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Comment by BitwiseFool | Hacker News Reader

I remember hearing a hypothesis that "junk DNA" acts as a kind of shielding against mutations. From a probability standpoint, the more non-coding DNA you have, the less likely an error is to hurt the lifeform. Taken to an extreme, if every DNA sequence did something a singe error would cripple the organism.

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However, the probability of the DNA getting hit would increase linearly with the amount of junk DNA, wouldn't it? So the chance of good DNA getting hit is exactly the same as without junk.

Not sure how it would help shielding the good DNA, unless it acted as a barrier.

Mutations from radiation come from particles literally slamming into the DNA and screwing it up. The more spaced out the important parts are, the lower the probability that the particle hits a piece of DNA that’s important.

If I place 4 cups of water out in the rain, does their spacing affect how much water lands in each cup?

It depends on how you place them, horizontaly on a table no, in a pyramid yes.

I see what you're getting at, but in this example there would be millions of cups and the individual raindrops would be spread out over a massive area of land.

That doesn’t change anything. The probability an important cup gets hit is always equal to the surface integral of the rain’s flux across the important cup openings. Whether there are unimportant cups nearby doesn’t change anything.

that makes intuitive sense but at least in the case of ionizing radiation it's a little like making a target circle bigger to protect the bullseye which remains the same size. there's still the same chance of hitting the bullseye, no matter how you change the chance of not missing the paper.

Think of it as a 3D sphere instead. You would get a shielding effect.

There is already one milligram of atmosphere over the cross sectional area of a cell nucleus, so it doesn't seem like it could change much for cosmic rays. All DNA in a human cell is about 6 picograms.

That's 166 million times more mass of atmosphere. The elements may be less effective etc. but it still seems like any junk dna shielding would be negligible.

Maybe cosmic rays aren't the main source of ionizing radiation throughout the evolutionary period though.

If the cell around is not shielding it, the little bit of extra DNA is not shielding it either. You need a lot more atoms to get a shielding effect.

I reject the term 'junk dna'. Until we understood the basics of DNA all of it was 'junk', then bit by bit we are making headway into understanding some of it. If we're ever done I predict there will be no junk DNA left. Nature is much too efficient to spend that much energy on something that it ends up not using.

> Nature is much too efficient to spend that much energy on something that it ends up not using.

And yet nature somehow finds it acceptable to grow a nerve all the way from a giraffe's head down its neck, loop around its heart and then go all the way back up through its neck until reaching the head. [0]

[0] https://rationalwiki.org/wiki/Laryngeal_nerve

Yes, and there are a number of other examples like that. But that doesn't detract from the general case in the least: if energy can be saved then it usually will be saved, especially if a mechanism lasts for a long time. The idea that the base mechanism of encoding life patterns is going to be on the order of 70% or more 'junk' requires a very good explanation. Why the argument presented here is invalid I'll leave for you to figure out.

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