The expression of the genetic information into proteins does not proceed directly from DNA. For the language of DNA to be transferred into the language of proteins, an intermediate molecule is necessary. This molecule is a special class of RNA, called messenger RNA, and the process of its synthesis is called transcription.
During transcription the sequence of a gene is copied into an mRNA, using one strand of the DNA molecule as template
(Figure 1.5). The rules of complementarity in DNA/RNA double-strands are much the same as in DNA, with rA pairing with dT (“r” stands for RNA and “d” for DNA), rU pairing with dA, rG with dC, and rC with dG. According to the complementary rules, an exact copy of the gene is made in the form of an mRNA. This messenger RNA, containing all the necessary signals for protein synthesis, namely the “start” and “stop” signals, is carried to the appropriate place in the cell and used as template for protein synthesis. For example, in eukaryotes, the mRNAs synthesized in the nucleus must go to the cytosol where the machinery of translation is located.
Figure 1.5. Relationship of the DNA to the mRNA. Note that the mRNA is complementary to the DNA strand from which it is transcribed. Note also that there are sequences upstream of the start signal and sequences downstream of the stop signal that will not be translated into amino acids. The sequence shown here is unrealistically short to illustrate both the start and the stop signals.
As I said earlier, the need for an mRNA intermediate makes sense in the environment of the cell, but is of little use in simple computer systems such as GEP, in which small and simple genomes are currently devoid of sophisticated mechanisms of regulation of gene expression. Perhaps in the near future we will see sophisticated GEP-like computer systems with more complex genomes and capable of complex somatic differentiation. Maybe then an mRNA-like intermediate will be necessary to ensure a differential pattern of gene expression.
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