Page 11 - Science Focus (issue20)

Mitochondria : 方便的方法追踪祖先。由於線粒體 DNA 僅是從母親那裡

So Much More Than
the Powerhouses of the Cell

線粒體： 遠不只是細胞的發電站
線粒體 DNA 不僅在臨床上很重要，而且還提供了一種

發電站和自殺式炸彈
在 1987 年發表的一項研究中，科學家從不同族群採集了
By Kit Kan 簡迎曦
147 個樣本，並根據其線粒體 DNA 序列的相似度畫成了
如果你對生物學有基本了解，你可能已經知道線粒體是
一棵演化樹。他們找到了證據證明目前所有的線粒體 DNA
細胞的發電站（註一）。這些擁有雙層薄膜的結構是細胞進
均源自非洲的一名婦女，這發現正正支持受普遍認同的非
行呼吸作用的地方，在過程當中糖和氧被用於生產腺苷三 繼承，因此科學家可以使用此特點追踪我們的母系血統。​
Powerhouse and Suicide Bomb Unlike nuclear DNA, 磷酸（ATP），亦即是我們細胞使用的「能量貨幣」。精子含 洲起源說​[3]。在人類遺傳學中，所有活著的人類的母系最
t h e i n he r i t a n c e o f
Fo r those of you w ho have some ba s ic 有大量線粒體產生能量來支持長距離游泳以尋找卵子，肌 近的共同祖先通常被稱為「線粒體夏娃」。然而，這並不是
understanding of biology, you may already know the m i t o c h o n d r i a l D N A i s 肉細胞需要線粒體生產的 ATP 來收縮並引起運動，神經元 說線粒體夏娃是當時唯一活著的女性；我們的細胞核內的
mitochondria as the powerhouses of the cell. These strictly maternal. During 利用線粒體產生的能量來發送電和化學信號，但其實線粒 DNA 表明我們確實還有其他女性祖先 [4]。換句話說，我
fertilization, mitochondria
double-membrane structures serve as sites for cellular from the sperm are actively
respiration, in which sugar and oxygen are used to removed. Therefore, no 體的功能並不只是為細胞提供能量。 們都有一個共同的母系曾……曾祖母，但同時我們也有其他
produce adenosine triphosphate (ATP), the energy one inherits mitochondrial 女性祖先，只是不在母線（direct​female​line）而已。
currency that our cells use. Sperms have plenty of DNA from his or her father. 我們的細胞有時會進行稱為細胞凋亡（apoptosis）的
mitochondria for their long distance swim in search of Mitochondrial DNA encodes for genes that are 過程，簡單來說就是細胞的自殺。在胚胎發育過程中，一些 在你使用線粒體產生的能量閱讀本文，並驚嘆於這些
an egg. Muscle cells need mitochondria to contract essential to the oxidative phosphorylation process 細胞必須死亡才能形成我們現在擁有的身體結構，例如如 複雜的結構之後，也許你可以去感謝你的母親，祖母和曾祖
and bring about movement. Neurons use the energy in the mitochondria. Since the mitochondria mainly 果我們的手指和腳趾之間的細胞不發生凋亡，那麼我們的 母，因為她們把健康的線粒體遺傳給你！
generated by mitochondria to send electrical and generate energy by oxidative phosphorylation, the 手和腳就會像青蛙的掌一樣有蹼。
chemical signals. However, powering the cell is not all mutation in mitochondrial DNA can lead to disorders
that the mitochondria do. 由於凋亡也是我們身體消除潛在癌
in tissues with high energy demands, such as muscles 細胞的一種方法，故不正常的凋亡
Our cells engage in a process called apoptosis, and brain [2].
which is essentially suicide for cells. During embryonic Mitochondrial DNA is not just clinically important, 可導致癌症。細胞凋亡的其中一種
development, some cells have to die to carve out but also provides a convenient way for ancestry 途徑涉及線粒體。線粒體含有一種
the body structures that we now have. For instance, tracing. Since mitochondrial DNA is only inherited from 叫做細胞色素 c（cytochrome​c）
our hands and feet would be webbed like frog’s the mother, scientists can trace our maternal ancestry 的蛋白質，當被釋放到細胞質時，它
if the cells between our fingers and toes did not using this feature. In a study published in 1987, taking 將與其他蛋白質結合形成凋亡小體
undergo apoptosis. As apoptosis is also a way for 147 samples from different populations, scientists
our body to eliminate cells that are potentially mapped the samples onto an evolutionary tree based （apoptosome）並促進細胞凋亡。
cancerous, defective apoptosis can result in cancer. on the similarity of their mitochondrial DNA sequence. 簡而言之，線粒體不僅是發電廠，而
