Thursday, July 26, 2007

Why are there variations in Genome Size?

The genome size varies considerably from one species to another. The enormity of the c. 40 000-fold range in genome size have intrigued scientists for over half a century. Scientists have asked how and why genomes vary so extensively and whether it matters. But recent findings have extended a paleogenomics dimension to these questions. By using the size of fossil dinosaur bone cells as proxies for genome size, they have attempted to trace the evolution of genome size in reptiles over 200 million years. By analyzing currently available sequence data from a range of reptiles and birds, they have aimed to shed light on the genomic makeup of dinosaur genomes.

Size doesn't matter
Back in the later 1940's estimates of genome size were made, but as data increased, it soon became clear that there was a huge disparity between organismal complexity and genome size. In other words, a complex animal does not guarantee a large genome. In fact, the lowly liverwort has 18 times as much DNA as we have, and the slimy, dull salamander known as Amphiuma has 26 times our complement of DNA'.

Where does Genome size diversity come from?
Since then, there has been much progress in understanding the molecular basis behind how genomes vary so extensively in size. It is now widely accepted that genome size diversity arises from differences in the amount of non-coding repetitive DNA (e.g. pseudogenes, retrotransposons, transposons satellite repeats, and so on.).

Where does the size of the Genome come from?
The actual genome size of an organism is determined by the differential activity of mechanisms generating increases such as retrotransposon amplification, polyploidy, segmental duplications or generating decreases like illegitimate and unequal recombination, or differences in double-strand break repair in the DNA amount.

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