The genes you don't get from your parents (but can't live without) - Devin Shuman

譯者: Lilian Chiu 審譯者: Amanda Zhu

Inside our cells, each of us has a second set of genes

我們每個人的細胞中， 都還有第二組基因，

completely separate from the 23 pairs of chromosomes

不包括在我們從父母遺傳 得到的二十三對染色體之內。

we inherit from our parents.

And this isn’t just the case for humans—

且不只有人類是如此，

it’s true of every animal, plant, and fungus,

所有的動物、植物、真菌類，

and nearly every multicellular organism on Earth.

以及地球上幾乎 所有的多細胞有機體也一樣。

This second genome belongs to our mitochondria,

這第二組基因位在粒線體內，

an organelle inside our cells.

粒線體是細胞中的一種胞器。

They’re not fully a part of us, but they’re not separate either—

它們不完全是我們的一部分， 也不完全不是。

so why are they so different from anything else in our bodies?

它們為什麼會如此不同於 我們體內的所有其他東西？

Approximately 1.5 billion years ago,

科學家認為，

scientists think a single-celled organism engulfed the mitochondria’s ancestor,

大約十五億年前，一種單細胞有機體

吞噬了粒線體的祖先，

creating the predecessor of all multicellular organisms.

創造出了所有多細胞有機體的前身。

Mitochondria play an essential role:

粒線體扮演了重要的角色：

they convert energy from the food we eat and oxygen we breathe

我們吃的食物和吸的氧氣中的能量

into a form of energy our cells can use, which is a molecule called ATP.

就要靠它們轉換成 細胞可以運用的能量形式，

即一種叫做三磷酸腺苷 ——即 ATP——的分子。

Without this energy, our cells start to die.

沒有這種能量，細胞就會開始死亡。

Humans have over 200 types of cells,

人類的細胞類型超過兩百種，

and all except mature red blood cells have mitochondria.

除了成熟的紅血球細胞之外，

全部都有粒細體。

That’s because a red blood cell’s job is to transport oxygen,

原因是紅血球細胞的 工作是在運送氧，

which mitochondria would use up before it could reach its destination.

如果有粒細體在， 就會在抵達目的地前就把氧用光。

So all mitochondria use oxygen and metabolites to create energy

所以，所有的粒線體

都會用氧和代謝產物來創造能量，

and have their own DNA,

且擁有它們自己的 DNA，

but mitochondrial DNA varies more across species than other DNA.

但粒線體 DNA 在不同物種間的差異性

比其他 DNA 更高。

In mammals, mitochondria usually have 37 genes.

哺乳類動物的粒線體 通常有三十七個基因。

In some plants, like cucumbers, mitochondria have up to 65 genes,

至於一些植物，如黃瓜，

其粒線體有高達六十五個基因，

and some fungal mitochondria have only 1.

有些真菌類的粒線體則只有一個基因。

A few microbes that live in oxygen-poor environments

少數幾種生活在稀氧環境中的微生物

seem to be on the way to losing their mitochondria entirely,

幾乎快完全失去粒線體，

and one group, oxymonad monocercomonoides, already has.

而有一類屬於類單鞭滴蟲屬的微生物，

已經沒有粒線體了。

This variety exists because mitochondria are still evolving,

會有這樣的多樣性，

是因為粒線體仍然在演化，

both in tandem with the organisms that contain them,

包括與它們所屬的有機體一起演化，

and separately, on their own timeline.

以及依它們自己的時間表單獨演化。

To understand how that’s possible,

要了解怎麼可能會如此，

it helps to take a closer look at what the mitochondria inside us are doing,

可以先細究粒線體 在我們體內做些什麼，

starting from the moment we’re conceived.

從懷孕的那一刻開始談起。

In almost all species, mitochondrial DNA is passed down from only one parent.

幾乎所有物種的粒線體 DNA

都是由父母其中之一所傳下來。

In humans and most animals, that parent is the mother.

就人類和大部分動物而言，是母親。

Sperm contain approximately 50 to 75 mitochondria in the tail,

精子尾部含有大約 五十到七十五個粒線體，

to help them swim.

用以協助它們游泳。

These dissolve with the tail after conception.

受孕之後，這些粒線體 就會和尾部一起溶解。

Meanwhile, an egg contains thousands of mitochondria,

一個卵子則含有數千個粒線體，

each containing multiple copies of the mitochondrial DNA.

每個都含有多個粒線體 DNA。

This translates to over 150,000 copies of mitochondrial DNA

算起來就有超過十五萬個粒線體 DNA

that we inherit from our mothers,

都是來自我們的母親，

each of which is independent and could vary slightly from the others.

每個粒線體都是獨立的， 且可能互相都有些微不同。

As a fertilized egg grows and divides,

隨著受精卵成長和分裂，

those thousands of mitochondria are divvied up into the cells

那數千個粒線體會分裂並分配到 發展中胚胎的各細胞當中，

of the developing embryo.

By the time we have differentiated tissues and organs,

當細胞已分化出組織和器官之後，

variations in the mitochondrial DNA are scattered at random throughout our bodies.

粒線體 DNA 的差異

就會隨機散佈到我們全身各處。

To make matters even more complex,

更複雜的是，

mitochondria have a separate replication process from our cells.

粒線體和細胞的複製過程是分開的。

So as our cells replicate by dividing, mitochondria end up in new cells,

所以，當細胞透過分裂來複製時，

粒線體就會進入到新的細胞中，

and all the while they’re fusing and dividing themselves,

而在此過程中，粒線體 也一直依循著自己的時間表

on their own timeline.

進行融合和分裂。

As mitochondria combine and separate,

隨著粒線體結合和分離，

they sequester faulty DNA or mitochondria that aren’t working properly for removal.

它們會把有缺陷、無法妥當運作的 DNA 或粒線體隔離並除去。

All this means that the random selection of your mother’s mitochondrial DNA

也就是說，你在出生時 從母親身上隨機選擇

you inherit at birth

而繼承的粒線體 DNA，

can change throughout your life and throughout your body.

在你一生中、在你體內各處 都有可能會改變。

So mitochondria are dynamic and, to a degree, independent,

所以粒線體會不斷變化， 某種程度上也是獨立的，

but they’re also shaped by their environments: us.

但它們的環境，即我們， 也會改變它們。

We think that long ago,

我們認為，很久以前，

some of their genes were transferred to their host’s genomes.

它們的一些基因 被轉移給其宿主的基因組。

So today, although mitochondria have their own genome

所以現今雖然粒線體 有它們自己的基因組，

and replicate separately from the cells that contain them,

且複製過程和它們 所屬的細胞是分開的，

they can't do this without instruction from our DNA.

沒有我們 DNA 的指示， 它們也無法複製。

And though mitochondrial DNA is inherited from one parent,

且雖然粒線體 DNA 來自父母其中之一，

the genes involved in building and regulating the mitochondria

用來建造和管理粒線體的基因

come from both.

則是來自雙親。

Mitochondria continue to defy tidy classification.

粒線體始終無法井然有序地分類。

Their story is still unfolding inside of each of our cells,

存在於我們每個細胞裡的粒線體 仍有許多待解的謎團，

simultaneously separate and inseparable from our own.

它們的故事和我們對人體的了解 是既分離又無法分離的。

Learning more about them can both give us tools

進一步了解它們，一方面 能讓我們保護未來人類的健康，

to protect human health in the future, and teach us more about our history.

二方面也能讓我們 更認識我們演化的歷史。