A fruit fly.
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Jorge Fardels
A fruit fly.
ScienceOnly Human

Why I Keep Harping on the Dynamic Genome

This week one of my readers, Lauren, suggested that epigenetics is a leftist plot:

The entire concept of epigenetics is being highjacked today for the Left’s eternal propaganda of neo-Lysenkoism — that “Nurture over Nature”, environment and not heredity, is what most significantly determines human qualities.

It’s essential to understand that the “methylome” is NOT — as the Left would like to give us the impression — an alternate kind of biological code created by the environment. It is not a competing genome that can overrule inherited DNA. It’s only a part of the chemical apparatus produced BY the DNA, to operate the cell.

I wouldn’t bother giving Lauren any attention except that her comment really gets under my skin. I never said or implied that environmental influences determine human qualities, for one thing. But worse than that, implicit in her remark is the opposite claim, which is just as insidious and, unfortunately, much more commonly argued: that genes determine our fate, that the genome is fixed, that environments don’t matter.

I’ve ranted before about the perils of genetic determinism and how it has contributed to an overblown fear of all things genetic. Where does the idea come from? And why does it have such a stronghold on our culture? Should I blame social Darwinism? Nazis? Gattaca? Or maybe it starts in biology class.

I’d guess that the vast majority of people, if they think about the genome at all, think of it as a static, abstract code, a string of letters that you’re born with and can’t do anything about. That’s sort of true, but sort of not true, and this complexity is something I tend to harp on in blog posts.

The super-long DNA ‘code’ is a physical molecule. It wraps around itself in a certain way so that it can fit into each of our cells. That wrapping affects which genes are turned on and made into proteins and which stay silent. DNA is littered with methyl groups, and these, too, turn genes on and off. And yes, Lauren, methylation itself is genetically encoded, leading to distinct ‘methylation landscapes’ in different tissues. But methylation is also influenced by environmental exposures, diet, and age.

Even if you put those epigenetic influences aside, the underlying genome isn’t fixed. DNA mutations frequently crop up during cell division, making the daughter cell and all of its progeny genetically distinct.

And then there are jumping genes, the subject of a cool neuroscience paper that came out yesterday in Science. Nearly 45 percent of the human genome is made of transposons, or pieces of DNA that can ‘jump’ randomly around the genome. Sometimes jumping genes use a cut-and-paste approach, other times a copy-and-paste, and these insertions can cause coding disruptions that can contribute to disease.

The new study shows that transposon activity is quite variable from one cell type to another. Analyzing the brains of adult fruit flies, the researchers showed that jumping genes are particularly active in ‘alpha-beta neurons’, which are important for processing smell-related memories. Many of these jumping DNA pieces insert themselves near or inside of active genes, likely affecting their function. These “disruptive insertions,” the researchers speculate, could accumulate throughout life, ultimately contributing to memory decline (or, if you’re into hyperbole, to fly “personalities“).

So why am I so obsessed with the dynamic genome? Mostly because it’s fascinating stuff. But I also wish that more people appreciated the unbelievable complexity of DNA and its dances with an ever-changing environment. If you think of the genome as moving and flexible, then it’s hard to fall into the trap of genetic determinism. Isn’t it?


Photo by Jorge Fardels, via Flickr