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VIRAL CHARACTERISTICS
1.Extremely small (20-14,000 nm in length)
2.Obligate intracellular parasites (only reproduce within a
living cell)
3.Not cells
4.Not sensitive to antibiotics
5.No independent metabolism --> few or no enzymes
6. Host range
•Spectrum of host cells that virus can infect
•Virus recognizes host cell by “lock & key” fit
between protein on outside of virus & specific
receptor sites on cell surface
•Some have broad host range
ex. Rabies  numerous mammalian species
•Some have narrow host range
a) Infect only one species
ex. Phages of E. coli
b) Infects only a single tissue type of one species
ex. Rhinovirus (cold)  upper respiratory
tract cells
ex. AIDS  specific receptors on certain
white blood cells
VIRAL STRUCTURE
1.Nucleic Acid
 Contain either DNA or RNA
 May be double-stranded or single-stranded
 Smallest viruses have only 4 genes - largest have
several hundred
 Nucleic acid is circular or linear
 Some viruses have several separate segments
ex. Influenza
2.Capsid
 Protein coat that surrounds nucleic acid of virus
 Composed of subunits called capsomeres
 Structure may be rod-shaped, helical, polyhedral, or
complex
 Most complex capsids found among bacteriophages
 Have icosohedral (polyhedral with 20
triangular facets) head which encloses the
nucleic acid
helical tail sheath with tail fibers
3.Envelope
 Membrane that covers the capsid in some viruses
 Some viral envelopes are covered by spikes
 Help virus to infect their host
Lytic & Lysogenic Cycles Diagram
Lytic Cycle
Viral replication that results in death or lyse of the host
cell
Virulent bacteriophages reproduce only by a lytic
replication cycle
Virulent Phages
•Virus that lyse (& therefore kill) their host cells
Steps of the Lytic Cycle
1.Attachment
 Phage attaches to host cell
 Attachment site on virus attaches to receptor site on
host cell
 Chemical interaction in which weak bonds are
formed between the attachment & receptors sites
2. Penetration
 Phage penetrates host cell & injects its DNA into
host cell
 Phage’s tail releases lysozyme (enzyme) which
breaks down bacterial cell wall
 Tail sheath of phage contracts & tail core is driven
thru cell wall  acts like a syringe
 Capsid remains outside the host cell
3. Biosynthesis
 Phage DNA directs synthesis of viral components
by the host cell
 Host protein synthesis is stopped by viral enzyme
produced by the host, which degrades the host DNA
4. Maturation
 Viral components are assembled into viruses
 Occurs spontaneously
5. Release
 Host cell lyses & new viruses are released
 Lysozyme, which digests cell wall, is made within
the cell
 Osmosis causes cell swell & finally burst
 Released phages then infect other host cell

**Lytic Cycle Only Takes 20-30 Minutes**
Lysogenic cycle
viral replication cycle that involves the incorporation of
the viral DNA into the host DNA
Temperate Virus
 Virus that can integrate their genome into a host
chromosome & remain latent until they initiate a
lytic cycle
Example: phage (lambda)
Steps of the Lysogenic Cycle
1. Phage attaches to host cell &injects DNA
2. Phage DNA forms circle & either begins a lytic or
Lysogenic cycle
3. During Lysogenic cycle, phage DNA integrates within
the bacterial chromosome by recombination 
prophage
*most prophage genes are inactive*
4. Lysogenic bacterium reproduces normally  also
replicates the prophage
5. Prophage may excise (“pop out”) from the bacterial
chromosome  rare spontaneously event, radiation,
chemicals
This initiates the lytic cycle.
Lysogenic Cycle
 Host cell carrying a prophage in its chromosome