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Viruses
Characteristics of Viruses
• Virus – a small infectious agent of genetic material encased in protein
• Not made of cells
• Uses the host cell’s energy and machinery to copy itself
• Cannot reproduce on their own
• Non-living, not considered biotic
• Cannot metabolize their own energy
• Not considered microorganisms
• Different structure than cells
• Smaller than cells
• Contain few specialized structures
• Genetic material can be DNA or RNA
• Capsid – the protein coat encasing the genetic material of a virus
• Primary Types
• Helical
• Ex) Tobacco mosaic
• Polyhedral
• Ex) Adenovirus
• Enveloped
• Lipid layer from host cell
• Bacteriophage – a virus that
infects bacteria
• Complex virus
• Structure: protein tail with fibers
• Irregularly shaped
• Contain double-stranded DNA
Biology 15.1 – Viruses
Viruses
Reproduction of Viruses
• Parts of a virus enter the host cell
• Metabolic machinery is hijacked
• Viral DNA or RNA instructs the host cell to make copies
of the virus
• Viral proteins and genetic material are reproduced
• Lytic pathway – a type of viral reproduction where the virus
lyses, or breaks open the host cell membrane in order to
release the newly replicated viruses
• New viruses leave host cell to infect other nearby cells
• Lysis – viruses burst the cell membrane as they leave
• Results in death of host cell
• Lysogenic pathway – a type of viral reproduction
involving the incorporation of the viral genes into the
host cell chromosomes
• Host cell passively becomes genetic viral factory
• Viral genes copied each time cell replicates via
mitosis
Biology 15.1 – Viruses
Viruses
Diseases Caused by Viruses
• Helper T cells – type of white blood cells that play an
important role in the immune system
• HIV (human immunodeficiency virus) – a virus which
causes AIDS and results in the loss of T cell and immune
system functioning
• Helper T cells become useless in fighting disease
• AIDS (acquired immune deficiency syndrome) – a
syndrome characterized by the loss of the immune system
and caused by HIV, the human immunodeficiency virus
• Die from secondary infections
• No vaccine
• Limiting and avoiding transmission of virus can
prevent infection
• HIV exits by budding from the host cell
• Envelope of virus fuses to plasma membrane of host cell
• Virus exits cell wrapped in host cell membrane
• Initial infection: number of helper T cells drops
• Large amount of HIV genome produced
• Clinical latency: virus enters lysogenic pathway
• Viral genomes lay mostly dormant
• Lytic cycle resumes
• Steady decrease in helper T cells
• More HIV viruses are produced
• More immune cells infected
• When enough helper T cells are destroyed, the patient is
diagnosed with AIDS
Biology 15.1 – Viruses
Viruses
• Influenza – a viral disease commonly known as “the flu”
• Symptoms of fever, fatigue, respiratory infections
• May cause death
• Caused by a variety of influenza viruses
• Strains mutate often
• Receiving seasonal vaccine can help prevent
• Washing hands frequently and avoiding contact with those infected can reduce transmission
• Common cold – an infection caused by a variety of viruses, most commonly rhinoviruses
• Symptoms similar to the flu but less intense
• Currently no vaccine
• Reduce transmission by washing hands frequently and avoid contact with those infected
• Hepatitis A – a virus that causes inflammation of the liver
• Produces a jaundice, or yellow, appearance of skin and eyes
• Virus is carried through infected food or water
• Vaccine can prevent
• Transmission can be limited by washing hands and food and using clean water
Biology 15.1 – Viruses