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.
But we shouldn’t be fooled into thinking that individuals that look quite different to the observer must have vastly different genomes. Imagine walking along a busy shopping street on a Saturday afternoon. If you’re anything like me, you’ll find the simple observation of passers-by to be an endlessly fascinating pastime – an ever-changing mix of physical appearance and behaviour. Yet, if we scratch the surface, this seemingly endless variety belies a resoundingly similar genome. I mentioned in my previous post how the human genome is made up of around 3 billion (3,000,000,000) building blocks. However, between individual humans, our DNA differs by just 0.2%: around 2,994,000,000 of our building blocks (or 99.8%) are the same, and just 6,000,000 are different. And that handful of differences – that 0.2% – is enough to generate the huge variety we see in the population around us. It’s pretty amazing (to my mind, at least).
The completion of the draft of the human genome (a result of the human genome sequencing project) was announced jointly by the then President of the US Bill Clinton and Prime Minister of the UK Tony Blair ten years ago, and the intervening years have witnessed the publication of a range of other genome sequences – from mouse, to dog, to chicken. As more genome sequences become available so we begin to learn more and more about just how similar (or different) the variety of life on Earth is at the level of our genes – all of which provide further clues to our evolution, and the evolution of the other species with which we share this planet. But perhaps that’s a topic for another post.