Genes And Knowledge


Stripped to its core, a living organism is nothing more than the packet of information recorded in its genes. And yet, if the very essence of life is information, one has to wonder why a column of numbers or a line of words isn't alive. Obviously, when digits or letters are arranged in a particular sequence, they convoy information. But just as clearly, information, in and of itself, is not alive.


說到底,生物只不過是一包記錄在基因中的信息。但是,如果生命的基本要素是信息的話,人們不禁要問,為什麼一列數字或一行文字卻沒有生命?顯然,數字和字母按特定的順序排列可傳輸信息;但是,同樣也很清楚的是,信息本身卻沒有生命。


Genetic information is special because it alone can make copies of itself. This remarkable ability is the basis of all the other differences that distinguish the living from the nonliving. Even a crystal of table salt is a form of information. Its sodium and chlorine atoms are arranged in a precise order, but a salt crystal cannot duplicate itself. Of all the substances on earth, only DNA, the molecule that carries genetic information, can orchestrate its own replication.


遺傳信息的特殊之處在於它可以獨自自我復制。這一非凡的能力是區分生物和非生物的各種差別的基礎。即使一顆食鹽晶體也是一種信息,其中鈉原子和氯原子按精確的順序排列。但是,食鹽晶體卻不能自我復制。地球上所有物質中,惟有攜帶遺傳信息的DNA(脫氧核糖核酸)能夠實現自我復制。


DNA's capacity to self-copy, as well as its ability to encode information, stems from its peculiar shape. First described in 1953 by James Warson and Francis Crick, the structure of the DNA (deoxyribonucleic acid) molecule is a double helix, a shape that looks like a long ladder twisted into a corkscrew. Each rung is a letter in a chemical alphabet limited to just four symbols. Arranged in varying but exact sequences, incredibly long strings of these four letters spell out the instructions for building and operating all living things. Every organism that has ever lived on this planet, from the greatest dinosaurs to the tiniest viruses, is a product of information recorded in its own particular version of the DNA molecule.


DNA的自我復制能力及其編碼能力來源於其獨特的形狀。一九五三年,詹姆斯.沃森和弗朗西斯.克里克首次描述了DNA分子的結構。其形狀為一個雙螺旋,象是一個長梯子被擰成了開塞鑽似的。每個梯級為有限的四個化學符號中的一個字母,這四個字母排列變化有序,由這些字母連成的極長的字母串給出形成生命和維持生命的指令。曾在地球上生活過的所有的生物,從最大的恐龍到最小的病毒,都是記錄在其特殊的DNA分子中的信息的產物。

No one knows how nature happened to settle on a coding system of four symbols. The simplest possible way of recording information, called binary notation, needs just two symbols--1 and 0. Each binary symbol conveys one binary digit, or bit, of information. Like a simple yes or no answer, a bit is the smallest fragment of information one can receive and still learn anything at all. All the information flowing through the circuits of digital computers is encoded in immensely long strings of binary 1s and 0s.


誰也不知道自然界為什麼採用這一四個符號的編碼系統。最簡單的記錄信息的方法,即二進制法,僅需兩個符號──1和0。每個二進制符號表達一個二進制數位信息。如同一個簡單的是或否的回答一樣,一個數位是能表達一定意義的最小信息傳輸單元,數字電腦中所有的信息流都是由很長很長的一串串1和0的代碼來表示的。


Whether a chunk of information happens to be recorded by the four symbols of DNA or the digits used by computers, the basics of information processing are much the same. Meaning is captured in a linear sequence of a few simple symbols arranged in a precise order. And even though it is somewhat more complicated than the binary system, DNA's method has worked four billion years.


無論信息是用DNA的四個符號記錄的或是用電腦上的二進制數碼記錄的,信息處理的基本原理卻是相同的。要傳遞的信息包含在由排列有序的簡單符號所組成的線性序列中。DNA的四符號編碼方法雖然比二進制編碼複雜,但它從誕生到現在已有四十億年了。



The letters in genetic code (A, T, G, and C) are read off in groups called codons just as computers read 1s and 0s in groups called bytes. Each codon stands for one amino acid, the building block of proteins. For example, if DNA's letters are arranged in order TGG AAG ATC, the first codon-TGG-will be interpreted by the cell's machinery to mean, "Place the amino acid tryptophan here." The next codon, AAG, codes for the amino acid lysine. And so on. One after another, like beads on a string, amino acids are assembled into the proteins that make up living tissue.


遺傳編碼中的字母(A、T、G和C)是按密碼子識讀的,就象電腦識讀二進制數中由1和0組成的字節一樣。每個密碼子代表一個氨基酸。氨基酸是組成蛋白質的最小結構單元。舉例來說,如果DNA的排列順序為TGG AGG ATC,那麼,第一個密碼子TGG就會被細胞 機構解釋為“在這里放一個色氨酸”。下一個AAG代表賴氨酸,等等。氨基酸象串珠上的珠子一樣,一個接一個地被安放到構成生 物組織的蛋白質中。

DNA's double-helix architecture also endows the molecule with the ability to make precise copies of the information it contains. Each rung in DNA's spiral ladder is actually formed by a pair of its chemical letters-adenine (A), thymine (T), guanine (G), and cytosine (C). A and T fit each other perfectly, as do G and C. Consequently, the four chemical letters always form two rungs, AT and GC. With only two kinds of rungs, it might seem that DNA uses a two-symbol code. But, in biochemistry, physical orientation makes a difference. Viewed form the vantage point one of the ladder's side rails, the TA rung is read as a T, while the same rung flipped over, AT, represents an A.


DNA的雙螺旋結構還使得分子具有準確復制其本身所包含的信息的能力。螺旋梯上的每一個梯級實際上是由兩個字母組成的。DNA結構中的四個化學字母分別為﹕腺嘌呤(A)、胸腺嘧啶(T)、鳥嘌呤(G)及胞嘧啶(C)。A和T可完美地對接,G和C也是如此。因此,這四個化學符號總是形成兩個梯級AT和GC。由於只有兩個梯級,看起來好象DNA用得是兩符號系統。實際上,在生物化學中,氨基酸的取向是非常重要的。從梯子的一個邊框方向看去,TA梯級被識讀成T,若將同一梯級翻過來,也就是AT,則被識讀為A。

When a DNA molecule copies itself, its rungs split down the middle. Each A lets go of its T and each G releases its C. The side rails of the molecule zipper apart, and the spiral ladder becomes two separate spirals, each with severed half-rungs hanging free. Because A will only bond to T and G will only cling to C, the sequence of broken rungs on each of these half-molecules is a mirror image of the other. From the chemical soup floating around the replicating DNA, unattached letters link up with the mates that are still hanging to the side rails. When this process is completed, two new DNA molecules appear. Each is an exact replica of the parent molecule.


DNA分子復制時,其梯級從中間斷開,每個A松開與之相連的T,每個G放開與之相連的C。於是,分子的兩個邊框就象拉鏈似的拉開。由於A只能和T連接,G只能和C連接,每個斷開的半分子梯框互成鏡像關係。這時,復制DNA周圍的化學溶液中未結合的自由字母就與掛在半梯框上的配對字母相連接。當這一過程完成時,兩個新的DNA分子就形成了,每一個都是母分子的精確復制品。