BIO1011 Lecture Notes - Lecture 25: Rna Virus, Dna Virus, Escherichia Coli
Viruses
Virus structure
•The tiniest viruses are only 20nm in diameter - they are smaller than ribosomes
•Viruses are not cells
•A virus is called a DNA virus or an RNA virus depending on the kind of nucleic acid that
makes up its genome
•Viruses usually contain a single linear or circular molecule of nucleic acid, although some
can consist of multiple
•The smallest viruses have only three genes in their genome, while the largest have
several hundred to 2000
•The protein shell enclosing the viral genome is called a capsid
•The capsid may be rod shaped, polyhedral or more complex in shape
•Capsids are built from a large number of protein subunits called capsomeres, but the
number of different kinds of proteins in a capsid is usually small
•Some viruses have accessory structures that help them infect their hosts
•For instance, a membranous envelope surrounds the capsids of influenza viruses and
many other viruses found in animals
•These viral envelopes contain host cell phospholipids and membrane proteins
•Many of the complex capsids are found among the viruses that affect bacteria, known as
bacteriophages
•The first phages studied included those that infect E.coli
Viruses replicate only in host cells
•Viruses only consist of a nucleic acid surrounded by a protein coat, and therefore lack
metabolic enzymes and machinery for making proteins, such as ribosomes
•Each particular virus can infect cells of only a limited number of host species, called the
host range of the virus
•This host specificity results from the evolution of recognition systems by the virus
•Viruses usually identify host cells by a “lock and key” fit between viral surface proteins
and receptor molecules on the outside of cells
•A viral infection begins when a virus binds to a host cell and the viral genome makes its
way inside
•Once the viral genome is inside, the proteins it encodes can commandeer the host,
reprogramming the cell to copy the viral genome and manufacture viral proteins
•After the viral nucleic acid molecules and capsomeres are produced they spontaneously
self-assemble into new viruses
•The simplest type of viral replicative cycles ends with the exit of hundreds or thousands
of viruses from the infected host cell, a process that often damages or destroys the cell