is recognised to only a fraction of the same extent. (I, for one, had never heard of it until just a few months ago.)
Rett syndrome is a delayed onset disorder — something whose effects only become apparent with time. When Carys was born, she appeared perfectly healthy, and developed in much the same way as any other healthy infant. Just as she began to master her first few words, however, she lost the power of speech, and soon lost the use of her hands too. The effects of Rett syndrome were beginning to be felt.
Over time, Rett syndrome robs young girls of their motor control: they lose the ability to walk, to hold or carry objects, and to speak. But there be other complications too: there may be digestive problems; difficulties eating, chewing, and swallowing; and seizures and tremors. It is a truly debilitating disorder.
So what causes Rett syndrome? What’s happened inside the body of young girls like Carys? We know that the syndrome is caused by as little as a single error (a mutation) in a single gene. (As I mention in a previous post, it’s quite unsettling to realise that just one error in the tens of millions of letters that spell out the sequence of our genomes is sufficient to cause certain diseases. Sometimes there’s very little room for error.) The normal, healthy gene (called MECP2) contains the instructions for the cell to manufacture a particular protein; the mutated gene produces a broken form of this protein, which no longer functions as it should.
But how can a single protein affect so many processes – from speech to the movement of limbs? The answer lies in the way the protein interacts with other genes, particularly in brain cells. Essentially, the protein acts like a cellular librarian by helping the cells in the brain to make use of the information stored in their genomes (their libraries of genes). If the protein is broken, the cells can no longer make use of all of the genetic information needed for them to work properly (a bit like trying to use an instruction manual with some of the pages blacked out), so normal processes begin to break down. The broken protein doesn’t just affect the ability of the brain cells to use one or two other genes, but a whole range of them – and that’s why the effects of Rett syndrome are so wide-ranging.
But the story of Rett syndrome runs deeper than this. The mutation that causes Rett syndrome occurs in sperm; it happens after the sp
By: Kirsty,
on 4/25/2011
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This is the third post in our latest regular OUPblog column: SciWhys. Every month OUP editor and author Jonathan Crowe will be answering your science questions. Got a burning question about science that you’d like answered? Just email it to us, and Jonathan will answer what he can. Today: How is the information in a gene used by a cell?
By Jonathan Crowe
In my last two posts I’ve introduced the notion that DNA acts as a store of biological information; this information is stored in a series of chromosomes, each of which are divided into a number of genes. Each gene in turn contains one ‘snippet’ of biological information. But how are these genes actually used? How is the information stored in these genes actually extracted to do something useful (if ‘useful’ isn’t too flippant a term for something that the very continuation of life depends upon).
Many (but not all) genes act as recipes for a family of biological molecules called proteins: they literally tell the cell what the ingredients for a particular protein are, and how they should be combined to create the protein itself. (Proteins have a range of essential roles in the human body. Some act as building materials for different components of the body, such as the keratin we find in our hair and nails. Others act as molecular transporters: haemoglobin, which is found in our red blood cells, carries oxygen from our lungs to other parts of the body. A family of proteins called the enzymes are arguably the most important, however. Enzymes cajole different chemicals in our body into reacting with one another. Without enzymes, our bodies would be unable to generate energy from the food we eat (and you’d not be reading this blog post).)
So, somehow, the information stored in a DNA molecule is deciphered by the cell and used as the recipe for a protein. But how?
To answer this question, let’s take a journey inside the cell. We can imagine a cell to be like a factory, but one that has been divided into a series of physically separated compartments. Unlike a factory filled with air, a cell is filled with a jelly-like fluid called the cytoplasm, which surrounds the various compartments enclosed within it. In an earlier post I likened a genome to a biological library. And, inside the cell, this library is stored within a particular compartment called the nucleus.
I mentioned earlier that genes often act as recipes for proteins. But here comes a bit of a quandary: chromosomes – and the genes they contain – are locked away inside the cell’s nucleus. By contrast, proteins are manufactured by the cell in the cytoplasm, outside of the nucleus. So, for the genetic information to be used, it has to get out of nucleus and into the cytoplasm. How does this happen? Well, if we’re in a library with a book that contains information we really need, but we’re unable to take the book out of the library, we might make a photocopy of the page that holds the information we’re after. To get the information it needs out of the nucleus and into the cytoplasm the cell does something remarkably similar. The chromosome containing the gene of interest has to stay inside the nucleus, so the cell makes a copy of the gene – and that copy is then transported to where it is to be used: out of the nucleus and into the cytoplasm.
The copy of the gene generated during this cellular photocopying is made not of DNA but of a close cousin called RNA. RNA is made of three of the same building blocks as DNA – A, C and G. Instead of the T found in DNA, however, RNA uses a different block represented by the letter U (for ‘uracil’). Despite this
By: Kirsty,
on 3/28/2011
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This is the second post in our latest regular OUPblog column: SciWhys. Every month OUP editor and author Jonathan Crowe will be answering your science questions. Got a burning question about science that you’d like answered? Just email it to us, and Jonathan will answer what he can. Today: What are genes and genomes?
By Jonathan Crowe
I described in my last blog post how DNA acts as a store of biological information – information that serves as a set of instructions that direct our growth and function. Indeed, we could consider DNA to be the biological equivalent of a library – another repository of information with which we’re all probably much more familiar. The information we find in a library isn’t present in one huge tome, however. Rather, it is divided into discrete packages of information – namely books. And so it is with DNA: the biological information it stores isn’t captured in a single, huge molecule, but is divided into separate entities called chromosomes – the biological equivalent of individual books in a library.
I commented previously that DNA is composed of a long chain of four building blocks, A, C, G, and T. Rather than existing as an extended chain (like a stretched out length of rope), the DNA in a chromosome is tightly packaged. In fact, if stretched out (like our piece of rope), the DNA in a single chromosome would be around 2-8 cm long. Yet a typical chromosome is just 0.00002–0.002 cm long: that’s between 1000 and 100,000 times shorter than the unpackaged DNA would be. This packaging is quite the feat of space-saving efficiency.
Let’s return to our imaginary library of books. The information in a book isn’t presented as one long uninterrupted sequence of words. Rather, the information is divided into chapters. When we want to find out something from a book – to extract some specific information from it – we don’t read the whole thing cover-to-cover. Instead, we may just read a single chapter. In a fortuitous extension of our analogy, the same is true of information retrieval from chromosomes. The information captured in a single chromosome is stored in discrete ‘chunks’ (just as a book is divided into chapters), and these chunks can be read separately from one another. These ‘chunks’ – these discrete units of information – are what we call ‘genes’. In essence, one gene contains one snippet of biological information.
I’ve just likened chromosomes to books in a library. But is there a biological equivalent of the library itself? Well, yes, there is. Virtually every cell in the human body (with specific exceptions) contains 46 chromosomes – 23 from each of its parents. All of the genes found in this ‘library’ of chromosomes are collectively termed the ‘genome’. Put another way, a genome is a collection of all the genes found in a particular organism.
Different organisms have different-sized genomes. For example, the human genome comprises around 20,000-25,000 genes; the mouse genome, with 40 chromosomes, comprises a similar number of individual genes. However, the bacterium H. influenzae has just a single chromosome, containing around 1700 genes.
It is not just the number of genes (and chromosomes) in the genome that varies between organisms: the long stretches of DNA making up the genomes of different organisms have different sequences (and so store different information). These differences make sense, particularly if we imagine the genome of an organism to represent the ‘recipe’ for that organism: a human is quite a different organism from a mouse, so we would expect the instructions that direct the growth and function of the two organisms to differ.
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