A cellular infectious agents Viruses, viroids,

1
A cellular infectious agentsViruses, viroids,
prions
Department of Applied Science King Saud
University/ Community College By Murad Sawalha
2
Characteristics of Viruses

• Complete virus particles, called virions, are
  very small and simple in structure
• Seen only by electron microscope
• No type of organisms is safe from viral
  infections
• Virus consists of a genome of either DNA or RNA,
  surrounding by a capsid (protein coat), which is
  composed of many small protein units called
  capsomeres.
• No ribosomes for protein synthesis

3
Characteristics of Viruses

• No sites of energy production
• Use another living cells for functioning and
  reproduction.
• Some viruses (called enveloped viruses) have an
  outer envelope composed of lipids and
  polysaccharides.
• Bacterial viruses may also have a tail, sheath,
  and tail fibers

4
Viruses

• 5 specific properties that distinguish viruses
  from living cells
• They possess either DNA or RNA
• They are unable to replicate on their own
• They dont divide by binary fission, mitosis, or
  meiosis.
• They lack the genes and enzymes necessary for
  energy production
• They depend on host cell for protein nucleic
  acid production

5
Viruses

• Are classified by the following characteristics
• Type of genetic material DNA or RNA
• Shape of the capsid
• Number of capsomeres
• Size of capsid
• Presence or absence of an envelope
• Type of host that it infects
• Type of disease produced
• Target cell
• Immunologic properties

6
DNA viruses herpes simplex virus (HSV)
7
RNA viruses Influenza
8
Viruses

• Origin of viruses (2 main theories)
• 1) The viruses existed before cells.
• 2) The other theory states that cells came first
  and that viruses represent ancient derivatives of
  degenerate cells or cell fragments

9
Bacteriophages

• Are the viruses that infect bacteria
• Must enter a bacterial cell to replicate
• There are 3 categories of bacteriophages, based
  on their shape
• Icosahedron bacteriophages spherical shape, with
  20 triangular faces
• Filamentous bacteriophages helical structure.
• Complex bacteriophages icosahedral heads
  attached to helical tails.
• Bacteriophages can be categorized by the events
  that occur following invasion the bacterial cell
• 1) Virulent bacteriophages
• 2) Timperate bacteriophages

10
Animal viruses

• Viruses that infect humans animals
• DNA or RNA surrounded by a protein coat (capsid)
• There are 6 steps in the multiplication of
  animal viruses
• Attachment
• Penetration
• Uncoating
• Biosynthesis
• Assembly
• Release
• Inclusion bodies are collection of viruses,
  seen in infected cells, and used as a diagnostic
  tool to identify certain viral diseases
• May be found in the cytoplasm (cytoplasmic), or
  within the nucleus (intranuclear)

11
Latent virus infections

• Infected persons harbor the latent virus in his
  body, under certain circumstances, virus become
  active and cause infection.
• Are usually limited by the defense systems of the
  human body.
• Examples
• Cold sores (fever blisters)
• Shingles

12
Antiviral agents

• Chemical materials interfere with virus specific
  enzymes virus production by either disrupting
  critical phases in viral cycles or inhibiting the
  synthesis of viral DNA, RNA, or proteins.

13
Oncogenic viruses

• Viruses that cause cancer.
• Epstein-Barr virus (a type of herpes virus)
• Human herpes virus 8
• Hepatitis B C viruses
• Human papillomavirus
• A retrovirus

14
Human immunodeficiency virus (HIV)

• The cause of acquired immune deficiency syndrome
  (AIDS)
• Is an enveloped, double stranded RNA virus.
• Retroviruses family
• Invade cells bearing receptors that the virus
  recognizes

15
Plant viruses

• Usually transmitted via insects, nematodes,
  infected seeds, cuttings, tubers, and
  contaminated tools

16
Viroids

• Smaller and less complex than viruses
• Short single stranded RNA
• Interfere with the metabolism of plant cells, and
  the growth of the plant
• Animal diseases have been discovered that are
  caused by viroids

17
Prions

• Infectious protein
• No DNA or RNA
• Cause fatal neurologic diseases in animals
• Convert normal protein molecules into
  nonfunctional ones