A virus, the simplest physical object in biology, consists of a protein shell called the capsid, which protects its nucleic acid genome -- RNA or DNA. The capsid can be cylindrical or conical in shape, but more commonly it assumes an icosahedral structure, like a soccer ball. Capsid formation is one of the most crucial steps in the process of viral infection. If the virus is small, the capsid forms spontaneously. Larger spherical viruses, however, such as the herpes simplex virus or infectious bursal disease virus, need the assistance of naturally produced "scaffolding proteins," which serve as a template guiding the capsid's formation. How these large viral shells assemble into highly symmetric structures is not well understood. This understanding may help researchers interrupt viruses' formation, containing the spread of viral diseases. Relying on a theory called the continuum elasticity theory, the researchers studied the growth of large spherical capsids. They showed that the template guides the formation of the capsid's protein subunits -- the individual building blocks of the shell -- in a way that is error-free and results, ultimately, in a highly symmetric, stable icosahedral structure.
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