The DNA in just one of your cells gets damaged tens of thousands of times per day.
在你的一个细胞里的DNA每天都受到上万次的破坏。
Multiply that by your body's hundred trillion or so cells, and you've got a quintillion DNA errors everyday.
乘以你身体中数以百万亿的细胞，每天你将得到百万的三次方的DNA错误。
And because DNA provides the blueprint for the proteins your cells need to function,
并且因为DNA为你身体所需的蛋白质提供蓝图，
damage causes serious problems, such as cancer.
DNA的破坏会造成严重的后果，比如患上癌症。
The errors come in different forms.
有很多种不同的错误。
Sometimes nucleotides, DNA's building blocks, get damaged,
有的时候核苷酸，DNA的基础组成部分，受到了破坏；
other times nucleotides get matched up incorrectly, causing mutations,
有时核苷酸配对错误，造成了突变，
and nicks in one or both strands can interfere with DNA replication, or even cause sections of DNA to get mixed up.
一条或者两条DNA链中片段缺失会影响DNA链的折叠，甚至会引起DNA片段配对混乱。
Fortunately, your cells have ways of fixing most of these problems most of the time.
幸运地是，你的细胞在绝大多数情况下有改正这些问题的能力。
These repair pathways all rely on specialized enzymes.
这些修护的方法都依赖于特定的酶。
Different ones respond to different types of damage.
不同的酶对应于不同种种的破坏。
One common error is base mismatches.
一种常见的情况是碱基错配。
Each nucleotide contains a base, and during DNA replication,
每一个核苷酸含有一个碱基，在DNA复制阶段，
the enzyme DNA polymerase is supposed to bring in the right partner to pair with every base on each template strand.
DNA聚合酶是用来确保正确的配对的，以保证模板链上的碱基配对成功。
Adenine with thymine, and guanine with cytosine.
腺嘌呤配胸腺嘧啶，鸟嘌呤配胞嘧啶。
But about once every hundred thousand additions, it makes a mistake.
但是每增加十万次的配对，就会产生一个错误。
The enzyme catches most of these right away,
聚合酶能够及时找到大部分的错误，
and cuts off a few nucleotides and replaces them with the correct ones.
并且减掉一小段核苷酸然后用正确的片段去替代。
And just in case it missed a few, a second set of proteins comes behind it to check.
为了以防万一它错漏了一些，又会有一组蛋白链返回检查。
If they find a mismatch, they cut out the incorrect nucleotide and replace it.
如果它们确实找到了错配的，它们会切断那段错误片段并用正确的片段来替代。
This is called mismatch repair.
这就是错配修复。
Together, these two systems reduce the number of base mismatch errors to about one in one billion.
这两种检测系统将碱基错配的数目减少到十亿分之一。
But DNA can get damaged after replication, too.
但是DNA在复制之后也可能遭到破坏。
Lots of different molecules can cause chemical changes to nucleotides.
很多不同的分子可能使核苷酸产生化学变化。
Some of these come from environmental exposure, like certain compounds in tobacco smoke.
一些变化是来自环境暴露，比如烟草烟雾中的特定化合物。
But others are molecules that are found in cells naturally, like hydrogen peroxide.
但是其他的是细胞中自带的分子，比如过氧化氢。
Certain chemical changes are so common that they have specific enzymes assigned to reverse the damage.
某些化学变化实在是太常见了，以至于它们有着特定的酶来扭转破坏。

But the cell also has more general repair pathways.
但是细胞同样有普通的维护方式。
If just one base is damaged, it can usually be fixed by a process called base excision repair.
如果只有一个碱基遭到了破坏，它通常能够被一种称为碱基切除修复的方式维护。
One enzyme snips out the damaged base, and other enzymes come in to trim around the site and replace the nucleotides.
一种酶剪掉受损的碱基，其他的酶来修整破坏点并更换核苷酸。
UV light can cause damage that's a little harder to fix.
紫外线造成的破坏会稍微难修复一点。
Sometimes, it causes two adjacent nucleotides to stick together, distorting the DNA's double helix shape.
有时，它会造成两个相邻的核苷酸粘在一起，使DNA双螺旋模型变形。
Damage like this requires a more complex process called nucleotide excision repair.
像这样的破坏需要更加复杂的修护过程，称为核苷酸切除修复。
A team of proteins removes a long strand of 24 or so nucleotides, and replaces them with fresh ones.
一组蛋白质会去除一长串大约24个左右的核苷酸，并且用新的核苷酸代替它们。
Very high frequency radiation, like gamma rays and x-rays, cause a different kind of damage.
频率十分高的射线，比如伽马射线和X-射线，引起另一种不同的破坏。
They can actually sever one or both strands of the DNA backbone.
它们可以分离DNA骨架中的单链或双链。
Double strand breaks are the most dangerous. Even one can cause cell death.
双链断裂是最危险的。即使是单链断裂也可能引起细胞死亡。
The two most common pathways for repairing double strand breaks
两种最常见的修复双链断裂破坏的方式，
are called homologous recombination and non-homologous end joining.
被称为同源重组和非同源末端连接。
Homologous recombination uses an undamaged section of similar DNA as a template.
同源重组利用一段未被破坏的相似DNA作为模板。
Enzymes interlace the damaged and undamgaed strands, get them to exchange sequences of nucleotides,
酶使破坏链和未破坏链交错，让他们交换核苷酸片段，
and finally fill in the missing gaps to end up with two complete double-stranded segments.
最终补充缺隙，得到两个完整的双链段。
Non-homologous end joining, on the other hand, doesn't rely on a template.
另一方面，非同源末端连接，不需要依赖于模板。
Instead, a series of proteins trims off a few nucleotides and then fuses the broken ends back together.
相反地，一系列的蛋白质剪切掉一些核苷酸，然后使断裂的端部融合在一起。
This process isn't as accurate. It can cause genes to get mixed up, or moved around.
这个过程并不那么得精确。它可能会造成基因错乱或者左右错动。
But it's useful when sister DNA isn't available.
但是当姐妹染色体不可获取时它很有作用。
Of course, changes to DNA aren't always bad. Beneficial mutations can allow a species to evolve.
当然，DNA的变化不总是有害的。有益的突变能使种族进化。
But most of the time, we want DNA to stay the same.
但是大多数情况下，我们希望DNA保持一致。
Defects in DNA repair are associated with premature aging and many kinds of cancer.
DNA修复中的错误，与过早衰老和许多种的癌症有关。
So if you're looking for a fountain of youth, it's already operating in your cells, billions and billions of times a day.
所以如果你在寻找“青春之泉”，它已经在你的细胞中了，每日数十亿倍地不停运行着。