MICROPARA 4

• Viruses


  • Complete virus particles (virions) are very small and simple in structure, ranging from 10 to 300 nm in diameter
  • Scientists were unable to see viruses until electron microscopes were invented in the 1930s
  • A negative staining procedure, developed in 1959, revolutionized the study of viruses
• Organisms infected by viruses

  • Humans
  • Animals
  • Plants
  • Fungi
  • Protozoa
  • Algae
  • Bacterial cells
• Viruses


  • Possess either DNA or RNA, unlike living cells which possess both
  • Unable to replicate on their own, their replication is directed by the viral nucleic acid once it has been introduced into a host cell
  • Do not divide by binary fission, mitosis, or meiosis
  • Lack the genes and enzymes necessary for energy production
  • Depend on the ribosomes, enzymes, and metabolites of the host cell for protein and nucleic acid production
• Virion


  Complete virus particle consisting of a genome of either DNA or RNA, surrounded by a capsid (protein coat)
• Viruses


  • Some have an outer envelope composed of lipids and polysaccharides
  • Bacterial viruses may have a tail, sheath, and tail fibers
• Characteristics used to classify viruses

  • Type of genetic material (DNA or RNA)
  • Shape of the capsid
  • Number of capsomeres
  • Size of the capsid
  • Presence or absence of an envelope
  • Type of host that it infects
  • Type of disease it produces
  • Target cell
  • Immunologic or antigenic properties
• Categories of viruses based on genome type

  • Double-stranded DNA
  • Single-stranded DNA
  • Double-stranded RNA
  • Single-stranded RNA
• Viral genomes


  • Usually circular molecules, but some are linear (having two ends)
• Capsids of viruses


  • May be polyhedral (many sided), helical (coiled tubes), bullet shaped, spherical, or a complex combination of these shapes
  • Polyhedral capsids have 20 sides or facets, referred to as icosahedrons
  • Each facet consists of several capsomeres, so the size of the virus is determined by the size of each facet and the number of capsomeres in each
• Enveloped viruses


  • The envelope around the capsid makes the virus appear spherical or irregular in shape in electron micrographs
  • The envelope is acquired by certain animal viruses as they escape from the nucleus or cytoplasm of the host cell by budding
• Theories of virus origin

  • Coevolution theory: viruses originated in the primordial soup and coevolved with bacteria and archaea
  • Retrograde evolution theory: viruses evolved from free-living prokaryotes that invaded other living organisms, and gradually lost functions which were provided by the host cell
  • Escaped gene theory: viruses are pieces of host cell RNA or DNA that have escaped from living cells, and are no longer under cellular control
• Bacteriophages


  Viruses that infect bacteria
• Categories of bacteriophages based on shape

  • Icosahedron bacteriophages: almost spherical shape, with 20 triangular facets
  • Filamentous bacteriophages: long tubes formed by capsid proteins assembled into a helical structure
  • Complex bacteriophages: icosahedral heads attached to helical tails, may also possess base plates and tail fibers
• Bacteriophages categorized by nucleic acid type

  • Single-stranded DNA phages
  • Double-stranded DNA phages
  • Single-stranded RNA phages
  • Double-stranded RNA phages
• Virulent bacteriophages


  Always cause the lytic cycle, which ends with the destruction (lysis) of the bacterial cell
• Temperate bacteriophages


  Their DNA remains integrated into the bacterial cell chromosome, generation after generation
• Lytic cycle of bacteriophages

  1. Attachment (adsorption) of the phage to the surface of the bacterial cell

  2. Penetration: phage injects its DNA into the bacterial cell

  3. Biosynthesis: phage genes are expressed, resulting in production of viral pieces

  4. Assembly: viral pieces are assembled to produce complete viral particles (virions)

  5. Release: host cell bursts open and all of the new virions escape
• Bacteriophages are involved in lysogenic conversion and transduction, two of the four major ways in which bacteria acquire new genetic information
• Research into using bacteriophages to treat bacterial infections has been renewed due to the emergence of multidrug-resistant bacteria
• Animal viruses


