万物简史 第593期:生命的物质(19)（在线收听）

The shape of a DNA molecule, as everyone knows, is rather like a spiral staircase or twisted rope ladder: the famous double helix. The uprights of this structure are made of a type of sugar called deoxyribose, and the whole of the helix is a nucleic acid — hence the name "deoxyribonucleic acid." The rungs (or steps) are formed by two bases joining across the space between, and they can combine in only two ways: guanine is always paired with cytosine and thiamine always with adenine. The order in which these letters appear as you move up or down the ladder constitutes the DNA code; logging it has been the job of the Human Genome Project.

正如人人所知道的那样，DNA分子的形状像一个螺旋状楼梯或扭曲的绳梯：著名的双螺旋结构。这种结构的支柱是一种被称为脱氧核糖的糖组成的，整个双螺旋是一个核酸——因此取名为“脱氧核糖核酸”。横档（或梯级）由两个碱基跨越中间的空间相连而成。它们只以两种方式配对，腺嘌呤总是与胸腺嘧啶配对，鸟嘌呤总是与胞嘧啶配对。当你在梯子上上下下走动时，这些字母所排列的顺序就组成了DNA的密码；记录这些密码一直是“人类基因组工程”所要做的工作。

Now the particular brilliance of DNA lies in its manner of replication. When it is time to produce a new DNA molecule, the two strands part down the middle, like the zipper on a jacket, and each half goes off to form a new partnership. Because each nucleotide along a strand pairs up with a specific other nucleotide, each strand serves as a template for the creation of a new matching strand. If you possessed just one strand of your own DNA, you could easily enough reconstruct the matching side by working out the necessary partnerships: if the topmost rung on one strand was made of guanine, then you would know that the topmost rung on the matching strand must be cytosine. Work your way down the ladder through all the nucleotide pairings, and eventually you would have the code for a new molecule. That is just what happens in nature, except that nature does it really quickly — in only a matter of seconds, which is quite a feat.

且说DNA的绝妙之处在于它的复制方式。当需要产生一个新的DNA分子时，两条单链从中间裂开，就像夹克上的拉链一样，每条单链的一半脱离而去，形成新的组合。由于一条单链上的每一个核苷酸与另一个特定的核苷酸匹配在一起，每条单链成为创造一条与之匹配的新链的模板。如果你只有你自己DNA的一条单链，通过必要的组合，你就很容易重建另一条与之匹配的单链。如果一条单链的第一级是由鸟嘌呤构成的，你就会知道与之配对的另一条单链的第一级一定是胞嘧啶。要是你沿着所有核苷酸配对组成的阶梯往下走，最后你将获得一个新的分子的密码。这就是大自然中所发生的事，只不过这一切是以极快的速度，完成的——仅仅几秒钟时间，快得令人不可思议。