Wednesday, 23 July 2008

The Human Genome: An overview (Part 1)

The human genome is the sum total of the DNA in our 46 chromosomes (22 pairs plus two sex chromosomes). Approach it as a layman, however, and you may be in for some surprises.

There are about 3.2 billion base pairs in the human genome. In the previous post, we saw what a nucleotide looks like, with a nitrogenous base at one end. Our DNA is double-stranded, which means that it is structured with two strands facing one another, with the bases on the inside, each pointing towards the other. The bases pair up, though - an A must always face a T (and vice versa), and a C must always face a G (and vice versa). Thus we have "base pairs". Since they are complementary, each base carries as much information as a pair of bases (i.e. even if you only knew one half of the base pair, you could always work out what the other was) but most facts about nucleotides still quantify them in "base pairs" anyway and we'll follow this convention.

And what do these base pairs code for? Now comes the shock.

Almost everybody has heard of genes, and of course genes code for mRNA (and tRNA and rRNA). Of the mRNA that is destined to become a protein, only a fraction of it counts as useful - areas of the molecule called introns are excised before it leaves the nucleus, leaving only the remaining exons to be translated into a protein. Proteins are ubiquitous, and include the keratin of our skin, the haemoglobin in our red blood cells, and the collagen in our tendons and ligaments. Even more importantly, they also act as enzymes, controlling the reaction rate of almost every one of the chemical reactions that take place in our bodies. Thus, they fundamentally control our body's development and maintenance. In short, our exons are the basis of what makes us... us. (This doesn't exclude a substantial environmental say, naturally, nor large epigenetic effects.)

So, you'd expect our exons to comprise most of our genomes, right? After all, they are the single most important part of it. But, alas for our suppositions, it turns out that exons comprise only...


That's it. Really.

If you are fresh to this news, your mind will no doubt hastily throw up a follow-up question: what the hell is the rest of our genome for then? Why on earth would 98.5% of our genome appear to be useless?

We'll answer that next.

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