Download Bio-261--Chapter-13-Animal-Viruses

Animal viruses/other infectious agents.
Medgar Evers College
Prof. Santos
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• Animal viruses are divided into a number of
families, whose name end in –viridae.
• Examples of RNA and DNA animal viruses that
infect vertebrates are
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RNA viruses
1- picornaviradae
8- paramyxoviridae
2-caliciviridae
9-orthomyxoviridae
3-togaviridae
10-bunyaviridae
4- flaviviridae
11-arenaviridae
5-coronaviridae
12-reoviridae
6-rhabdoviridae
13-birnaviridae
7- filoviridae
14-retroviridae
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DNA viruses
1- hepadnaviridae
2- parvoviridae
3-papillomaviridae
4-polyomaviridae
5- adenoviridae
6- herpesviridae
7- poxviridae
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Reproductive cycle of animal viruses involved in infections
Steps are
1- attachment
2- penetration and uncoating
3- synthesis of protein and replication of nucleic acid
4- assembly
5- release
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Attachment
• Many glycoprotein receptors are on the surface of the cell
membrane to which the viral attachment proteins will bind
to.
• Example. HIV must bind to 2 key molecules on the cell
surface before it can enter the cell. These are CCR5 and
CD4 receptors.
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Penetration and uncoating
Naked viruses
the virus enters by the process of endocytosis in which
the cell membrane surrounds the viral particle and
forms a vesicle. After being engulfed, the virus
dissolves the vesicle and its released into the
cytoplasm.
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Enveloped viruses can enter the host by one of
two mechanisms;
1-Fusion with the host membrane
2- Endocytosis
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Entry following membrane fusion
• The envelope of the virus fuses with the cell membrane after
attachment to a host cell receptor. This is enhanced by fusion
proteins on the surface of the virus.
• Following fusion, the nucleocapsid is released directly into the
cytoplasm.
• The nucleic acid separates from the protein coat.
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Entry by endocytosis
• The host plasma membrane surrounds the
whole virion and forms a vesicle.
• The envelope of the virion then fuses with the
plasma membrane of vesicle.
• Nucleocapsid is released.
• Separation of nucleic acid from the capsid.
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Nucleic acid replication/protein synthesis
• Involves transcription or the production of
messenger RNA from either DNA or RNA.
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Assembly
• The capsids and genome are assembled
together.
• Most RNA viruses are assembled in the
cytoplasm while most DNA viruses are put
together in the nucleus.
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Release
• Enveloped viruses- budding
• Naked virus- are released when cell dies due
to the virus triggering apoptosis
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2 main categories of relationships between virus and host
1- Acute
2- Persistent
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Acute
• Short lasting
• The Immune system eventually clears the virus
from the body
• Unlike a phage infecting a bacteria during a
lytic infection, the host can survive.
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Persistent infection. 2 types
1. Latent infection in which a symptomless
period is followed by reactivation of the virus.
The agent can’t be detected until the disease
is activated.
•
Examples are Herpes simplex 1, Herpes
simplex 2, and Chickenpox (varicella).
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2. Chronic infection- an infection that can be
demonstrated at all times.
• Disease symptoms may be present or absent
during an extended period of time or may
develop late.
• In the carrier state the infectious agent is
constantly produced and can be detected in the
bloodstream.
• Examples are Hepatitis B, Hepatitis C, and
Congenital Rubella.
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Viroids and Prions: The Simplest Infectious Agents
• Viroids
– Are circular RNA molecules that infect plants
and disrupt their growth.
– Consists of a single strand of RNA
– The viroid RNA is circular and resistant to
nucleases.
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• Prions
– Are slow-acting, virtually indestructible
infectious proteins that cause brain diseases in
mammals.
– Propagate by converting normal proteins into
the prion version.
Prion
Original
prion
Many prions
Normal
protein
New
prion
Figure 18.13
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• Prions can cause infections in sheep, humans,
cows, deer, elk, and the mink.
• Most agents affect the central nervous system.
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