Читать книгу Cell Biology - Stephen R. Bolsover - Страница 16

Viruses occupy a unique position between the living and nonliving worlds. On the one hand they are made of the same molecules as living cells. On the other they are incapable of independent existence, being completely dependent on a host cell for reproduction. Almost all living organisms have viruses that infect them. Human viruses include polio, influenza, herpes, rabies, smallpox, chickenpox, HIV, and SARS‐CoV‐2, the causative agent of COVID‐19. Viruses are submicroscopic particles consisting of a core of genetic material enclosed within a protein coat called the capsid. Some have an extra membrane layer called the envelope. Viruses are inert until they enter a host cell, whereupon their genetic material directs the host cell machinery to produce viral protein and viral genetic material. Viruses often insert their genome into that of the host, an ability that is widely made use of in molecular biology research (Chapter 8). Bacterial viruses, bacteriophages, are used by scientists to transfer genes between bacterial strains. As we will see, human viruses are used as vehicles for gene therapy.

Figure 1.2. Organization of prokaryotic and eukaryotic cells.

Figure 1.3. The tree of life. The diagram shows the currently accepted view of how the different types of organism arose from a common ancestor. Many minor groups have been omitted. Distance up the page should not be taken as indicating complexity or how “advanced” the organisms are. All organisms living today represent lineages that have had the same amount of time to evolve and change from the last universal common ancestor.