(Intro)

Michael: For the past 120 years, the Nobel Prize has honored scientists for discoveries that the committee feels most benefit mankind.

And today we're gonna talk about this year's winners for physics and chemistry whose discoveries are solving the mysteries of both the universe and our DNA.

The Nobel Prize for physics was awarded to Takaaki Kajita and Arthur B. McDonald who led separate teams of scientists who each proved neutrinos have mass.

Neutrinos are subatomic particles produced when radioactive elements decay. They're the second most abundant particle in the universe after photons.

Billions of them are streaming through you right now!

We know there are at least three flavors, or types, of neutrinos - electron, muon, and tau. All named after which charged particles they can interact with.

But they're very difficult to detect and thus incredibly hard to study. But if neutrinos are so hard to detect, how did Kajita and McDonald make their discovery?

Both scientists work in teams attached to a Neutrino Detector. Kajita works with a Super-Kamiokande Detector in Japan, and McDonald works with a Sudbury Neutrino Observatory in Canada.

Both detectors work in pretty much the same way, looking for tiny flashes neutrinos make when they pass by protons and electrons in just the right way.

For a long time, we thought the flavor of a neutrino was built in and never changed. But by studying the flashes of light observed in the Super-Kamiokande Detector, Kajita discovered in 1998 that muon neutrinos were turning into electron neutrinos and then back to muon neutrinos as they traveled.

A few years earlier, McDonald's team had found that about two-thirds of the neutrinos from the sun, which all start out as electron neutrinos, were turning into muon or tau neutrinos on their way to Earth.

Now according to Quantum Theory, neutrinos should only be able to change flavor is they have mass. Not a lot of mass - the new data suggests the upper limit is about one-one thousandth the mass of an electron.

But the fact that they have mass at all means that there are some major things we don't know about some of the most abundant particles in the universe. And neutrinos are now a real contender for dark matter, the universe's so-called "missing mass."

The Nobel Prize for chemistry was awarded to three scientists, again all part of separate teams who helped figure out how the cells in your body repair damaged DNA.

DNA is fragile. Our genetic material is constantly damaged - up to ten thousand times per day, per cell. There is no way that life based on such a fragile molecule could exist if your cells couldn't repair that damage.

And it turns out that different types of DNA damage are repaired by different mechanisms in your cells.

Tomas Lindahl's work in 1974 led to the discovery of Base Excision Repair. This is the mechanism that repairs the most common type of damage, which is spontaneous decay of one of the bases that make up our genetic code.

For example, cytosine (the DNA base that normally pairs with guanine) can spontaneously lose part of its molecule to form uracil. This happens all the time actually, and when it happens, the newly formed uracil pairs with adenine instead. So that section of DNA looks all different.

Lindahl discovered the first DNA glycosylases were pair proteins that removed the incorrect or damaged bases. These proteins recognize specific types of DNA damage, clip them out of the DNA strand, and replace them with their original bases. So, if say, they see a uracil bonded to an adenine, they'll know to replace it with a cytosine. 

In 1992, Aziz Sancar discovered another way our bodies fix DNA called Nucleotide Excision Repair. It takes care of damage done to DNA by UV rays as well as chemical substances that cause mutations like nicotine.

That kind of damage changes the shape of the DNA helix, so NER moves not just the base but the entire nucleotide, which is the base plus the backbone of the helix it's attached to.

Sancar found that an enzyme known as an exonuclease cuts out the damage, taking a piece about twelve nucleotides long in total.

He then mapped out how two other enzymes are involved - DNA polymerase, which fills in the gap, and DNA ligase, which stitches the pieces back together again. The DNA of a normal skin cell exposed to sunlight would end up with thousands of errors everyday if this process didn't remove them.

The third winner, Paul Modrich, discovered Mismatch Repair in 1989, which is exactly what it sounds like. It repairs mismatched bases introduced to a strand of DNA during cell division.

DNA proofreading during cell division is very good, but it's not perfect. About one in every hundred million bases is copied incorrectly, and MMR fixes 99.9% of those.

Modrich demonstrated how an enzyme called demethylase marks damaged DNA, which shows other enzymes which pieces to cut out. Then DNA polymerase and ligase again fill in and repair the cut strand. But we still don't know how the enzyme knows which strand needs to be repaired.

Thanks to this year's winners though, we do know a lot more about how our bodies keep their genetic instructions clear. Plus, we have a few more clues about how the universe works.

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