Unit 4 bacteria and viruses pt.2
Summary of Viral Biology and Infection Mechanisms
This content provides a comprehensive overview of viruses, their structure, infection cycles, and
their biological nature, as well as related infectious agents such as viroids and prions. It also
addresses the debate regarding whether viruses are living organisms, their impact on health, and
their specificity to host cells.
Key Concepts and Definitions
Term Definition
Virus A particle composed of nucleic acid (DNA or RNA) enclosed in a protein
coat called a capsid; some viruses also have a lipid envelope.
Capsid The protein coat surrounding the viral nucleic acid; made up of subunits
called capsomeres.
Bacteriophage A virus that infects bacteria specifically.
Lytic Cycle A viral life cycle in which the virus enters the host cell, takes over its
machinery to replicate viral DNA/RNA and proteins, produces new viruses,
and causes the host cell to burst (lyse), releasing new viruses.
Lysogenic Cycle A viral life cycle where viral DNA integrates into the host genome and
replicates along with it without producing new viruses immediately,
potentially for a long time before switching to the lytic cycle.
Retrovirus A virus that begins with RNA instead of DNA; uses reverse transcriptase to
convert RNA into DNA after infection (e.g., HIV).
Viroid A small infectious agent consisting solely of a single-stranded RNA molecule
without a protein coat, interfering with host cell growth.
Prion An infectious protein that causes disease by inducing misfolding of normal
proteins, forming damaging protein clumps.
Virus Structure and Properties
● Viruses consist of DNA or RNA nucleic acid enclosed within a protein capsid.
● Some viruses have a membranous envelope with glycoproteins used to attach and enter
host cells.
, Unit 4 bacteria and viruses pt.2
● Virus sizes vary widely, e.g.:
● Tobacco mosaic virus: ~18 × 250 nm
● Adenoviruses: ~80 × 225 nm
● Influenza viruses: ~80–200 nm diameter
● Bacteriophage T4: Head ~70–90 nm diameter, tail sheath ~18 × 250 nm
Virus Type Size (nm) Notable Feature
Tobacco mosaic virus 18 × 250 Rod-shaped, infects plants
Adenoviruses 80 × 225 Non-enveloped, infect vertebrates
Influenza viruses 80–200 diameter Enveloped; responsible for flu
Bacteriophage T4 Head 70–90 diameter Complex structure; infects bacteria
● Viruses are not considered complete cells because they lack cellular components required
for independent life and reproduction.
● The protein capsid can trick host cells into allowing viral entry.
● Viruses depend entirely on host cells for replication and do not carry out metabolic processes
independently.
Viral Infection Cycles
Lytic Cycle (primarily in prokaryotes)
● Virus attaches to the host cell and injects its DNA.
● Viral DNA commandeers host machinery to produce viral components.
● New viruses are assembled inside the host.
● Host cell lyses (bursts), releasing new viruses.
● Result: Host cell death and rapid virus proliferation.
Lysogenic Cycle
● Viral DNA integrates into the host’s genome.
● Viral DNA replicates passively with host DNA during cell division.
● No immediate production of new viruses.
● Can persist for extended periods.
● Eventually triggers the switch to the lytic cycle.
Eukaryotic Virus Behavior (Notably Different)
Summary of Viral Biology and Infection Mechanisms
This content provides a comprehensive overview of viruses, their structure, infection cycles, and
their biological nature, as well as related infectious agents such as viroids and prions. It also
addresses the debate regarding whether viruses are living organisms, their impact on health, and
their specificity to host cells.
Key Concepts and Definitions
Term Definition
Virus A particle composed of nucleic acid (DNA or RNA) enclosed in a protein
coat called a capsid; some viruses also have a lipid envelope.
Capsid The protein coat surrounding the viral nucleic acid; made up of subunits
called capsomeres.
Bacteriophage A virus that infects bacteria specifically.
Lytic Cycle A viral life cycle in which the virus enters the host cell, takes over its
machinery to replicate viral DNA/RNA and proteins, produces new viruses,
and causes the host cell to burst (lyse), releasing new viruses.
Lysogenic Cycle A viral life cycle where viral DNA integrates into the host genome and
replicates along with it without producing new viruses immediately,
potentially for a long time before switching to the lytic cycle.
Retrovirus A virus that begins with RNA instead of DNA; uses reverse transcriptase to
convert RNA into DNA after infection (e.g., HIV).
Viroid A small infectious agent consisting solely of a single-stranded RNA molecule
without a protein coat, interfering with host cell growth.
Prion An infectious protein that causes disease by inducing misfolding of normal
proteins, forming damaging protein clumps.
Virus Structure and Properties
● Viruses consist of DNA or RNA nucleic acid enclosed within a protein capsid.
● Some viruses have a membranous envelope with glycoproteins used to attach and enter
host cells.
, Unit 4 bacteria and viruses pt.2
● Virus sizes vary widely, e.g.:
● Tobacco mosaic virus: ~18 × 250 nm
● Adenoviruses: ~80 × 225 nm
● Influenza viruses: ~80–200 nm diameter
● Bacteriophage T4: Head ~70–90 nm diameter, tail sheath ~18 × 250 nm
Virus Type Size (nm) Notable Feature
Tobacco mosaic virus 18 × 250 Rod-shaped, infects plants
Adenoviruses 80 × 225 Non-enveloped, infect vertebrates
Influenza viruses 80–200 diameter Enveloped; responsible for flu
Bacteriophage T4 Head 70–90 diameter Complex structure; infects bacteria
● Viruses are not considered complete cells because they lack cellular components required
for independent life and reproduction.
● The protein capsid can trick host cells into allowing viral entry.
● Viruses depend entirely on host cells for replication and do not carry out metabolic processes
independently.
Viral Infection Cycles
Lytic Cycle (primarily in prokaryotes)
● Virus attaches to the host cell and injects its DNA.
● Viral DNA commandeers host machinery to produce viral components.
● New viruses are assembled inside the host.
● Host cell lyses (bursts), releasing new viruses.
● Result: Host cell death and rapid virus proliferation.
Lysogenic Cycle
● Viral DNA integrates into the host’s genome.
● Viral DNA replicates passively with host DNA during cell division.
● No immediate production of new viruses.
● Can persist for extended periods.
● Eventually triggers the switch to the lytic cycle.
Eukaryotic Virus Behavior (Notably Different)
