Embryonic demethylation and remethylation:
Congratulations. Anybody who would look at something with that title must be a highly educated expert or a person with the courage of a tiger. Either way you win. Even I can hardly bear to think about it.
And I can’t say I know very much. But let me give you a perspective. One day you are strolling along the sidewalk and hear a hiss from a stranger. Brave soul that you are – we know that – you go over and ask what’s up. He says, “Look buddy. I need to sell my flying saucer factory. You got fifty bucks?”
“That’s absurd. Something like that would be priceless.”
“Here’s my driver’s license.”
You glance it over; it seems true bill. “All right, that’s your name, so what?”
“So get on your cell phone and verify that I own the property.” You punch up City Hall, and sure enough he owns a lot with a building. A sale has been negotiated. It will take place automatically day after tomorrow. The purchaser has contracted to have the site demolished immediately. The contents remain the property of your acquaintance.
“All right. Why the low price?”
“I’m leaving for Mars tonight. All I need is enough cash for dinner and a cab ride to the pickup point. After that your money would be worthless to me anyway.”
You verify the building on Google Earth and decide there might be fifty dollars worth of fun in it however it turns out. The problem is that you have to be out of town all the next day. The Martian has the paperwork all ready. Money changes hands. You call a friend and ask him to go to the factory and collect whatever is valuable there and put it in a storage unit you have. The Martian walks off, and you go your way. The next night by agreement you meet your friend at the storage unit. He is burbling with delight.
“You just made a fortune. You have supercomputers. You have laser alignment tools. You have automatic lathes, assembly robots, hammer mills that can play the xylophone or shape an anvil into the Venus de Milo …”
You ask, “Where are the directions? When do I turn these tools on and off? I don’t care about the resale value. I want to make a flying saucer.”
“Oh.”
In a manner of speaking your genes are those wonderful tools. They code for information to make the proteins that the cell uses to accomplish tasks that make it possible for you to live. But when do you turn them on and off? The answer, in part, lies in the epigenetic coding on the genes.
The mechanism they talk about is called “methylation.” You remember the title? All right then, you remember methane. It’s swamp gas. It’s in natural gas. It’s also in the gas you pass. It’s just a single carbon atom with four hydrogen atoms attached. Pluck off a hydrogen atom, leaving an uncompleted bond, and you have a methyl group. Find an organic compound, pluck off a hydrogen atom, put the methyl group in it’s place, and you have methylated the compound.
When the body does that to DNA, it generally hooks the methyl group onto a bit of the DNA called a cytosine base – not just any cytosine, but one next a couple others, which can vary a bit. And the methyl group is faithfully copied, most of the time, and is passed along to the daughter cells. The presence of the methyl group is a signal that usually says, “Slow this one down a tad.” So as the embryo develops, genes are turned on and off as the methylation pattern changes, and voila you have something that one day will pay taxes.
Of course there is more to it than that. After all you are making something to which a flying saucer’s complexity pales in comparison. (Would pale, if one existed.)
And as you now can understand from the title, there is a time during reproduction when there is massive removal of methyl groups, which are later replaced, more or less. (Wolf Reik and Gavin Kelsey, Cellular Memory Erased in Human Embryos, NATURE, vol. 511 no. 7511 July 31, 2014 page 540 and two accompanying articles)
But it doesn’t all get stripped. Methyl patterns can persisit from generation to generation. They aren’t as stable as DNA, at least not as stable as DNA supported by its usual repair mechanisms. (Dash it. I never can understand how your repair information.) But persist it does, and the weight of current understanding is that it probably accounts for the kin recognition that underlies inbreeding depression and outbreeding depression.
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