Читать книгу Astrobiology - Charles S. Cockell - Страница 114

The DNA molecule seems a thing of remarkable peculiarity with its four bases and the code that links the amino acids to particular codons. However, is all this a “frozen accident” as Francis Crick once famously called it, or is there something more behind it? There are possible alternative bases that might be used in a genetic code. For example, the base pairs xanthosine with 2,4-diaminopyrimidine and isoguanine with isocytosine might offer alternative base structures. However, research work testing the base pairing of a range of other bases across a molecular landscape of possibilities found them to be limited. By making RNA molecules with alternative bases, base pairs can be tested for their efficacy. Some bases, such as the hexopyranoses, which have a six-carbon ring in them, instead of the familiar five carbon ring, are too large and do not allow binding. Other bases were found to allow for even stronger base pairing than our own RNA, but in these cases, the pairing may be too strong, failing to provide the flexibility needed. These experiments suggest that the bases used in DNA and RNA may not be a frozen accident, but may have been selected by evolution from a wide range of possibilities. Explore the literature on the incorporation of new bases into DNA and discuss whether you think the genetic bases are a matter of chance or a frozen accident, or whether they were selected specifically by the evolutionary process.

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