The synthesis of RNA is called transcription because it is simply the copying of DNA “language” into RNA. Like the transcription of spoken language into written language, the units of information (nucleotides in nucleic acids, words in speech and writing) are the same. Translation—the conversion of one language to another—is much more difficult, whether in human language or in biochemical language. Translation can't be too literal and has to preserve the context of information as well as its symbols. The information problem of biological translation is the way in which a protein sequence can be encoded by a nucleic acid sequence.
The correspondence between nucleic acid information and protein information is given by the genetic code. which is a set of rules giving the correspondence between mRNA and protein sequence information. See Figure 1 .
The genetic code can be thought of as a dictionary giving the equivalents for words from one language to another. However, the dictionary isn't enough. Just as the translation of one language into another requires a translator, the genetic code requires an adaptor molecule. Transfer RNA is that adaptor molecule for biological information. Secondly, the error frequency of the process must be kept to a minimum. The wrong amino acid in a protein could, in principle, lead to the death of the cell, just as the wrong word in translation of a diplomatic message could lead to a war. Both cases need a proofing mechanism to check that the information transfer is accurate. Thirdly, punctuation and reading frame selection are essential components of the process. Because the genetic code is a triplet code, two of the three “messages” in a string of nucleic acids are usually meaningless. In fact, a number of severe genetic diseases are caused by mutations that cause a “frameshifted” protein whose information is meaningless. A biological translation system must know where messages start and stop.
A protein molecule's amino acid sequence determines its properties. This has been shown for many proteins, which can be denatured and then refolded to re-form active protein tertiary structure. The biological translator thus has a somewhat easier task than the translation of human languages, because the mRNA and protein sequences are colinear. Important parts of the information don't rearrange from one language to another in contrast to the way, for example, that verbs occur at different positions in German and English sentences.
The conversion of nucleic acid into protein information doesn't completely solve the problem of translation. Proteins must be targeted to their appropriate locations, either inside or outside the cell. In eukaryotes especially, proteins must be broken down at appropriate rates; some proteins have longer half-lives than others do. These steps are all possible points for cellular control.
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