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Chapter 19 – Viruses
Definitions:
 Virus – particles of nucleic acid, protein (and in some
cases lipids) that can reproduce only by infecting living
cells
 Virulent – virus causes a disease immediately
 Temperate – virus does not cause a disease immediately
Structure (of a typical virus):
 Core composed of genetic material
o Either DNA or RNA
 Core surrounded by a protein coat
o Called a capsid
Characteristics:
 Not considered cells (or alive), must reproduce via
host cell
 Are parasitic – cause harm to host cell for its own
benefit
 Can be polyhedral, rod shaped, round
2 Major Classifications:
 DNA viruses – DNA codes directly for RNA which
produces viral proteins (ex. Chicken pox)
 RNA viruses – “Retroviruses” – RNA makes DNA
which then makes viral proteins (ex. HIV)
Most commonly studied…
 Bacteriophage (aka phage) – virus that infects
bacteria
 Reproduction – 2 methods
o Lytic Cycle – virus enters cell, makes copies
of itself, and causes cell to burst or lyse
(“break open”)
1. Adsorption – virus attaches to a host
cell
2. Entry – capsid contracts, injecting DNA
into bacterial cell
3. Replication – viral DNA takes over,
directing the cell to make more viral
DNA and proteins
4. Assembly – new viruses are put together
5. Release – cell wall is digested and new
virus particles are released which can
then infect other cells
o Lysogenic Cycle – virus embeds its DNA into
the DNA of the host cell and is replicated
along with the host cell’s DNA