How Space Might Have Shaped Our DNA
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How Space Might Have Shaped Our DNA

[♩INTRO] The DNA double helix is one of the more iconic
images in all of science. Slapping a twisty piece of DNA on something
is guaranteed to make it look more… science-y. But there’s a reason it looks like that. It turns out our cells have a finely tuned sense of aesthetics. The DNA inside our cells is almost exclusively
right-handed. This doesn’t mean it never has to fight
over the single lefty desk in the lecture hall. Instead, it means the double helix twists
from lower left to upper right whether you’re holding it upside down or
right side up. Now, just to be clear, our DNA can temporarily
form left-handed helices under incredibly specific conditions. But those lefty molecules look pretty bizarre, and while they might crop up in cells in specific
situations, the majority of the time our genetic code
is stored in a right-handed twist. And that’s the kind of helix our cells are
adapted to decode if our genetic information were left-handed,
our cells wouldn’t be able to use it! This biological preference for right-handed
DNA is very strict. In fact, it holds across all forms of life…
which is kind of odd. Why does DNA have to twist the same way in
every living organism? The leading hypothesis to explain why DNA
always twists to the right has to do with the shape of its building blocks,
or nucleotides. And nucleotides are chiral, a word used to
describe things that are mirror images of each other, like
your left and right hand. No matter how much you flip or twist a pair
of chiral molecules, you can’t superimpose them. The double helix twists toward the right. But when we talk about left- or right-handed when
talking about a nucleotide, it just means one of those two mirror-image
forms. Our cells only have the left-handed form of
these DNA nucleotides, and they only make right-handed helices. The mirror images of our nucleotides would
theoretically make left-handed helices. So the question becomes: why do all living cells only use left-handed
nucleotides? The short answer is that there may just have
been more of them around 4 billion years ago when life first evolved. But the longer, more interesting answer might
come from space. Specifically, from the high-energy radiation
known as cosmic rays. When cosmic rays shower down on Earth’s
atmosphere, they cause gas molecules in the atmosphere to break down into atomic particles like electrons. In the breakdown process, the cosmic rays
put a bit of a spin on the electrons. Which, once again, means they can go left
or right. But a funny quirk about electrons from degraded
atoms is that they are more likely to spin towards
the left than the right. In fact, while rightward spinning electrons
can be generated in a lab, they have never been observed in nature. And physicists have yet to come up with a
good reason why. Regardless, about fifty years ago, scientists
tried to reconcile this electron asymmetry with the lack of left-handed
DNA helices in nature. They proposed that these lefty electrons could preferentially destroy chiral molecules of
a single handedness, like the right-handed nucleotides that form
left-handed DNA helices. Then, in 2015, scientists demonstrated that
that’s physically possible — maybe. They shot a beam of left- or right-handed
electrons at a gas and found a slight difference. The right-handed electrons destroyed 0.03% more right-handed gas molecules than
lefty ones. That’s a tiny number. But researchers believe it’s possible that
over billions of years, that miniscule percentage could add up or that this reaction could be amplified somehow
through other means. Regardless, if left-handed DNA nucleotides
are even that tiny bit more resistant to space radiation, they’d still be an evolutionary advantage
for right-handed helices. Of course, this experiment only demonstrates
the general chemical idea that electrons might interact differently
with different-handed molecules. It didn’t actually show anything to do with
DNA directly. But it is a first step toward understanding
why our cells have such an ingrained bias against southpaw
DNA. Thanks for watching this episode of SciShow, and thanks to all the awesome patrons who
support what we do. If you’d like to help out, as well as join
an awesome community of people and maybe earn some sweet perks,
check out [♩OUTRO]

100 thoughts on “How Space Might Have Shaped Our DNA

  1. 2:56 "The right-handed electrons destroyed 0.03% more right-handed gas molecules than lefty ones[.]"
    But you just said a few seconds earlier that right handed electrons have never been found in nature…
    So how does measuring right handed particles prove anything? Wouldn't it make more sense to evolve right-handed nucleotides where no right spinning electrons will destroy them?
    I think this experiment just gives us more questions than answers but really doesn't solve anything like why all living things DNA spirals in a right handed manner.

  2. Any time I see a picture of the Sun's radiation being fended off by the Earth's magnetic field, like at 1:57, I see a giant "Spider Earth". Now you can't unsee it.

  3. Funny thing is, this matches up pretty well with the lore of the game Mass Effect on DNA Chirality. It explains why there are so few Dextro DNA species in Mass Effect.

  4. This is a crazy idea: We know the earth has Birkeland currents on both poles, which also follow the rules of left/right handedness, always twisting in a particular direction. Maybe they set handedness for life on Earth. The double helix even kinda looks like a tiny Birkeland current, too. So, basically, directionality would be set by Earth's own EM fields.
    Too obvious?

  5. Michael has been getting chubby over the years. Look into intermittent fasting, try it out, experience the easy success, and do a video on it

  6. Much too complex thinking for a very simple questions: Nucleotides are chiral, this means that they interact differently with other chiral molecules. This means: if the first self-replication molecule were just by pure change (50% probability) made up of L-form molecules and not of R-Form molecules, this automatically determined what molecules would further be used, until today.
    Assuming that both forms were present in exactly the same quantity, it was just by pure chance. (if the other forms would by chance have been used, we would all have left-handed DNA and you'd ask the same question). Evolution has a strong random component included, live with it.

