April 26, 2012
Take a pair of twins. Look at them – matching brown hair, brown eyes, freckles across the nose. Matching birthmarks on the elbow, except one wears it on the left and the other carries it as a mirror image. The eldest (by six minutes) is slightly tanned, but she’s just been on holiday. They’re identical in almost every way, except one: their personality.
Take the same pair of twins. Look at them again – the brown hair is the same shade, but one sister wears hers long, whilst the other one keeps hers religiously styled in a harsh bob. The freckles? One covers them up every day with make-up. The eldest (by six minutes) has just come back from volunteering in Borneo, which explains the tan. Her sister would rather spend the weekend in a boutique hotel in Edinburgh.
Actually, they’re not identical at all. Some of the differences, like the haircut, are purely environmental. The more interesting and challenging question is why one twin chose the harsh bob whilst the other stuck to the more natural look. One explanation may lie in transposable elements, tiny pieces of DNA which can change their relative position within the human genome.
If we consider the genome to be like an extremely long Word document, the transposable element can either be a ‘copy and paste’ or a ‘cut and paste’. As with the Word document, this function can either create a masterpiece or reduce what has been written to unintelligible nonsense.
Previously, transposable elements had almost exclusively been considered junk DNA, DNA without purpose. The transposable element had one aim, which was to cause havoc within the cell, to reduce our document to nonsense by inserting this junk disruptively in the middle of a sentence. This idea was supported by the fact that porphyria (think the Madness of King George), immunodeficiency and predisposition to cancer have all been linked to transposable elements.
More recent research, however, suggests that there may be a more positive developmental role for the transposable element. These roving pieces of ‘junk’ could insert themselves into the cells of the body, including the brain. If this happens outside the germ line, then the changes in DNA architecture will not be transmitted into the next generation. If the changes occur after the original fertilized cell has split, then two individuals who started out exactly the same may end up being different.
Developmentally, these two twins started with identical stem cells. These stem cells contain all the genetic information needed to create a full human being, and at this stage, the information tells the same story. In the womb, however, some of these stem cells started to develop into neuronal progenitor cells; cells which have started the path to becoming important parts of the brain. During this development, the DNA found within the cell starts behaving differently, with genes that lead to neuronal proteins being switched on, and those that are more appropriate for a muscle being turned off.
During this genetic rearrangement, it is possible for transposable elements to move around. This has the ability to change the genetics of the individual, even just slightly. Muotri and colleagues at the University of Sao Paolo, term this development a ‘genetic mosaic signature’, implying that each stamp is different in each individual. The team also recognise that this information has a huge potential to change the way that we view the genome, as previously science had viewed genetic information as static.
Although research into the transposable element may have redefined certain areas of genetics, it remains to be seen how much this type of DNA may actually affect us in daily life. The team at the University of Sao Paolo suggest that these elements could affect anything from cognition to health, and may present us with a way of determining why some identical twins can be so different. This could be particularly informative in twins who have relatively similar environments, as non-genetic environment is also important in the development of personal differences.
Take another look at our twins. They seem so similar on the surface, but they really are very different. Although no-one knows quite why, transposable elements could be key in determining their cognitive differences. They might even explain our own.
I was watching a film with the honourable Mr. Justin Timberlake in yesterday, and I realised something. I really do not find him attractive. Sure, I never have, but before I just chalked it up to him being handsome in a generic way; just “not my type”. This time I really didn’t find him attractive, and I actually kind of thought he looked kinda foetal. So why do I feel this way?