Download Lecture 18

VIRUSES - General Principles;
Focus on Bacterial viruses
•  Non-living obligate intracellular
parasites
•  OR the simplest living things there are
(depends upon your definition of “life”) Viruses
•  They ALWAYS contain nucleic acid (DNA or
RNA) and a protein coat (the “capsid”), and
SOMETIMES have an envelope made of lipids,
carbohydrates, and proteins (basically similar in
structure to an enhanced plasma membrane, but
with a DIFFERENT FUNCTION!)
•  Viruses are very specific regarding the types of
cells they can infect; they generally infect certain
types of cells in a particular species. This is their
HOST RANGE.
•  Viruses can be seen by electron microscopy
ONLY and range from 20 to 1,000 nM in length.
Viral Structure
•  A VIRION is a mature, infective particle consisting of
nucleic acid and protein coat (and envelope depending upon
the species).
•  Viruses can have many arrangements of nucleic acids as
their genomes. They may have either RNA or DNA (never
both), and the nucleic acid may be single or doublestranded, and linear or circular.
•  The protein coat is referred to as the CAPSID, which
consists of subunits called capsomeres.
•  If the virus has an envelope, spikes are usually present
(carbohydrate-lipid complexes whose function we will
discuss later.)
General Morphology
•  HELICAL VIRUSES (capsomeres are assembled in a
helical configuration) usually resemble long rods, and their
capsids are hollow cylinders surrounding the nucleic acid.
•  POLYHEDRAL viruses are many-sided; usually the capsid
is an icosahedron (20 triangular faces).
•  ENVELOPED viruses may be helical or polyhedral, and
acquire their envelope by taking part of the plasma
membrane of the host cell as they “bud” from the cell (leave
by exocytosis). If a virus lacks an envelope, it is said to be
“naked”. Enveloped viruses are usually animal viruses.
•  COMPLEX viruses have a polyhedral head with a complex
tail attached - bacteriophages are complex viruses.
Taxonomy of viruses
•  Based on
–  Type of nucleic acid, life-cycle strategy, and
morphology.
•  Viruses are not assigned a place in the “normal” overall
classification system; however, they are generally
grouped into families, genera, and species. (Exactly how
depends upon who you ask!)
•  A viral species is a group of viruses sharing the same
genetic information and ecological niche.
•  Viral species are not given binomial names, they are
given normal names (like influenza virus, HIV) that do
not have to be underlined or italicized.
Taxonomy Examples
•  RNA VIRUSES
–  Species: HIV (Human Immunodeficiency Virus) •  Family: Retroviridae; Genus: Lentivirus
–  Species: Poliovirus
•  Family: Picornaviridae; Genus: Enteroviridae –  Species: Rabies Virus
•  Family: Rhabdoviridae; Genus: Lyssavirus
•  DNA VIRUSES
–  Species: HPV (Human papillomavirus)
•  Family: Papovaviridae; Genus: Papillomavirus
–  Species: HSV (Herpes simplex virus)
•  Family: Herpesviridae; Genus: Simplexvirus
Viral cultures
•  To grow a virus in the lab, its host cells
must be present. •  It is easiest to grow bacteriophages; phages
are incubated on a lawn of bacteria and
plaques develop where they are multiplying.
•  Animal viruses may require a living host in
order to grow; otherwise, cell cultures of
host cells can be used.
Viral replication strategies:
“life cycles ”
•  Viruses do not have enzymes for key metabolic
reactions (e.g. protein synthesis or energy
production).
•  Therefore, they must take over a host cell, and use
its cellular machinery to produce viral enzymes
and other components. Viral infection turns a cell
into a factory for producing more viruses.
Focus on Bacteriophages:
Viruses that infect bacteria
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Discovered in 1915
Bacteriophage means “Bacteria-eater”
Term is often shortened to “phage”
Almost all carry their genomes as double-stranded
DNA.
•  Almost always have a “complex” morphology.
•  Most widely studied are the viruses of E. coli,
including the “T-even” phages and phage lambda.
Bacteriophages: Lytic and
Lysogenic Cycles
•  LYTIC CYCLE: A phage causes the lysis and death of its
host bacterium as it replicates.
•  T-even phages, (T2 and T4 are widely studied), use the
lytic cycle.
•  In the LYSOGENIC CYCLE, a phage incorporates its
nucleic acid into the host cell chromosome and remains
dormant for a period of time.
•  Phage lambda is a popular example of this type. These
are called TEMPERATE phages.
Stages in the lytic cycle
•  Attachment - Sites on the bacteriophage tail fibers
attach to receptor sites on the bacterium.
•  Penetration - The tail sheath contracts to force the
tail core through the cell wall, and phage DNA
enters the cell (the capsid stays outside!!)
•  Biosynthesis - transcription, translation, and
replication of viral DNA occurs.
•  Maturation - New Phage DNA and capsids are
assembled into virions.
•  Release - Phage lysozyme destroys the cell wall,
the cell bursts, and the new virions are released.
Stages in the lysogenic cycle
•  Attachment/ Penetration - same as in lytic cycle
•  Viral DNA recombines with the bacterial
chromosome to form a PROPHAGE.
•  The prophage replicates with the bacterial DNA,
and will continue to do so until something (an
adverse environment for example) triggers it to
excise itself; it then continues with the steps of the
lytic cycle.
•  Thus, the steps specific to the lysogenic cycle can
be viewed as additional steps added into the middle
of the lytic cycle.