The Standing Invitation

Alu and You

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Selfish gene theory is a gene-centred view of natural selection. Your genome is made up of thousands of individual genes, each one of which has only one goal: replication. And since the resources available for replication are finite, the genes compete.

One way in which a gene might ensure it gets reproduced is to gang up with some other genes to make a body. This body will act as a temporary, disposable vehicle, to be thrown away once it has had children, but worth designing carefully. The genes try to make the body survive at least to childrearing age, by equipping it with sharp teeth or keen eyes. Genes have to learn to work together: a gene for light bones might do well to pair up with a gene for wings; genes for gills and flippers go hand in hand. All of the wonderful good design of life comes from genes cooperating to compete ­– building bodies that enhance their chances of replication.

But even within a body, the competition between genes is still going on.

Imagine your genome as a sequence of letters, T, C, A and G – 2.9 billion letters arranged in a line, maybe on a tape of paper. Every generation, this piece of paper gets transcribed and copied. Sometimes, mistakes are made.

About 65 million years ago, early in the evolution of primates, an error in gene replication created a monster called Alu.

Alu is a transposon: a short piece of DNA ­– about 300 letters long ­– that is able to reproduce itself within the genome. It appeared when a gene necessary for protein synthesis was mistranscribed. It only had to appear once. Since then, Alu has been quietly copying itself in the genomes of primates – humans included.

Its success has varied over history. Right now it is believed to create one extra copy of itself in the genome every 200 generations or so; in times it has been more successful. In those periods, almost every child had one more Alu unit than its parent.

Like other genes, it is copied from one generation to the next; the effects are cumulative. Over the immensely long course of its existence, Alu has done well. Imagine huge segments of your DNA existing as the same sequence of letters repeating over and over again – not just hundreds of times, but millions of times.

65 million years after it first appeared as a mutation in a single primate, Alu now occupies 10% of your genome. 10% of your DNA is made up of these 300 letters, meaningless junk repeated over and over again, simply because in the great competition of evolution, Alu found a way to cheat.

But even Alu can mutate, adapt, evolve, and now there are subtle variations of Alu, superfamilies that compete with each other, trying to out-copy one another…

And the game goes on.

 

REFERENCES

Richard Dawkins: The Selfish Gene and The Ancestor’s Tale

The numbers vary from one paper to another; I took the above from “Alu Repeats and Human Genetic Diversity”, Batzer and Deininger; see also Molecular Biology of the Cell, second edition.

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Written by The S I

August 21, 2011 at 11:59 pm

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