生物学 1958

大肠杆菌中 DNA 的复制

马修·梅塞尔森与富兰克林·斯塔尔

DNA 复制时，留一条旧链、造一条新链——靠「称重」得到证明。

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In depth · the introduction

当 DNA 复制自身时，它是留住旧的，还是从头全新造一份？两位年轻的科学家，靠「称量」这个分子、看它停在哪里，回答了这个问题。

核心想法

沃森与克里克说过，DNA 是两条扭在一起的链，彼此互为镜像。他们猜测：要复制自身，这个分子会像拉链一样解开，让每条旧链充当模板，去造出一条新的搭档。如果真是如此，那么每一份新拷贝都将是一半旧、一半新。可那毕竟只是猜测，没人真正看见过。

梅塞尔森与斯塔尔找到了一个能让答案现身的办法。他们先把细菌养得 DNA 变「重」，再让它们在普通的「轻」食物里生长，一代一代地盯着 DNA 到底有多重。结论干净利落：经过一轮复制，每个分子都恰好是半重的——留住一条旧链，造出一条新链——正如沃森与克里克所预言的那样。

它是如何诞生的

1950 年代中期，双螺旋发表之后，生物学家面对一个尖锐的悬而未决之问：像 DNA 这样的分子，究竟是怎样复制的？看上去有三种可能的答案，却无从在它们之间作出抉择。加州理工的两位年轻研究者马修·梅塞尔森与富兰克林·斯塔尔，恰好发明了一件新工具：把 DNA 放进盐溶液里高速旋转，让盐形成一道密度梯度，于是每个分子都会漂到与自身「体重」相称的那一层。

他们意识到，这能为那场争论一锤定音。先给细菌喂一种重的氮，再切换成普通的轻氮，他们就能眼睁睁看着重的旧物质被轻的新物质稀释——而条带的图样，会揭示自然采用的是哪一种复制方案。1958 年的这个结果如此清晰而优雅，以至于它常被称为「生物学中最美的实验」。

它为何重要

双螺旋是一个美丽的结构，但结构并不等于机制。这个实验把一个满怀希望的猜测，变成了确凿的事实：DNA 复制自身，确实是在每个新分子里留住一条旧链。这一个结果，锚定了我们对遗传、突变、DNA 修复，以及每一个活细胞中那套复制机器的理解——它也作为一个范例，示范了如何把实验设计得如此干净，使答案几乎无可争辩。

一个可以想象的画面

想象一条拉链，由两半组成，而这两半只与彼此咬合。要造出两条新拉链，你不会把两半都从头做起。你会把拉链拉开，给每一半旧的，都配上一半全新的、能与它咬合的。于是你有了两条完整的拉链，而每一条都是一半旧、一半新。再来一次，最初那两半旧的，依然还在其中——完好无损地被传了下去，每一代各得一半——而周围则环绕着越来越多全新的物质。那种对一半旧物的忠实保留，正是梅塞尔森与斯塔尔抓拍到的、这个分子所做的事。

它的位置

这个实验，是本馆两座里程碑之间的桥梁：它印证了沃森与克里克 1953 年双螺旋所暗示的机制，也为后来的分子机器——聚合酶、修复酶——以及最终像 CRISPR 这样、去读写正是此处被证明会被保留的那些链的工具，铺平了道路。如果说沃森与克里克给出了结构，那么梅塞尔森与斯塔尔，便给出了这个结构在生命中如何运作的证明。

The original document

Original source text

问题所在

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——给 DNA 贴标签并「称重」

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.

[ … ]

条带说明了什么

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.

两条结论

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