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32 Cards in this Set

  • Front
  • Back
Explain a Virus.
• Viruses are noncellular, obligate
intracellular parasites. Obligate
means that viruses cannot replicate
unless they enter a host cell. Some
properties are listed in Table 34.1.
T or F?
Because viruses are not organisms, they are referred to as particles or agents and are not given scientific names.
What is an epidemic?
An epidemic is a disease that infects a
large number of people at the same time.
Define virulent.
meaning that a virus tends to cause severe disease.
How does HIV cause disease?
• HIV parasitizes and destroys helper
T cells and macrophages, components
of the human immune system
(Figure 34.3).
•When the T-cell count drops, the
body is less able to fight infections.
What Is the Scope of the
AIDS Epidemic?
• AIDS has already killed 25 million
people. It is estimated that 42 million
people worldwide are infected with
HIV, and an additional 5 million are
infected each year (Figure 34.4). If
present trends continue, 80 million
more people will be infected by 2010.
T or F?
Viruses are big relative to eukaryotic or even bacterial cells.
• Viruses are very small relative to
eukaryotic or even bacterial cells
Describe Nonenveloped Viruses.
they are enclosed by just a shell of protein called a capsid.
Describe Enveloped Viruses.
they are enclosed by both a capsid and a membrane-like envelope.
-HIV is an enveloped virus.
Lytic and Lysogenic Growth
• Viruses infect their host cells in one of two ways:
lytic growth or lysogenic growth. All viruses
undergo lytic growth, but some also grow
Lytic replication cycle
viral particles are made and released from the
cell, usually resulting in the death of the host.
Lysogenic growth cycle.
viral genes are incorporated into the host's
chromosome and are transmitted to the host's
daughter cells.
Phases of the Lytic Cycle.
• Five phases are common to lytic
growth in virtually all viruses:
1. entry into a host cell,
2. replication and transcription of the viral
3. production and processing of viral
4. assembly of a new generation of virions,
5. exit from the infected cell.
How Do Viruses Enter a Cell?
• Many viruses are transmitted by insects, and some
gain entry by binding to specific cell receptors.
•Research has shown that HIV particles can enter
cells only if the virions bind to a membrane protein
called CD4 and to a second membrane protein,
called a co-receptor, in addition to CD4.
How Do Viruses Copy Their Genomes?
• Most DNA viruses copy their genomes by using their
own DNA polymerase enzyme.
•Most RNA viruses use a viral enzyme called RNA
replicase. RNA replicase is an RNA polymerase that
synthesizes RNA from an RNA template.
•In other RNA viruses, the genome is transcribed from
RNA to DNA by a viral reverse transcriptase. Reverse
transcriptase is a DNA polymerase that makes a doublestranded
complementary DNA (cDNA) from a singlestranded
RNA template. Retroviruses use reverse
T or F?
Viruses must exploit the host cell's biosynthetic machinery to make viral proteins.
_______ proteins are
synthesized on the rough ER, then inserted into the
plasma membrane (Figure 34.11a).
_____ proteins are synthesized by ribosomes in the
cytoplasm and cut into pieces by an enzyme called
protease, then assemble near the host cell's plasma
membrane (Figure 34.11b).
How Are Viruses Transmitted
to New Hosts?
• Viruses leave a host cell by budding
from the cell membrane or by bursting out
of the cell (Figure 34.13).
•This ends the infection cycle, and the viral
particles can infect new cells—either
within the same organism or in another
What Themes Occur in the
Diversification of Viruses?
• In addition to being identified as
enveloped or non-enveloped, viruses can
be categorized by the nature of their
hereditary material—the type of molecule
their genes are made of.
• The single most important aspect of viral
diversity is the variation that exists in their
genetic material.
T or F?
Viruses only have single-stranded DNA or RNA genomes.
They can also have double-stranded genomes.
Describe positive-sense viruses.
the genome contains the
same sequences as the mRNA required to produce
viral proteins.
Describe negative-sense viruses.
the base sequences in
the genome are complementary to those in viral
Describe Ambisense viruses.
they contain both positive- and negative-sense sections.
Where Did Viruses Come From?
• The escaped-genes hypothesis proposes that
viruses descended from clusters of genes that
physically escaped from bacterial or eukaryotic
chromosomes long ago.
•The degeneration hypothesis suggests that
organisms gradually degenerated into viruses by
slowly losing the genes required to synthesize
ATP and other compounds.
• Hantavirus and Ebola are examples of _______
diseases—new illnesses that suddenly affect
significant numbers of individuals in a host
•Hantavirus and Ebola were considered emerging
viruses because they had switched from their
traditional host species to a new host—humans.
Double-Stranded DNA (dsDNA) Viruses. Describe and give examples.
• The dsDNA viruses are a large group that includes smallpox
• Viral genes must enter the nucleus to be replicated, so they can
infect only cells that are actively dividing (e.g. epithelium).
• Hosts include all types of organisms, except land plants
• Pox viruses, herpesviruses, adenoviruses
RNA Reverse-Transcribing Viruses (Retroviruses). Describe.
• The genomes of the RNA reverse-transcribing viruses are
composed of single-stranded RNA. Virus particles have two
copies of their single-stranded RNA genome (diploid).
• Retroviruses contain reverse transcriptase. The enzyme
synthesizes a viral cDNA that is then made double-stranded and
integrates into the host chromosome.
• Parasitize only vertebrates, birds, fish or mammals
• Rous sarcoma virus and mouse mammary tumor virus cause cancer.
Describe Double-Stranded RNA (dsRNA) Viruses.
• Once in the host cell, the double-stranded RNA
synthesis of viral RNAs, which are then translated into viral proteins.
• Wide variety of hosts, fungi, plants, animals, bacteria
• Some are important crop viruses
Describe Negative-Sense Single-Stranded RNA
([–]ssRNA) Viruses
• Once in a host cell, a viral RNA polymerase uses the negative-sense
template to make viral mRNAs. The viral mRNAs are then translated to
form viral proteins and new negative-sense single-stranded RNA.
• Many plants and animals are hosts
• In humans, flu, mumps, measels, Ebola, Hanta, rabies
Describe Positive-Sense Single-Stranded RNA ([+]ssRNA) Viruses.
• Because the sequence of bases in a positive-sense RNA virus is the same
as mRNA, it does not need to be transcribed before proteins are
• Most of the important plant viruses, mosaic viruses
• In humans, common cold, polio, hepatitis A, C, and E
Famous Viruses in the news?
•Bird Flu
•West Nile