Saturday, June 25, 2016

Evolution, transposons, retrotransposons.

We all like to think mutation as a random occurrence, an occurrence only due to chance which is mostly harmful and may be evolutionary significant once in a blue moon.
There are actually, contrary evidences to all that. Evidences which will force us to rethink all our notions and accept Jean Baptiste Lamarck as our hero(yeah the Giraffe-neck guy).

How,do you ask? Well, we'll have to begin by learning what 'junk' DNA is. Junk DNA (better call it noncoding DNA) is a vast amount of nucleic acid lying unused in the nucleus. Actually, only 3% of the 'normal' DNA codes at a given time. Rest of it lies unused.

With me still here? Great. Now, we come to 'jumping genes' or 'transposons'. What initially was considered as a rigid blueprint, the DNA is so not like it. In fact, its a dynamic entity. Genes moving here and there, cutting,copying, and  pasting themselves within the strands.
These frisky genes are the transposons. In experiments conducted by Barbara MC Clintock, it was discovered that when corn plants were subjected to environmental stresses, the genes arranged themselves so as to confer them with a survival benefit. Yes, a sort of intentional mutation was done by the plant itself!
Later, in experiments conducted by Harvard researcher John Cairns on certain lactophobic strains of E coli, where the bacteria were deprived of all the nutrition except for lactose, it was observed that those bugs lost their milk fear rather readily, quicker than mere chance would have allowed to mess with their genetics.
Lamarck doesn't sound too stupid now, does he?!

And now, coming to the most interesting part, does all this magic occur in us, as well? Hell yeah. And wait till I mention the name of the partner in crime - Retroviruses.Yep.You read that right.
These are the viruses which can penetrate the Weissman barrier - a barrier which prevents traits acquired by the parent's somatic cells to pass to the germ cells.The idea is to prevent harmful acquired traits like radiation induced DNA damage from passing to the offspring.

Coming to viruses again,we all know how they work their charm. Using the host machinery and then in the process, sometimes ending up as a part of our own DNA- the junk DNA about which I wrote earlier.Today,it is a known fact that at least 8% of our DNA can be traced back to retroviruses. With this vast noncoding (junk) DNA, the space to play for the jumping genes opens up many folds, and hence a higher number of permutations and combinations become available for an acceptable mutation to occur.

Now what the hell are 'retrotransposons' in the title? Well, a subtype of jumping genes. Normal transposons work by cutting and pasting within the genome, while these retrotransposons are copy pasters.
But more interestingly, their work resembles very, very close to retroviruses. A retrotransposon initially copies itself on an RNA strand, travels,and uses reverse transcriptase to paste itself into a new location on the DNA strand. Does this mean retrotransposons are descended from retroviruses?! You bet!

This has led many to theorize that viruses have actually accelerated human evolution by endowing us with acres of junk DNA and retrotransposons, while we continued to provide them a warm, cozy environ. 
Some thought innit?


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