Viruses

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Viruses

• The Ultimate Parasite

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• Characteristics of Viruses. . .
• They are not cellular
• They cannot carry on metabolic activities
  independently
• They contain either RNA or DNA- not both (true
  cells contain both RNA and DNA)
• They do not do protein synthesis - they lack the
  ribosomes and enzymes necessary for the process
• They reproduce only within the cells they infect

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• Classification of viruses. . .
• They are not cells - or considered to be living
• They are not classified in any of the kingdoms
• No classification system for viruses has been
  agreed upon
• Viruses are usually grouped according to size,
  shape, presence or absence of an outer envelope,
  and whether they contain DNA or RNA and whether
  it is single-stranded or double-stranded
• They are sometimes classified according to the
  diseases they cause or their mode of transmission

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Viral Structure. . . A virus is a tiny particle
(virion) consisting of a core of DNA or RNA
surrounded by a protein coat (capsid).
The capsid (protein coat) is constructed of
capsomeres.
Bacteriophages are viruses that infect bacterial
cells. They are used a great deal in research.
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A virion is a single, infective virus particle.
The envelope is a membranous layer surrounding
some viruses. It contains proteins, lipids,
carbohydrates and traces of metals. The nucleic
acid core is either RNA or DNA. Bacteriophages
are viruses that infect bacteria. These phages
are used widely in viral research. They are a
good model to work with. Virulent (lytic) viruses
cause lytic infections. They cause the host cell
to lyse. Temperate (lysogenic) viruses remain
relatively dormant within the host and do not
kill the host cell. Lysogenic viruses may become
lytic.
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• Viral Replication in Lytic Infections. . .
• Attachment - phage attaches to receptor sites on
  bacterial cell wall.
• Penetration - nucleic acid is injected through
  the cell membrane into the cytoplasm of the host
  cell the capsid remains outside the cell.
• Replication - the virus takes over the hosts
  metabolic machinery and replicates its own
  macromolecules viral genes contain all
  information necessary to produce new viruses.
• Assembly - viral components are assembled into
  new viruses.
• Release - lysozyme produced by viruses degrades
  the host cell wall, causing release of about 100
  phages.

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Lytic Cycle

• Attachment
• Insertion
• Replication of viral parts
• Assembly
• Release

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Lysogenic Infections. . . When viral DNA is in
the lysogenic mode it is called a prophage. In
lysogenic infections, the temperate viruses
integrate their DNA into the host cells DNA.
When the bacterial DNA replicates, the viral DNA
prophage also replicates. Lysogenic Conversion is
the display of new properties by bacterial cells
containing temperate viruses toxins are only
produced by many disease-causing bacteria when
infected with a phage. Diphtheria, Scarlet
Fever, and Botulism are all lysogenic bacteria
toxins.
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Lysogenic Cycle

• Attachment
• Insertion
• Integration
• Every new cell is infected
• May convert to a lytic cycle

