生物學 1958

大腸桿菌中 DNA 的複製

馬修·梅塞爾森與富蘭克林·斯塔爾

DNA 複製時，留一條舊鏈、造一條新鏈——靠「秤重」得到證明。

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當 DNA 複製自身時，它是留住舊的，還是從頭全新造一份？兩位年輕的科學家，靠「秤量」這個分子、看它停在哪裡，回答了這個問題。

核心想法

沃森與克里克說過，DNA 是兩條扭在一起的鏈，彼此互為鏡像。他們猜測：要複製自身，這個分子會像拉鍊一樣解開，讓每條舊鏈充當模板，去造出一條新的搭檔。如果真是如此，那麼每一份新拷貝都將是一半舊、一半新。可那畢竟只是猜測，沒人真正看見過。

梅塞爾森與斯塔爾找到了一個能讓答案現身的辦法。他們先把細菌養得 DNA 變「重」，再讓牠們在普通的「輕」食物裡生長，一代一代地盯著 DNA 到底有多重。結論乾淨俐落：經過一輪複製，每個分子都恰好是半重的——留住一條舊鏈，造出一條新鏈——正如沃森與克里克所預言的那樣。

它是如何誕生的

1950 年代中期，雙螺旋發表之後，生物學家面對一個尖銳的懸而未決之問：像 DNA 這樣的分子，究竟是怎樣複製的？看上去有三種可能的答案，卻無從在它們之間作出抉擇。加州理工的兩位年輕研究者馬修·梅塞爾森與富蘭克林·斯塔爾，恰好發明了一件新工具：把 DNA 放進鹽溶液裡高速旋轉，讓鹽形成一道密度梯度，於是每個分子都會漂到與自身「體重」相稱的那一層。

他們意識到，這能為那場爭論一錘定音。先給細菌餵一種重的氮，再切換成普通的輕氮，他們就能眼睜睜看著重的舊物質被輕的新物質稀釋——而條帶的圖樣，會揭示自然採用的是哪一種複製方案。1958 年的這個結果如此清晰而優雅，以至於它常被稱為「生物學中最美的實驗」。

它為何重要

雙螺旋是一個美麗的結構，但結構並不等於機制。這個實驗把一個滿懷希望的猜測，變成了確鑿的事實：DNA 複製自身，確實是在每個新分子裡留住一條舊鏈。這一個結果，錨定了我們對遺傳、突變、DNA 修復，以及每一個活細胞中那套複製機器的理解——它也作為一個範例，示範了如何把實驗設計得如此乾淨，使答案幾乎無可爭辯。

一個可以想像的畫面

想像一條拉鍊，由兩半組成，而這兩半只與彼此咬合。要造出兩條新拉鍊，你不會把兩半都從頭做起。你會把拉鍊拉開，給每一半舊的，都配上一半全新的、能與它咬合的。於是你有了兩條完整的拉鍊，而每一條都是一半舊、一半新。再來一次，最初那兩半舊的，依然還在其中——完好無損地被傳了下去，每一代各得一半——而周圍則環繞著越來越多全新的物質。那種對一半舊物的忠實保留，正是梅塞爾森與斯塔爾抓拍到的、這個分子所做的事。

它的位置

這個實驗，是本館兩座里程碑之間的橋樑：它印證了沃森與克里克 1953 年雙螺旋所暗示的機制，也為後來的分子機器——聚合酶、修復酶——以及最終像 CRISPR 這樣、去讀寫正是此處被證明會被保留的那些鏈的工具，鋪平了道路。如果說沃森與克里克給出了結構，那麼梅塞爾森與斯塔爾，便給出了這個結構在生命中如何運作的證明。

The original document

Original source text

問題所在

Matthew Meselson & Franklin W. Stahl · Proc. Natl. Acad. Sci. USA 44(7) (1958): 671–682 · Communicated by Max Delbrück, May 1958

Watson and Crick had proposed in 1953 that DNA is two complementary strands wound into a double helix, and suggested that each strand could serve as a template for a new partner. That implied a precise prediction about how the molecule should divide when it copies itself — but in 1958 no experiment had yet shown which of the competing schemes nature actually used.

Three schemes were on the table. In semiconservative replication each daughter double helix keeps one whole parental strand and pairs it with a freshly built one. In conservative replication the parent stays wholly intact and an entirely new double helix is made alongside it. In dispersive replication the parental material is broken up and scattered piecemeal through both daughters.

方法——給 DNA 貼標籤並「秤重」

Escherichia coli was grown for many generations on a medium whose only nitrogen source carried the heavy isotope N15, so that the bacteria's DNA became uniformly dense. The culture was then abruptly switched to ordinary light N14 medium, and samples were taken after successive generations of growth.

Each sample's DNA was spun for some twenty hours in a concentrated cesium chloride solution. Under the immense centrifugal force the salt forms a smooth density gradient, and every DNA molecule drifts to the level where its own buoyant density matches the surrounding fluid, settling into a sharp band photographed by ultraviolet absorption.

[ … ]

條帶說明了什麼

Fully N15-labelled DNA banded at a higher density than ordinary N14 DNA, the two differing in buoyant density by about 0.014 g/cm³ — a separation wide enough to resolve cleanly. After one generation of growth in N14, all of the DNA banded at a single intermediate position, exactly halfway between heavy and light: every molecule was now half-heavy. After two generations, two bands of equal amount appeared — one half-heavy, one fully light.

A half-heavy molecule, heat-denatured into single strands, separated into one heavy strand and one light strand — direct evidence that the intermediate band was a hybrid of one old and one new strand, not a uniform blend.

兩條結論

Conclusions (verbatim)

1. The nitrogen of a DNA molecule is divided equally between two subunits which remain intact through many generations.

2. Following replication, each daughter molecule has received one parental subunit.

The result is in exact accord with the expectations of the Watson–Crick model for the duplication of DNA.

Gates and Crellin Laboratories of Chemistry, California Institute of Technology · 1958