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  • Symmetry is everywhere in nature,

  • and we usually associate it with beauty:

  • a perfectly shaped leaf,

  • or a butterfly with intricate patterns mirrored on each wing.

  • But it turns out that asymmetry is pretty important, too,

  • and more common than you might think,

  • from crabs with one giant pincer claw

  • to snail species whose shells' always coil in the same direction.

  • Some species of beans only climb up their trellises clockwise,

  • others, only counterclockwise,

  • and even though the human body looks pretty symmetrical on the outside,

  • it's a different story on the inside.

  • Most of your vital organs are arranged asymmetrically.

  • The heart, stomach, spleen, and pancreas lie towards the left.

  • The gallbladder and most of your liver are on the right.

  • Even your lungs are different.

  • The left one has two lobes, and the right one has three.

  • The two sides of your brain look similar, but function differently.

  • Making sure this asymmetry is distributed the right way is critical.

  • If all your internal organs are flipped, a condition called situs inversus,

  • it's often harmless.

  • But incomplete reversals can be fatal,

  • especially if the heart is involved.

  • But where does this asymmetry come from,

  • since a brand-new embryo looks identical on the right and left.

  • One theory focuses on a small pit on the embryo

  • called a node.

  • The node is lined with tiny hairs called cilia,

  • which tilt away from the head and whirl around rapidly,

  • all in the same direction.

  • This synchronized rotation pushes fluid from the right side of the embryo

  • to the left.

  • On the node's left-hand rim,

  • other cilia sense this fluid flow

  • and activate specific genes on the embryo's left side.

  • These genes direct the cells to make certain proteins,

  • and in just a few hours,

  • the right and left sides of the embryo are chemically different.

  • Even though they still look the same,

  • these chemical differences are eventually translated into asymmetric organs.

  • Asymmetry shows up in the heart first.

  • It begins as a straight tube along the center of the embryo,

  • but when the embryo is around three weeks old,

  • the tube starts to bend into a c-shape

  • and rotate towards the right side of the body.

  • It grows different structures on each side,

  • eventually turning into the familiar asymmetric heart.

  • Meanwhile, the other major organs emerge from a central tube

  • and grow towards their ultimate positions.

  • But some organisms, like pigs, don't have those embryonic cilia

  • and still have asymmetric internal organs.

  • Could all cells be intrinsically asymmetric?

  • Probably.

  • Bacterial colonies grow lacy branches that all curl in the same direction,

  • and human cells cultured inside a ring-shaped boundary

  • tend to line up like the ridges on a cruller.

  • If we zoom in even more,

  • we see that many of cells' basic building blocks,

  • like nucleic acids, proteins, and sugars, are inherently asymmetric.

  • Proteins have complex asymmetric shapes,

  • and those proteins control which way cells migrate

  • and which way embryonic cilia twirl.

  • These biomolecules have a property called chirality,

  • which means that a molecule and its mirror image aren't identical.

  • Like your right and left hands, they look the same,

  • but trying to put your right in your left glove proves they're not.

  • This asymmetry at the molecular level is reflected in asymmetric cells,

  • asymmetric embryos,

  • and finally asymmetric organisms.

  • So while symmetry may be beautiful,

  • asymmetry holds an allure of its own,

  • found in its graceful whirls,

  • its organized complexity,

  • and its striking imperfections.

Symmetry is everywhere in nature,

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B2 US TED-Ed asymmetric embryo node tube embryonic

【TED-Ed】Why are human bodies asymmetrical? - Leo Q. Wan

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    SylviaQQ posted on 2016/02/26
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