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Viruses: Key Ideas
• Why is a virus not
considered a living
organism?
• What two structures are
characteristic of
viruses?
• What are two ways that
a virus can reproduce?
Is a Virus Alive?
• Viruses are not considered living because they are
missing key characteristics of living organisms.
• Viruses do have genetic material, but they cannot
reproduce on their own.
• Viruses reproduce by infecting cells. Viruses use
the cell’s ribosomes, ATP, enzymes, and other
molecules to make more viruses.
Is a Virus Alive?, continued
• Viruses do not grow. Instead, they are
assembled into their full size within a cell.
• Viruses do not carry out any metabolic
activities, do not have any cytoplasm or
organelles, and do not maintain homeostasis.
Viral Structure
• The structure of a virus is relatively simple with
two structures that are characteristic of all viruses.
• All viruses have nucleic acid and a capsid. A
capsid is a protein covering that surrounds the
nucleic acid core in a virus.
• In addition to a capsid, viruses may have an
envelope. An envelope is a membranelike layer
that covers the capsid of some viruses.
• Viruses that infect bacteria also have tail fibers.
Types of Viruses
Plant Viruses
They are similar to animal viruses in most basic
characteristics but they can also be markedly different.
Most plant viruses have RNA as the genetic material.
Tobacco
Mosaic
Virus
Enteroviruses
cause many common illnesses such as polio,
meningitis, encephalitis, common cold, and tonsillitis,
Rotavirus
Viral gastroenteritis is the second most common
clinical disease in developed countries
Bacteriophages
viruses that attack bacteria
Virus Reproduction
• A viral infection begins when the genetic material
of a virus enters a host cell.
• Once inside the cell, a virus can reproduce by two
different processes.
• Viruses can reproduce by a lytic life cycle and a
lysogenic life cycle.
The Lytic and Lysogenic Life Cycle