By, Mackenzie Pabst
VIRUSES;
SECTION 18-1
What is a Virus?
 disease-causing,
nonliving particle
composed of an inner
core of nucleic acids
surrounded by a capsid
 A virus replicates inside
living cells called host
cells
Why are Viruses Non-Living?
 Viruses are nonliving because:
 They don't exhibit all the criteria for life
 Criteria for life includes:
 Respiration, growth and development
 All viruses can do is replicate
 Replication is making exact copies of
themselves with the help of living cells
How are Viruses Named?
 Some Viruses are
named after the disease
they cause
 Example: Rabies Virus
 Other viruses were
named for the organ or
tissue they infect.
 Example: The
Adenovirus effects
the adenoid tissue
How does a Virus attach to a
Host Cell?
 For a virus to replicate,
it must enter a host cell
 When a virus’ proteins
interlock with the
receptor site, the virus is
attached to its host cell
 The attachment process
is like two pieces of a
jigsaw puzzle fitting
together
Virus Attachment
 Each virus has a specifically shaped
attachment protein
 Viruses are species specific
 Some Viruses are cell-type specific
 Example: polio viruses normally infect only
intestinal and nerve cells.
Lytic Cycle:
A. Attachment:
The Virus attaches to the
Host Cell
B. Entry:
The Bacteriophage injects
its nucleic acid into the
bacterial cell
C. Replication :
The host’s metabolic
machinery makes the virus’
nucleic acid and proteins
Lytic Cycle:
D. Assembly:
New Virus Parts are made
E. Lysis and Release:
The host cell breaks open
and releases new viruses
•
This results in the killing
of the host cell
Another Viral Replication
Process…
Of an Enveloped Virus:
 After attachment, the
membrane of the host
cell surrounds the virus
 This produces a virusfilled vacuole inside the
host cell's cytoplasm.
 The virus bursts out of
the vacuole and releases
its nucleic acid into the
cell.
The Lysogenic Cycle
The Lysogenic Cycle:
 Begins the same way as the Lytic Cycle
 The virus attaches to the host cell and its
nucleic acid is injected into the cell.
 Then the viral DNA is integrated into the host
cell's chromosomes
 This is done instead of immediately taking over
the host's genetic material (As in the Lytic Cycle)
 This Cycle does not kill its host cell
RNA Viruses:
 Many Viruses, including HIV, are RNA viruses
 This means their only nucleic acid is RNA
 The RNA virus with the most complex
replication cycle is the Retrovirus
 The Retrovirus - uses reverse transcriptase to
make DNA from viral RNA
 the retroviral DNA is then integrated into the
host cell's chromosome
Retrovirus Cycle:
Retrovirus Cycle:
 Once inside a host cell, the retrovirus
makes DNA from its RNA with reverse
transcriptase
 Reverse transriptase produces doublestranded DNA from the viral RNA.
 The double-stranded viral DNA becomes part
of the host cell’s chromosome and it becomes
a provirus
Human Immunodeficiency Virus
(HIV)
 HIV infects white blood cells in humans
 The infected cells still function normally
 Because the viral genetic material is a provirus
that only produces a small number of new viruses
at a time
 An infected person will not appear sick
 Can infect others with the virus through body fluids
Human Immunodeficiency Virus
(HIV)
 Most people with HIV
will get AIDS
 Over time, more white
blood cells are infected
and killed
 White Blood cells are
essential to our immune
system
 Their destruction reduces
the body’s ability to
protect itself from other
viruses
Virus’ link to Cancer:
 Some viruses have
been linked to certain
cancers in humans and
animals
 Viruses can disrupt the
normal growth and
division of cells in a
host
 This causes abnormal
growth and creates
tumors
Prions:
 Are composed of
proteins but have no
nucleic acid to carry
genetic information
 Are thought to act by
causing other proteins to
fold themselves
incorrectly, resulting in
improper functioning
 Responsible for many
animal diseases
 Example: Mad Cow
Disease
Viroids:
 Composed of a single
circular strand of RNA
with no protein coat
 Shown to cause
infectious diseases in
several plants.
 Have less RNA than
most viruses
Plant Viruses:
 There are more than
400 plant viruses
 Plant Viruses can cause
stunted growth and
yield losses in their
host plants.
 The first virus to be
identified as a plant
virus was the tobacco
mosaic virus
Plant Viruses:
 Do not undergo lytic or
lysogenic phases
 Plant Viruses are named
according to their host
plant.
 Not all viral plant
diseases are fatal or even
harmful.
 The Mosaic virus, as seen
in the following picture,
cause patterns of color in
the flowers of plants.