Astrobiology. Charles S. Cockell

evolved. Life on Earth exhibits a vast variety of cellular structures and shapes, and it interacts with non-cellular structures such as viruses. A question that emerges across all these types of cellular structures is: Where do they get their energy to grow and reproduce? This is the subject of the next chapter.

      Questions for Review and Reflection

      1 Give at least one reason why compartmentalization (for example using a membrane) has been regarded as a defining characteristic of life. Do you agree that it is an essential feature of a living thing?

      2 Describe three differences between a prokaryotic and eukaryotic cell.

      3 Microbes are sometimes said to be “simple.” Discuss this statement with respect to what you know about microbes, their internal structures, and other capacities. Do you think microbes today reflect the characteristics of the earliest life forms?

      4 Explain the process of translation, describing quantitatively the concept of the “degeneracy of the genetic code.”

      5 An alien genetic code has been found that has six bases in its code and only two positions in each codon. Can it encode for the same diversity of amino acids as terrestrial life? Compare the degeneracy of this code to the terrestrial genetic code. What about an alien code with two bases and four positions in its codons?

      6 Compare a plasmid and a virus.

      7 Some microbes are motile. Discuss how this attribute could be useful in an extreme environment, such as inside a deep ice sheet. Why might this characteristic have evolved in the first place?

      8 Explain the importance of endosymbiosis in evolution.

Bibliography

       Books

      1 Allen, T. and Cowling, G. (2011). The Cell: A Very Short Introduction. Oxford: Oxford University Press.

      2 Pross, A. (2014). What Is Life? How Chemistry Becomes Biology. Oxford: Oxford University Press.

       Papers

      1 Abisado, R.G., Benomar, S., Klaus, J.R. et al. (2018). Bacterial quorum sensing and microbial community interactions. mBio 9. https://doi.org/10.1128/mBio.02331-17.

      2 Albers, S.-V. and Meyer, B.H. (2011). The archaeal cell envelope. Nature Reviews Microbiology 9: 414–426.

      3 Belin, B.J., Busset, N., Girud, E. et al. (2018). Hopanoid lipids: from membranes to plant–bacteria interactions. Nature Reviews Microbiology 16: 304–315.

      4 Bell, S.P. and Dutta, A. (2002). DNA replication in eukaryotic cells. Annual Reviews of Biochemistry 71: 333–374.

      5 Bray, D. (1995). Protein molecules as computational elements in living cells. Nature 376: 307–312.

      6 Jacob, E.B., Becker, I., Shapira, Y. et al. (2004). Bacterial linguistic communication and social intelligence. Trends in Microbiology 12: 366–372.

      7 Koonin, E.V. and Novozhilov, A.S. (2009). Origin and evolution of the genetic code: the universal enigma. Life 61: 99–111.

      8 Kovacs, G.G. and Budka, H. (2008). Prion diseases: from protein to cell pathology. American Journal of Pathology 172: 555–565.

      9 Le Romancer, M., Gaillard, M., Geslia, C. et al. (2007). Viruses in extreme environments. Reviews in Environmental Science and Biotechnology 6: 17–31.

      10 Lombard, J., López-García, P., and Moreira, D. (2012). The early evolution of lipid membranes and the three domains of life. Nature Reviews Microbiology 10: 507–515.

      11 Lurland, C.G., Collins, L.J., and Penny, D. (2006). Genomics and the irreducible nature of eukaryotic cells. Science 312: 1011–1014.

      12 Martin, W., Roettger, M., Kloesges, T. et al. (2012). Modern endosymbiotic theory: getting lateral gene transfer into the equation. Journal of Endocytobiosis and Cell Research 23: 1–5.

      13 Satkudo, A., Ano, Y., Onodera, T. et al. (2011). Fundamentals of prions and their inactivation. International Journal of Molecular Medicine 27: 483–489.

      14 Shapiro, J.A. (1998). Thinking about bacterial populations as multicellular organisms. Annual Reviews of Microbiology 52: 81–104.

      15 Strahl, H. and Errington, J. (2017). Bacterial membranes: structure, domains, and function. Annual Review of Microbiology 71: 519–538.

      16 Young, K.D. (2006). The selective value of bacterial shape. Microbiology and Molecular Biology Reviews 70: 660–703.