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Ch 24
Characteristics of Viruses
 Viruses are not alive and they are not cells.
 Viruses ARE – Nonliving particles that cause disease (Flu,
HIV, colds, warts)
 Do not exhibit all criteria of life such as growth and
development.
 Once inside a host cell…they exhibit two characteristics of
life
 They replicate/reproduce inside a host cell.
 Heredity & are able to mutate.
 Viruses are named after the diseases they cause or the
tissues/organs they infect.
 bacteriophage –viruses that infect bacteria cells
Structure of
Viruses
 Capsid-outer protein coat
 Inner core- DNA or RNA
contains instructions for
copying viruses.
 The tail and tail fibers help
viruses bind to the host
cell
 Envelope – outer coat with
a structure similar to the
plasma membrane. Only
on some viruses.
Bacteriophage
Kinds of viruses
How do viruses attack?
 Viruses recognize and attach to a receptor site on a host
 a virus latches onto a host cell with its “tails”.
 Viruses are species and cell specific. This prevents the spread of
disease. Viruses just attack one particular kind of cell. (HepB & Liver Cell)
 Viruses inject their genetic material DNA/RNA into the host cell.
 An enveloped virus is engulfed by the host cell then burst the
vacuole to release nucleic acid into the host
Lytic cycle
 Viruses take over the metabolic activities of the host cell.
 The host cell’s energy, enzymes, and organells are used to
replicate viral genes and new virus particles are produced.
 Once the DNA of the virus is inside the cell, it takes control of it and forces it,
using its resources, to produce more of the same virus DNA and parts of the virus.
 The new virus particles burst out of the host cell and then infect
other cells.
 When most of the cell is used up to produce more virus material, it becomes is so
weakened that it bursts like a soap bubble, releasing the new replicated viruses
which immediately seek out new host cells to invade and destroy.
 5 Step Process:
 A. Attachment
 B. Entry
 C. Replication
 D. Assembly
 E. Release
Attachment
Lysis and
Release
Entry
Assembly
Replication
Lysogenic Cycle
 Replication cycle in which the virus’s nucleic acid
becomes part of the host cell’s genetic material.
 Viral DNA that is integrated into the host cell’s
chromosomes is called a provirus or temperate virus.
 The host cell will continue its activities. (Growth &
Replication)
 Virus replicates along with the host cell.
 Lysogenic cycle can continue for years until the
provirus/temperate virus is activated and enters a lytic
cycle.
 Examples: HSV1 – Cold Sores; HSV2 – Genital
Herpes; HepB; Chicken Pox/Shingles
Lysogenic Cycle
Viruses exit host cell
and infect other cells.
 Lysis-viruses burst from cell killing the cell in the
process
 Exocytosis-secreted from cell by active transport
Examples of Proviruses
the virus that causes chickenpox
reactivates to cause shingles
Retrovirus
 Virus that contains the genetic material RNA
rather than the more common DNA
 Viral RNA is converted into DNA by reverse
transcriptase.
 When a retrovirus infects a cell, it injects its RNA
into the cytoplasm of that cell along with the
reverse transcriptase enzyme.
 Retroviruses then become provirus.
HIV
Lysogenic Stage
 Provirus, retrovirus
 Affects White Blood Cells
 Released into the bloodstream by exocytosis
 WBC’s fight disease; decrease in WBC’s leads to
the inability to resist disease.
 People do not appear sick and pass to others
 Onset of AIDS when cells enter lytic cycle and
WBCs are killed
Tumor viruses
(RETROVIRUS)
 Cause normal cells to become tumorous
 Warts – Human Papilloma Virus
 Cancer
 Human Popilloma Virus – Cervical Cancer
 Hepatitis B virus-liver cancer
 Epstein-Barr Virus – Burkits Lymphoma
 Human T-Lymplotrophic Virus - Lukemia
Plant Viruses
 400 identified plant viruses
 1000 plant diseases
 All are not harmful
 Some cause changes in coloration.
 Tobacco mosaic virus causes yellow spots on
tobacco plants
 Other plant viruses
cause
patterns of color
on some plants
Diseases caused by viruses
 Rhinovirus-common cold
 Herpes zoster virus-chicken pox
 Polio virus-polio (paralysis)
 HIV/AIDS
 Measles
 Mumps
 Hepatitis
 Herpes