  7. Fascinating episode! The chirality, i.e., "handedness", is indeed an important part of biochemistry. Although all DNA is "right-handed", I think the consensus right now is that this was a"binary" choice, which was passed down from the most ancient Last Universal Common Ancestor, (L.U.C.A. for short.) But we may never know for sure, (unless we can study life from exoplanets, etc.) Great video!

  8. Thanks for giving me one more thing to be pedantic about when I see photos on the internet… (I'll never be able to not see it now)

  9. It always twists to the right because you are observing it from left to right, as in the way we read.

    Arabic is read right to left, do they contend that DNA always twists to the left ?

  10. Its an interesting theory. This could indicate that DNA and bacteria originated from earth and not from space.

  11. Interesting. Do they think that the difference is due to the weak interaction breaking chirality?

    I would think it would be much more likely that whichever one occurred first would force the issue on all of its descendants. It's hard to imagine a mutation that would result in flip-flopping from right to left. if you had two competing populations, one with left-handed and the other with right-handed DNA, it's very doubtful that they could interbreed with each other or exchange DNA. At that point one or the other probably just died out at some point

  12. How can a person make a one-time donation ( I only find ways were I need to subscribe to give money on a regular basis)

  13. Couldn't it just be a coincidence? I mean if something works, evolution isn't going to fix it right? So if the first organism evolved with right-handed DNA using left-handed molecules as the building block then they would never evolve to have left-handed DNA unless it offers some sort of benefit. And I would think that left-handed DNA would mean a really big change so that would mean the chance is even smaller. So why is it that it couldn't just be a coincidence?

  14. So what if there was an extinction event "shortly" after right was by chance dominant over left even by a tiny percent. At that point it may be amplified to the point after the right vs left phenomenon, due to cosmic rays, it puts put extra force on the right and the right becomes dominant.

    Yes or no?

  15. Maybe the electrons only go right, is because maybe this is a duel universe? The other universe goes left, similar to the nucleotides.

  16. Ok I don’t tend to receive misinformation from this channel but if anyone is interested in the correct definition of a chiral center, I would recommend googling it.

  17. So if you invent or discover a way to send a person through a 4 dimensionally curved Möbius strip of 3d space, then they'd be a mirror image of their former self, and they'd be screwed. Their cells and DNA would not react properly with the air they breath and the food and water try eat/drink.

  18. That seems incredibly far fetched and 0.03% does not sound like a very significant amount, when talking about numbers of molecules. But we'll see what research into that topic may reveal.

  19. Would it not be the case that how DNA works, is a very early adaption that all organisms need to have and mirroring an entire organism including dna (by accident) is highly unlikely?

  20. Maybe one day we'll meet aliens and we'll have serious trouble with their biology because everything they have is left-handed. We know that many of the left-handed molecules of "normal" chemistry in our bodies is causing havoc. Bath salts, crystal meth and other drugs..

  21. As far as I know, most biological aminoacids are left-handed too, although sugars are right-handed (why?). Can cosmic radiation explain those forms in a similar way as it possibly do to nucleotides?

  22. Hey guys….i recently learned about supercoiling of DNA, which is like winding an already helical DNA….it as well has 2 orientations positive and negative….and just like preferential right handedness of dna helix…. negative supercoiling is observed preferential in most organisms….i wish u guys would make video about it😊

  23. It’d also be a benefit in a pre ozone earth, where it would make crippling mutations less of a threat than it’d be for right handed nucleotides

  24. The theory about the cosmic radiation dont really fit up with the theory aboute evolution through natural selection. But interesting thought.

  25. I've gotten to a "can't unsee it" place in terms of illustrations that show DNA with a left-handed helix. It's crazy common; I sometimes wonder if it might be a bit over 50% of the time. This video actually has it wrong more often than it has it right (half of them at 0:11, 0:58, 1:33) . Sometimes you see this in detailed molecular models, suggesting that an actual atom-by-atom 3D model was encoded, rendered, and then presumably flipped left-right for aesthetic reasons. It's been wrong on the cover of Nature literally dozens of times.

  26. Really interesting video, brings up as many questions as it answers. Like why the lack of right spin electrons in nature? And in the video it states that right spin electrons preferentially destroy right handed nucleotides, but does that really affect much if right spin electrons aren’t found in nature?

  27. Isn't it more logical for life to choose the right-handed dna, because it is easier for storage in the nucleus?

  28. DNA is right handed due to cosmic rays from space. I think it's a fine hypothesis. However folks, this wouldn't fit the theory that life started around vents in the deep ocean. Light doesn't penetrate down that far right? Never mind visible light but cosmic rays themselves. No cosmic rays would penetrate that deep right? They'd run into too much water molecules the deeper they'd go. The energy would be depleted and absorbed by these water molecules. Perhaps we should test if energy produced from vents could have the same effect, i.e.: producing enough right-handed electrons to have the same effect. Please note: I give no preference to either hypothesis- i.e.: life starting nearer to the surface or deep on the ocean floor. I simply would like to know which would be more probable in producing life. Excellent episode as ever folks. You have my mind buzzing!:D

  29. As if we needed any more proof that all lefties are FREAKS 😂! If you ain't right, then you

    ain't… Well, RIGHT! Repent, all ye spawn of Satan 😈, and we shall pray for thy souls. 😇

    (How many commentors do you want to bet will think I'm actually being serious?) 😉

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