Download Chapter 13: Characterizing and Classifying Viruses, Viroids, and

Chapter 13: Characterizing and Classifying Viruses, Viroids, and Prions
Characteristics of Viruses
A virus is a minuscule, acellular, infectious agent having one or several pieces
of nucleic acid (DNA or RNA, never both).
They have no plasma membrane (some have a membrane like envelope),
cytosol, or organelles.
They are not capable of metabolic activity on their own, outside of the cell,
called a virion.
Virions consist of a protein coat, a capsid, that surrounds the nucleic acid core.
The capsid or envelope (if present) provides protection and recognition, allows
for binding to host cells.
Viruses can be classified by their genetic material, cells they attack, size, nature
of capsid coat, their shape, and the presence or absence of an envelope.
Genetic Material
1. dsDNA2. ssDNA3. ssRNA4. dsRNAHosts of Viruses
-Most viruses infect only particular host’s cells (liver, lung, heart, etc)
-Some viruses are generalists, can infect many kinds of cells in many
different hosts (rabies).
-Viruses attack every type of living cell, first viruses found in tobacco
plants
Size of Viruses
-Smallest viruses are about 24 nm while the largest are 500 nm (smallest
bacteria)
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Nature of Capsid
-CapsomeresViral Shapes
-There are three basic shapes of viruses
Viral Envelope
-Enveloped viruses acquire their envelope from the host cell during viral
replication or release.
Classification of Viruses
All viruses can be placed in a family, but only 3 viral orders have been
established.
Family names are usually derived from special characteristics or important
members of the family.
-Picornaviridae = very small RNA virus
-Hepadnaviridae = DNA virus that causes hepatitis B
Species names are not Latinized, but use common English designations
Viral ReplicationLytic replication usually consists of five stages
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-Burst time-Burst size*Follow the steps in figure 13.8 to see the lytic cycle of the T4 bacteriophage*
Lysogenic replication- similar to lytic, but includes an additional portion.
*Follow the steps in figure 13.11 to see the lysogenic cycle of the Lambda phage*
Replication of animal viruses- very similar to bacteriophages, but differ in the
presence of envelopes around some viruses.
-There are three mechanisms of entry for animal viruses:
1. direct penetration2. membrane fusion3. endocytosis-
Synthesis of animal viruses- remember these two questions:
How is mRNA synthesized?
What molecule serves as a template for nucleic acid replication?
**See Biosynthesis of DNA and RNA Viruses Handout**
Assembly and Release of Animal Viruses
-Most DNA viruses assemble in the nucleus, whereas most RNA viruses
assemble in the cytosol.
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-Bacteriophages replicate much quicker than animal viruses; 25 min to 24
hrs.
-Enveloped animal viruses released via budding, where the mature virions
are enveloped by some membrane (nuclear, endoplasmic, or plasma) as
they are released from the cell.
-Infected host cells that gradually release enveloped viruses are called
persistent infections, host cell can live for a while as viruses are slowly
being released.
-Naked animal viruses can be released by exocytosis or cause lysis and
death of the cell.
Latency of Animal Viruses
-Latent animal viruses are similar to lysogenic bacteriophages, but differ in
that not all latent animal viruses are incorporated into host genome,
whereas lysogenic bacterial viruses always do.
-Proviruses become a permanent part of the host chromosome, induction
(excision of viral genome) does not happen.
Role of Viruses in Cancer
Multicellular animal cell division is under strict genetic control
-neoplasia-tumorSome tumors are benign; remain in one place and are not generally harmful.
Some are malignant; invade other tissues and travel throughout the body,
producing new tumors (metastisis)
Malignant tumors are also called cancers; which rob normal cells of space and
nutrients, and can cause pain.
Proto-oncogenes play a role in cellular division. When proto-oncogenes are
repressed, no cancer results.
Various factors can contribute to inactivation of oncogene repressors or
activation of oncogenes:
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Viruses can cause cancer in several ways
Culturing Viruses in the Laboratory
Viruses must be cultured in order to conduct research and develop vaccines and
treatments. Three main ways: whole organisms, embryonated eggs, and cell
cultures.
1. Culturing Viruses in Whole Organisms- can be in bacteria or plants and
animals.
2. Culturing Viruses in Embryonated Chicken Eggs
3. Culturing Viruses in Cell (Tissue) Culture
Other Parasitic Particles: Viroids and Prions
Viroids- extremely small, circular pieces of RNA that are infectious and
pathogenic in plants.
Prions- proteinaceous infective agents, contain no nucleic acid.
Cellular PrPPrion PrP-
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