1. A Brief Look at the Structure of Viruses Mr. Chapman Biology 20

2.  Viruses are much smaller than plant and animal cells, and also much smaller than bacterial.  Viruses range in size from 18 – 300 nm. Note that 1 nanometre = 1 x 10 -9 m.

4.  A virus is simply made of genetic material (either RNA or DNA) surrounded by a protein shell called a capsid.

5.  A single viral particle is called a virion.  Capsids (the protein structure on the outside of the virus) are different shapes for different viruses, and some are covered by a lipid (a.k.a. Fat) envelope.  The lipid envelope is the protective outer coat of a virus, from which spiky structures from proteins or sugars sometimes stick out.

6.  A virus is called a naked virus when it consists of only the genetic material surrounded by a capsid (no lipid envelope).

7.  In some viruses, capsids form a 20-sided polyhedron.  Other viruses are rod-like, and some strand- like viruses are shaped in coils, like a spring or a helix.  Unlike prokaryotes and eukaryotes, viruses have DNA or RNA, but never both.

9.  The structure and shape of particular viruses are very important.  A virus can only infect certain hosts, and it identifies its hosts by fitting its surface proteins to receptor molecules on the surface of the host cell.  The system is like a lock and key, and viruses need the correct key to infect a particular cell.

10. Believe it or not, the answer is not clear.

11.  Unlike other types of cells, a virus really has no structure to maintain at all.  Viruses don’t eat, excrete, use energy, or require oxygen at all. They don’t have organelles, either In fact, they do almost nothing that living organisms do – except reproduce.  Even so, a virus can only reproduce after it has infected a host cell.

12.  What are the three main components of a typical virus? Which one of these components is a naked virus missing?  How does the size of a virus compare to that of a plant or animal cell?  Why are the structures protruding from the lipid envelope important for bonding to host cells?  Why is it unclear as to whether or not a virus is alive?

13. Complete the rest of the class by reading section 18.2 of the Biology textbook, and complete questions 1 – 4 on page 551.