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One form of coexistence is budding. There are a
few animal and bacterial viruses that release new
viruses slowly without harming the host cell.
The host cell carries on its own metabolic
activities, using some energy to produce new
viruses. The new viruses appear to exit the cell
in a process that may be the reverse of
penetration.
An example is the herpes virus
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Plant Viruses and Viroids The tobacco mosaic
virus is one of several viruses that have certain
characteristics. They consist of only an RNA
core surrounded by a protein capsid. They are
elongate and lack an outer envelop. They attach
to the hosts ribosomes and are translated like
mRNA. Plant viruses cause serious agricultural
losses and cures are not known. Infected plants
are commonly burned. Virus resistant strains are
in the process of being developed.
http//webs.wichita.edu/mschneegurt/biol103/lectur
e11/lecture11.html
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Viroids are infective agents that are smaller and
simpler than viruses. They consist of a short
strand of RNA (250-400 nucleotides). (This
should be enough to code for a medium sized
protein). They do not have a protein coat. The
hosts enzymes are used to replicate the viroids
RNA. They are generally found in the nucleus and
cause disease by either interfering with intron
splicing or by interfering with the regulation of
the hosts genes. As far as we know, they only
infect plants (potato spindle tuber disease and
the stunting of chrysanthemums). They are
suspected to cause some animal diseases.
Avocado sunblotch viroid
http//www.ncbi.nlm.nih.gov/ICTVdb/Ictv/fs_avsun.h
tm
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Viruses that infect Animals There are hundreds of
different viruses that attack humans and other
animals. Virtually every type of cell is
susceptible to viral infection. Most viruses
cannot survive very long outside of a living
cell. Viruses may be quite specific due to
receptor sites and binding on surfaces. Some
viruses are not only specific to the organism
that they infect, but also to the tissue that
they infect. (Polio virus is specific to the
spinal cord, throat, and intestinal
cells.) Viruses are associated with cancer. The
virus, itself may contain oncogenes or may not
contain oncogenes, but may activate cellular
oncogenes. http//www.cdc.gov/std/hpv/STDFact-HPV-
vaccine.htm Oncogenes are genes which trigger
the growth process in non-growing tissue. It is
believed that proto-oncogenes are a normal part
of our genetic make-up. They can be triggered to
mutate to oncogenes by viral infection.
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Retroviruses are RNA containing viruses that also
contain reverse transcriptase. HTLV - retrovirus
linked to certain leukemias. HIV - the retrovirus
that causes AIDS
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DNA viruses have not been isolated from human
tumors, but several are linked with human
cancers Epstein-Barr virus - a type of
herpesvirus thought to cause Burketts lymphoma
(a lymphatic cancer), infectious mononucleosis,
and nasopharyngeal carcinoma among persons of
Chinese ancestry. Herpes simplex virus type 2 -
linked with cervical cancer. Hepatitis B virus -
associated with liver cancer. DNA viruses may
play a role in other cancers also (Hodgkins
Disease, breast cancer, Kaposis sarcoma).
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• The infection cycle of HIV - the AIDS-causing
  virus. . .
• The HIV virus responsible for AIDS attaches to a
  CD4 receptor protein on the surface of a T4
  lymphocyte cell, and enters the cell by
  endocytosis.
• The viral RNA is released into the cells
  cytoplasm.
• A DNA copy is made of the virus RNA (with the
  assistance of reverse transcriptase).
• The DNA copy enters the cells chromosomal DNA.
• After a long period (an average of 8 years), the
  virus genes initiate active transcription.
• Both HIV RNA and HIV proteins are made.
• Complete HIV virus particles are assembled.
• Some cells are lysed, releasing free HIV, whereas
  others bud out HIV by exocytosis.

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Prions - these are protein-like infectious
particles. They are even smaller than a viroid.
They consist of only glycoproteins with no
nucleic acids. Prions polymerize in infected
tissue. Mad Cow disease and Chronic Wasting
disease are forms of Spongiform encephalopathy.
Prions have recently been found in the meat of
deer with Chronic Wasting disease this is bad
news! http//www.fieldandstream.com/fieldstream/
hunting/article/0,13199,454228,00.html
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Treating viral infections. . . Vaccines can help
prevent viral infections Antibiotics kill
bacteria - not viruses Effective antiviral drugs
are being developed. http//pathmicro.med.sc.edu/l
ecture/vaccines.htm http//www.netpharmacy.co.nz/
tamiflu.html Acyclovir, used in the treatment of
genital herpes, is the most effective antiviral
drug to date it doesnt cure but it speeds
healing. More recent concern about The Bird flu
and possibility of pandemic infection has caused
stockpiling of other anti-viral
medications http//www.mayoclinic.com/health/bird
-flu/DS00566
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• Theories on the Origin of Viruses. . .
• Ancestors of viruses were primitive free living
  heterotrophs in the primordial sea. As free
  nutrients were depleted, they switched to a
  parasitic mode.
• Viruses evolved from cellular ancestors and
  became highly specialized parasites. In the
  process, they lost their cellular components.
• Viruses are bits of nucleic acids which escaped
  from cellular organisms. Some are from plants
  and some are from animals. This would help
  explain host specificity. (Most likely).

http//www.biologyjunction.com/virus_notes_b1.htm