  • May consist solely of nucleic acid surrounded by a protein coat (capsid), or may be more complex with an envelope and enzymes
  • The first step in multiplication is attachment (adsorption) to the cell
  • The entire virion usually enters the host cell, necessitating an uncoating step where the viral nucleic acid escapes from the capsid
  • Biosynthesis can be quite complicated depending on the virus type, and some animal viruses remain latent within the host cell for variable periods
  • Escape from the cell can involve destroying the host cell or budding
• Inclusion bodies


  Remnants or collections of viruses, often seen in infected cells and used as a diagnostic tool to identify certain viral diseases
• Important human viral diseases

  • AIDS
  • Chickenpox
  • Cold sores
  • Common cold
  • Ebola virus infections
  • Genital herpes infections
  • German measles
  • Hantavirus pulmonary syndrome
  • Infectious mononucleosis
  • Influenza
  • Measles
  • Mumps
  • Poliomyelitis
  • Rabies
  • SARS
  • Viral encephalitis
  • All human warts
• Latent virus infections


  Viruses remain dormant in the body, but can be triggered to reactivate and cause symptoms, such as cold sores and shingles
• Antibiotics do not directly target viruses, but may be prescribed to prevent secondary bacterial infections following a viral infection
• Antiviral agents


  Chemicals that interfere with virus-specific enzymes and virus production, disrupting critical phases in viral cycles or inhibiting synthesis of viral DNA, RNA, or proteins
• Oncogenic viruses


  Viruses that cause cancer
• Viral infections


  • Usually limited by the defense systems of the human body - phagocytes and antiviral proteins called interferons produced by virus-infected cells
• Shingles


  A painful nerve disease caused by a herpesvirus
• Latent viral infection


  1. Chickenpox virus remains latent in the human body for many years

  2. When the body's immune defenses become weakened, the latent chickenpox virus resurfaces to cause shingles
• Antibiotics


  Function by inhibiting certain metabolic activities within cellular pathogens, but viruses are not cells
• Antiviral agents


  Chemicals developed to interfere with virus-specific enzymes and virus production by disrupting critical phases in viral cycles or inhibiting the synthesis of viral DNA, RNA, or proteins
• Cancers caused by viruses

  • Nasopharyngeal carcinoma
  • Burkitt lymphoma
  • B-cell lymphoma (caused by Epstein-Barr virus)
  • Kaposi sarcoma (caused by human herpesvirus)
• HIV


  An enveloped, single-stranded RNA virus that is the cause of AIDS
• HIV


  • It is a member of the lentivirus genus in the Retroviridae family
  • It is able to attach to and invade cells bearing receptors that the virus recognizes, especially CD4+ cells like helper T cells
  • It can also invade macrophages and other cells with different receptors
• Mimivirus


  An extremely large double-stranded DNA virus that "mimics" bacteria and can be observed using a standard compound light microscope
• Mimivirus


  • It has a 7-nm-thick capsid with a diameter of 750 nm and an array of 80- to 125-nm-long closely packed fibers projecting outward
  • Its DNA is surrounded by two 4-nm-thick lipid membranes within the capsid
  • Its genome is at least 10 times larger than that of large viruses in the smallpox family and larger than the genome of some of the smallest bacteria
  • It possesses close to 1,000 genes, including some for functions previously thought to be exclusive to cellular organisms
• Megavirus


  An even larger double-stranded DNA virus with the largest capsid diameter (440 nm) and largest and most complex genome of all known viruses, predicted to encode over 1,000 different proteins
• More than 1,000 different viruses cause plant diseases, resulting in huge economic losses estimated to be in excess of $70 billion per year worldwide
• Viroids


  Short, naked fragments of single-stranded RNA that can interfere with the metabolism of plant cells and stunt their growth
• Prions


  Small infectious proteins that apparently cause fatal neurological diseases in animals and humans