Читать книгу Astrobiology - Charles S. Cockell - Страница 87

Research has shown that “unnatural” base pairs can be incorporated into cells. For example, d5SICS and dNaM are a synthetic base pair successfully incorporated into the DNA of the microorganism Escherichia coli. This research raises the question of whether the genetic code needs to have the four bases that we find in the natural terrestrial genetic code. Are they the serendipitous result of one evolutionary path? Is it possible that we could have an entirely different set of bases? A second, and related, question is whether the fundamental structure of DNA need be the same, even if the base pairs were different. You might like to discuss what sort of molecular structure an alternative information storage system could have, bearing in mind that it must replicate and be used to code information to construct other molecules (such as proteins). What required general characteristics does this impose upon an information storage system? Does such an information storage system have to be organic? Could you imagine such a system using minerals or regular crystal structures to store the quantity of information needed to build life?

Hoshika, S., Leal, N.A., Kim, M-Y. et al. (2019). Hachimoji DNA and RNA: A genetic system with eight building blocks. Science 363: 884–887.

Malyshev, D.A., Dhami, K., Lavergne, T. et al. (2014). A semi-synthetic organism with an expanded genetic alphabet. Nature 509: 385–388.

RNA is an important part of the architecture of reading the cell's information, and we look at it further in Chapter 5 when we investigate how the genetic code is read. RNA is also found in a range of viruses. The RNA molecule, on account of its different chemistry and orientation of its bases, is generally more reactive than DNA. We will see later that on account of these properties, RNA plays a central role in some theories about the origin of life.

At this point in the book, it is worth visiting yet again that question we thought about in Chapter 3. How universal would we expect the preceding discussion to be? Must life use proteins, carbohydrates, lipids, and nucleic acids to build itself? Must all information storage molecules have the same chemistry as DNA? Must all cell membranes be made of lipids? Certainly, even in terrestrial biology we see a great deal of diversity, for example in the different lipids and sugars used to assemble macromolecules. One point of view could be that the major classes of macromolecules used by life are universal, but their details are subject to great variation depending on how a particular biochemistry evolves. What do you think?