 Viruses are not alive
 A virus in an obligate intracellular
parasite
 Requires host cell to reproduce
 Can be seen at magnifications
provided by the electron
microscope (they are microscopic)
 1.) Contains a single type of nucleic acid: either DNA or RNA but not
both
 2.) Has a protein coat (capsid) surrounding the nucleic acid, some
also have a lipid envelope around the capsid
 3.) multiply inside living cells by using the synthesizing machinery of
the host cell
 4.) Cause the synthesis of specialized viral structures that can transfer
the viral nucleic acid to other cells
 5.) Have a specific host range
 Usually much smaller than bacteria
 must be smaller than the cells they infect:
 20-14,000nm in length
 Virion = infectious viral particle: completely assembled with
a protein coat surrounding the nucleic acid
 All viruses are made of at least 2 parts
 Inner core of nucleic acid
 Enclosed in protein capsid
 * Some also contain lipoprotein envelope
 1.) Nucleic Acids:
 Either DNA or RNA, but not both
 Single or Double Stranded (SS or DS)
 if RNA, it can be plus sense strand (has codons) or minus/antisense
(need to make complement sense strand for translation)
 If DNA- usually double stranded
 Linear or circular
 Genome is SMALL
 Only a few genes (most have 6-10 genes)
 2. Capsid – protein coat (protein shell)
 Surrounds the nucleic acid
 protects the virion in the external environment
 Aids in transfer between host cells
 Composed of subunits called capsomeres
 some capsids have protein-carbohydrate pointed projections
called pentons
 if pentons are present they are used for attachment to the host
cell
 *3. Envelope (not all viruses)
 Function is to protect the virion
 some viruses have an envelope around the capsid consisting of lipids,
proteins and carbohydrates (cell membrane like)
 with envelope = enveloped virus
 the envelope may be coded for by the virus or taken from the host cell plasma
membrane
 some envelopes have carbohydrate-protein complexes called spikes which are
used for attachment to the host cell
 if a virus does not have an envelope it is called a non-enveloped virus,
“naked”
Capsomere
protein
 The capsid can be distinct and sometimes identifies a
particular virus. It is constructed in a highly symmetrical
manner
 Helical
 Cylindrical capsid, hollow
 Can be rigid or flexible
 Made up of a helical structure of capsomeres with the nucleic acid
wound up inside
 Examples: Rabies virus, Ebola virus, tobacco mosaic virus (TMV)
Rabies Virus
 Polyhedral
 Most are icosahedrons (icosohedral)
 20 equilateral triangle faces and made from capsomeres
 12 corners made form capsomeres called pentons which contain 5
protomers each
 Appear spherical
 Examples: Adenovirus, Polio virus
Polio virus
 Complex
 Several types of symmetry in one virus
 Unique shape
 Examples:
 Bacteriophage: capsid and accessory structure
 Pox virus: no clear capsid, just several protein layers around the
nucleic acid
Glass sculpture of pox
virus
 Replication must occur in a host cell (multiply only when
inside a living cell)
 The viral genome codes for viral structural components
and a few viral enzymes needed for processing the viral
enzymes
 Everything else is supplied by the host:
 Ribosomes, tRNA, nucleotides, amino acids, energy etc.
 The DNA or RNA of the virus takes control of the host cell' metabolic
machinery and new viral particles are produced utilizing the raw
materials from the host cell.
 Replication of viruses is studied in great detail in
bacteriophages
 Bacteriophages are viruses that infect a specific bacteria
 Two possible types of infection cycles:
 1.) Lytic cycle (virulent)
 Ends with the lysis and death of the host bacterial wall
 2.) Lysogenic cycle
 Host cell remains alive, but carries the virus in its genome
http://sites.fas.harvard.edu/~biotext/animations/lyticcycle.html
 1.) Attachment- phage contacts a bacterium (attachment to
host) and uses the tail fibers to attach to proteins on the
bacterial cell wall
 2.) Penetration/Entry- the phage injects its DNA into the
bacterium
 The phage tail releases lysozyme to break down the bacterial
cell wall
 The sheath contracts to drive the tail core through the
weakened cell wall and plasma membrane
 The DNA is injected into the bacterium through the tail core
 Uncoating- During or before penetration
 3.) Synthesis of new virus particles (Multiplication)
 Once inside, host protein synthesis is stopped
 Virus has host make proteins and nucleic acid
 Virus directs viral nucleic acid replication and transcriptions
and translation of viral genes (host’s cell transcription stops)
 This results in a pool of viral genomes and capsid parts
 4.) Assembly
 “eclipse period” – the time of viral entry
 The bacteriophage DNA and capsid spontaneously assemble
into complete virons
 5-10 hrs DNA viruses
 2-10 hrs RNA viruses
 5.) Lysis- release of virus and death of host cell
 A single virus can give rise to up to 1000 new virus particles from on
host cell
 Virions will leave bacteria (host)
 Lysozyme encoded by viral genes causes the cell wall to break
down
 The bacteria lyses releasing the virions
 Cycle will then repeat with new phages
http://sites.fas.harvard.edu/~biotext/animations/lysogeny.html
 The lysogenic phage infects the cell, but remains inactive in
a stage called lysogeny
 1.) the phage attaches to the host cell and injects DNA
 2.) the phage genome circularizes
 At this point, the phage could begin a normal lytic cycle or it can
begin the lysogenic cycle/lysogeny
 Latency- “dormant” state- unpredictability
 Viral DNA/RNA  integrated into DNA of host = hidden
DNA=provirus
 Can be reactivated in the future
 Factors that influence: stress, other viral infections, UV light
 Example: fever blisters, chicken pox, HIV 2+ years
 Embryonated eggs
 Refer to handout given in class
 Refer to handout given in class
 Refer to handout given in class