One pathway of apoptosis involves mitochondria. 且亦是自殺式炸彈。
Mitochondria contain a protein called cytochrome c, They found proof that all current mitochondrial DNA
originated from one woman in Africa, supporting
which when released to the cytoplasm, binds to other
proteins to form apoptosomes and facilitate apoptosis. the popular “out-of-Africa” model [3]. The matrilineal 起源與遺傳
most recent common ancestor of all living humans is
In short, mitochondria are not just powerhouses, but
also suicide bombs. often called the Mitochondrial Eve in human genetics. 線粒 體 的 有趣 之 處 不僅在 於
However, it is not to say that Mitochondrial Eve was
the only woman alive at the time; our nuclear DNA 其 功 能，它 們 的 來 歷 更 是 稀 奇。
suggests that we do have other female ancestors 線粒 體 來 自 稱 為 內共 生 的 過 程
Origin and Inheritance [4]. In other words, we all share a common maternal （endosymbiosis）。科 學 家 相 信
Mitochondria are not only interesting in the great-great…grandmother, but we also have female 在超過 14.5 億年前​[1]，一種單細胞生物吞噬了另一種單
functions they serve, but also their origin. The ancestors, just not in the direct female line. 細胞生物，然後它們形成了共生關係，當中內共生體（被吞
mitochondria came from a process named After you use the energy generated by your 1​ 編按：在英文中，線粒體通常被比喻為「the​powerhouse​of​the​cell（細胞的
endosymbiosis. More than 1.45 billion years ago [1], mitochondria to read this article and marvel at the 噬的單細胞生物）變成了線粒體。關於宿主是原核生物還 發電站）」，外國網民甚至創作了不同迷因來揶揄這個一成不變的比喻，有人覺
a unicellular organism is believed to have engulfed beauty of these complex structures, maybe you 是發展得較成熟的真核生物，學術界存在兩種不同的理論， 得它代表了學校裡會教授但在現實生活上不太實用的知識。但無論如何，教育
another unicellular organism, and they formed a can go thank your mother, grandmother and great 但是宿主是原核生物的假說被更廣泛地接受，因為在真核 局似乎沒有把這個比喻帶進香港中學的中文課程裡。
symbiotic relationship. The endosymbiont (the one grandmother for the functional mitochondria that 生物是從原核生物的線粒體內共生進化而來的前提下，這
being engulfed) became the mitochondria. There they passed on to you!
are two different theories as to whether the host 種說法可以解釋為何線粒體普遍存在於真核生物中​[1]。線 References 參考資料：
was a prokaryote or a more full-fledged eukaryote, 粒體在許多方面與細菌相似。內共生的其中一個主要特徵
but the hypothesis that the host was a prokaryote 和證據是線粒體含有自己的環狀 DNA 染色體。 [1] Martin WF, Mentel M. The Origin of Mitochondria. Nat
Educ. 2010;3(9):58.
is more widely accepted because the ubiquity of
mitochondria in eukaryotes can then be explained, 與細胞核內的 DNA 不同，線粒體 DNA 是母系遺傳的。 [2] Chial H, Craig J. mtDNA and Mitochondrial Diseases. Nat
providing that eukaryotes were evolved from the 在受精過程中，來自精子的線粒體會被移除，因此沒有人能 Educ. 2008;1(1):217.
mitochondrial endosymbiosis in a prokaryote [1]. 遺傳父親的線粒體 DNA。線粒體 DNA 包含了線粒體中氧 [3] Cann RL, Stoneking M, Wilson AC. Mitochondrial DNA and
Mitochondria are similar to bacteria in many ways. 化磷酸化過程必不可少的基因。由於線粒體主要通過氧化 human evolution. Nature. 1987;325:31-36.
One major feature and evidence of endosymbiosis [4] Takahata N. Allelic Genealogy and Human Evolution. Mol
is that mitochondria contain their own DNA that is 磷酸化產生能量，因此線粒體 DNA 的突變可以導致一些 Biol Evol. 1993;10(1):2-22.
organized in circular chromosomes. 與肌肉和大腦等能量需求較高的組織相關的病症​[2]。
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