gets damaged tens of thousands of times per day.

一天之內就會受損數百萬次。

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

乘上你體內百萬多個細胞，

and you've got a quintillion DNA errors everyday.

也就是說一天內你會有一百萬的三次方處分子損壞。

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

而正因DNA提供了你細胞所需要製造的蛋白質的藍圖，

damage causes serious problems, such as cancer.

這些損壞會帶來嚴重後果，比如：癌症。

The errors come in different forms.

損壞有多種不同型態。

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

有時是核苷酸(組成DNA的基本單元)受損，

other times nucleotides get matched up incorrectly,causing mutations,

有時是核苷酸的配對錯誤導致的衰變，

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

而不論是單邊或雙邊的缺口都可能影響DNA的複製，

or even cause sections of DNA to get mixed up.

甚至使不同區段的DNA相混合。

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

但別擔心，通常，你的細胞都有辦法解決大部分的問題。

These repair pathways all rely on specialized enzymes.

這些修復途徑都需依賴特殊的酵素。

Different ones respond to different types of damage.

不同的酵素負責解決不同的損壞問題。

One common error is base mismatches.

最常見的一種損壞就是鹼基錯配。

Each nucleotide contains a base,

每個核苷酸分子都含有一個鹼基，

and during DNA replication,

而在DNA複製過程中，

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

DNA聚合酶應負責將正確的對象

to pair with every base on each template strand.

配對給模板鏈上的每個鹼基。

Adenine with thymine, and guanine with cytosine.

腺嘌呤配胸腺嘧啶，鳥嘌呤配胞嘧啶。

But about once every hundred thousand additions,

但在百萬分之一的機率裡，

it makes a mistake.

它還是會出錯。

The enzyme catches most of these right away,

酵素通常立即就會發現，

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

並將一些核苷酸去除並換上正確的。

And just in case it missed a few,

而為了確保它有可能出錯，

a second set of proteins comes behind it to check.

第二組蛋白質會接著檢查。

If they find a mismatch,

若他們找到一處錯配，

they cut out the incorrect nucleotide and replace it.

便將錯誤的核苷酸去除並換上正確的。

This is called mismatch repair.

這就稱為「錯誤配對修復」。

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

在兩組系統的合作下，鹼基錯配的數量，每一億個裡會大約下降至一個。

But DNA can get damaged after replication, too.

但DNA也會在複製後被損壞。

Lots of different molecules can cause chemical changes to nucleotides.

許多不同的分子都會造成核苷酸的化學變化。

Some of these come from environmental exposure,

有些是來自所暴露的環境，

like certain compounds in tobacco smoke.

比如某些菸草內含有的化合物。

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

但其他原因都是自然來自於細胞內的物質，比如過氧化氫。

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

有些化學變化非常普遍，因此有特定酵素被指派去修復它們的損壞。

But the cell also has more general repair pathways.

但細胞還有更多不同的修復途徑。

If just one base is damaged,

如果只有一個鹼基受損，

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

通常可以由稱為「鹼基切除修復」的程序修復。

One enzyme snips out the damaged base,

一個酵素將受損的鹼基取出，

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

其他酵素則在該處進行修剪，並換上正確得核苷酸。

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

紫外線造成的損壞，修復起來比較困難一點。

Sometimes, it causes two adjacent nucleotides to stick together,

有時，它會造成兩個鄰近的核苷酸黏在一起，

distorting the DNA's double helix shape.

扭曲DNA雙鼓螺旋的結構。

Damage like this requires a more complex process

如此的損壞需要更複雜的修復過程，

called nucleotide excision repair.

成為「核苷酸切除修復」。

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

一組蛋白質會將含有約24個核苷酸的其中一長股去除，

and replaces them with fresh ones.

並換上新的核苷酸。

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

諸如gamma射線或X射線這類高頻率的射線，

cause a different kind of damage.

則會造成其他不同的損壞。

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

他們可能會切斷一股甚至雙股的DNA主幹。

Double strand breaks are the most dangerous.

雙股斷裂是最危險的。

Even one can cause cell death.

就算只有一處也可能造成細胞死亡。

The two most common pathways for repairing double strand breaks

兩種最常見修復雙股斷裂的途徑

are called homologous recombination and non-homologous end joining.

稱為「同源性重組」和「同源性末端接合」。

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

「同源性重組」利用一段相似，而且未受損的DNA為範本。

Enzymes interlace the damaged and undamgaed strands,

酵素促使受損與未受損的兩股相互交織，

get them to exchange sequences of nucleotides,

並且使它們得以交換核苷酸的排序，

and finally fill in the missing gaps

然後填補消失的間隔，

to end up with two complete double-stranded segments.

最終產生兩段完整的雙股區段。

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

相反地，「同源性末端接合」，不需要依賴任何範本。

Instead, a series of proteins trims off a few nucleotides

而是一系列的蛋白質取出某些核苷酸，

and then fuses the broken ends back together.

然後融合斷裂的兩端股段。

This process isn't as accurate.

這個程序正確性較低。

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

可能會引發基因混合或位移。

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

但缺乏一組相似的DNA時，也是很有用的。

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

當然，DNA的轉換也不全是負面的。

Beneficial mutations can allow a species to evolve.

有效的轉換使物種得以進化。

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

但大多數時候，我們希望DNA保持原貌。

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

DNA修復的疏失與早性老化和數種癌症有關。

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

若你正在尋找青春之泉，

it's already operating in your cells,

其實它早已存在你細胞的運作中，

billions and billions of times a day.

一天之內會發生好幾億次呢。