The Genetic Code

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The genetic code by which DNA stores the genetic information consists of "codons" of three nucleotides. The functional segments of DNA which code for the transfer of genetic information are called genes. With four possible bases, the three nucleotides can give 4³ = 64 different possibilities, and these combinations are used to specify the 20 different amino acids used by living organisms.

The Genetic Code in DNA

The instructions for the construction of proteins is written in DNA using the genetic code. More specifically, the sequence of bases bonded to the sugar phosphate backbone of the double helix contains information in the form of three-base codons that specify the sequence of amino acids to be used in the construction of proteins.

The sequence of bases in DNA operates as a true code in that it contains the information necessary to build a protein expressed in a four-letter alphabet of bases which is transcribed to mRNA and then translated to the twenty-amino-acid alphabet necessary to build the protein. Saying that it is a true code involves the idea that the code is free and unconstrained; any of the four bases can be placed in any of the positions in the sequence of bases. Their sequence is not determined by the chemical bonding. There are hydrogen bonds between the base pairs and each base is bonded to the sugar phosphate backbone, but there are no bonds along the longitudional axis of DNA. The bases occur in the complementary base pairs A-T and G-C, but along the sequence on one side the bases can occur in any order, like the letters of a language used to compose words and sentences.

To further illustrate what is meant by a true code, consider the magnetic letters fixed to the magnetic board at right. The letters are held to the board by the magnetic forces, but those forces do not impose any specific ordering of the letters. The letters can be arranged to spell out a meaningful message in the English language (code) or to form a meaningless sequence like the one at bottom. 为进一步说明何谓真正的电码，考虑右侧磁性字母固定在磁性板上的情形。这些字母由磁力固定在板上，但这些力并不对字母施加任何特定的顺序。这些字母可以排列成一个有意义的英语消息（电码），也可以排列成一个无意义的序列，如底部所示。

The Genetic Code in Operation for Protein Construction

The use of a formal code to accomplish a purpose requires the receiver of the code to understand the rules and the meaning of the symbols, and be able to use the information received to accomplish a task. In the language of information science, the code must have a syntax and semantics. For the communication of information, the receiver must be in possession of that syntax and semantics, and possibly also a cipher to be able to decode the information. The receiver must also be able to carry out the task communicated.

The cipher in this case involves the agency of another complex structure which fixes the amino acid valine to the transfer RNAs which have the anti-codon CAC, even though these bases do not have any chemical or physical reason to be associated with valine. They are "formally" matched to follow the genetic code. The building blocks for proteins are the 20 amino acids used in life, and each is attached to a specific transfer RNA molecule so that protein building materials are available in the intracellular medium. The pattern for the construction of a protein is copied to the messenger RNA by transcription in the cell nucleus. Then the translation into a chain of amino acids takes place in a ribosome of the cell. The tRNAs transfer the appropriate amino acids to build the protein according to the blueprint carried by the mRNA. After this translation, further shaping of the protein takes place to fold and form it into its final functional configuration.

Meaning is given to combinations of symbols, like a language

Three-letter combinations from the four-letter alphabet of bases form the genetic code. These base triplets act as codons to specify one of the letters of the 20 letter alphabet of amino acids. With three letters out of four possible, there are 4³ = 64 possibilities. So the code is redundant, but in a purposeful way. The redundancy takes care of the majority of single-base errors in coding.