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  • The DNA in just one of your cells

  • gets damaged tens of thousands of times per day.

  • Multiply that by your body's hundred trillion or so cells,

  • and you've got a quintillion DNA errors everyday.

  • And because DNA provides the blueprint for the proteins your cells need to function,

  • damage causes serious problems, such as cancer.

  • The errors come in different forms.

  • Sometimes nucleotides, DNA's building blocks, get damaged,

  • other times nucleotides get matched up incorrectly,causing mutations,

  • and nicks in one or both strands can interfere with DNA replication,

  • or even cause sections of DNA to get mixed up.

  • Fortunately, your cells have ways of fixing most of these problems most of the time.

  • These repair pathways all rely on specialized enzymes.

  • Different ones respond to different types of damage.

  • One common error is base mismatches.

  • Each nucleotide contains a base,

  • and during DNA replication,

  • the enzyme DNA polymerase is supposed to bring in the right partner

  • to pair with every base on each template strand.

  • Adenine with thymine, and guanine with cytosine.

  • But about once every hundred thousand additions,

  • it makes a mistake.

  • The enzyme catches most of these right away,

  • and cuts off a few nucleotides and replaces them with the correct ones.

  • And just in case it missed a few,

  • a second set of proteins comes behind it to check.

  • If they find a mismatch,

  • they cut out the incorrect nucleotide and replace it.

  • This is called mismatch repair.

  • Together, these two systems reduce the number of base mismatch errors to about one in one billion.

  • But DNA can get damaged after replication, too.

  • Lots of different molecules can cause chemical changes to nucleotides.

  • Some of these come from environmental exposure,

  • like certain compounds in tobacco smoke.

  • But others are molecules that are found in cells naturally,like hydrogen peroxide.

  • Certain chemical changes are so common that they have specific enzymes assigned to reverse the damage.

  • But the cell also has more general repair pathways.

  • If just one base is damaged,

  • it can usually be fixed by a process called base excision repair.

  • One enzyme snips out the damaged base,

  • and other enzymes come in to trim around the site and replace the nucleotides.

  • UV light can cause damage that's a little harder to fix.

  • Sometimes, it causes two adjacent nucleotides to stick together,

  • distorting the DNA's double helix shape.

  • Damage like this requires a more complex process

  • called nucleotide excision repair.

  • A team of proteins removes a long strand of 24 or so nucleotides,

  • and replaces them with fresh ones.

  • Very high frequency radiation, like gamma rays and x-rays,

  • cause a different kind of damage.

  • They can actually sever one or both strands of the DNA backbone.

  • Double strand breaks are the most dangerous.

  • Even one can cause cell death.

  • The two most common pathways for repairing double strand breaks

  • are called homologous recombination and non-homologous end joining.

  • Homologous recombination uses an undamaged section of similar DNA as a template.

  • Enzymes interlace the damaged and undamgaed strands,

  • get them to exchange sequences of nucleotides,

  • and finally fill in the missing gaps

  • to end up with two complete double-stranded segments.

  • Non-homologous end joining, on the other hand, doesn't rely on a template.

  • Instead, a series of proteins trims off a few nucleotides

  • and then fuses the broken ends back together.

  • This process isn't as accurate.

  • It can cause genes to get mixed up, or moved around.

  • But it's useful when sister DNA isn't available.

  • Of course, changes to DNA aren't always bad.

  • Beneficial mutations can allow a species to evolve.

  • But most of the time, we want DNA to stay the same.

  • Defects in DNA repair are associated with premature aging and many kinds of cancer.

  • So if you're looking for a fountain of youth,

  • it's already operating in your cells,

  • billions and billions of times a day.

The DNA in just one of your cells

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TED-ED】DNAが傷つくとどうなる?- モニカ・メネシーニ (【TED-Ed】What happens when your DNA is damaged? - Monica Menesini)

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    janfuc   に公開 2021 年 01 月 14 日
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