VIRUSES are entities that:
contain a single type of nucleic acid, either DNA or RNA
contain a protein coat (sometimes enclosed itself by an
envelope of proteins, lipids, and carbohydrates) that
surrounds the nucleic acid
multiply inside living cells by using the synthesizing
machinery of the cell
cause the synthesis of specialized structures that can
transfer the viral nucleic acid to other cells
History
1886: Adolf Mayer showed that Tobacco mosaic disease
was transmissible from a diseased plant to a healthy
plant
1892: Dmitri Iwanowski filtered the sap of diseased
Tobacco plants through a porcelain filter designed to
retain bacteria
1935: Wendel M. Stanley isolated the tobacco mosaic
virus (Tobamovirus),making it possible for the first time
to carry out studies on pure virus
Viral Size
with the aid of electron microscope
range from 20 to 14,000 nm in length
Host Range
depends on the spectrum of host cells the virus can
infect
determined by the virus’ requirements for its specific
attachment to the host cell and the availability within
the potential host cell
Outer surface of the virus must chemically interact with
the specific receptor sites on the host cell surface
bacteriophages or phages = infect bacteria
General Morphology
Basis: capsid structure with the use of electron microscope
and a technique called X-ray crystallography
Helical Viruses
resemble long rods that may be rigid or flexible
hollow, cylindrical capsid that has a helical stucture
Ex. causing rabies and Ebola hemorhagic Fever
Viral Structure
Nucleic Acid
either DNA or RNA, but never both
can be single-stranded or double-stranded Polyhedral Viruses
can be linear or circular but can be in several separate Capsid shape is icosahedron having 20 triangular faces
segments (influenza virus) and 12 corners
total amount varies from a few thousand nucleotides (or Many-sided
pairs) to as many as 250,000 nucleotides Ex. Adenovirus; poliovirus
Capsid
protein coat that surrounds the nucleic acid
determined by the viral nucleic acid
accounts for most of the mass of the virus
each composed of protein subunits called capsomeres
arrangement of capsomeres is a characteristic of a
particular type of virus
Envelope
outer covering that surrounds some capsid
usually consists of some combination of lipids, proteins,
and carbohydrates
may or may not be covered with spikes (carbohydrate-
protein complexes that project from the envelope Enveloped Viruses
surface) roughly spherical
presence of spikes are reliable characteristic for Ex.
identifying viruses o Influenzvirus (enveloped helical virus)
nonenveloped viruses are those not covered by an o Herpex simplex virus (enveloped polyhedral virus)
envelope
with complicated structures
Ex. Bacteriophage; poxvirus
Complex Viruses
Phages multiply by 2 mechanisms:
Taxonomy of Viruses Lytic Cycle and Lysogenic Cycle
Symptomatology is the oldest classification of viruses;
however it is not scientifically acceptable. Lytic Cycle or Replication
ends with the lysis and death of the host cell
International Committee on the Taxonomy of Viruses (ICTV) Lysis is the destruction of a cell by the rupture of the plasma
-has grouped the viruses into families based on: membrane, resulting in a loss of cytoplasm. In disease, it is a
a) nucleic acid type gradual period of decline.
b) strategy for replication The replication cycle of a virus usually results in the death and
c) morphology lysis of the host cell. Because the host cell undergoes lysis near
the end of the cycle, this is called lytic replication.
How to name viruses?
Genus: –virus 5 Stages:
Family names: -viridae (1) Attachment of the virion to the host cell
Order names: -ales (2) Entry of the virion or its genome into the host cell
2 Approved Orders (3) Synthesis of nucleic acids and viral proteins by host cell’s
Nidovirales (Ex. Coronavirus) enzymes and ribosomes
Mononegavirales (viruses with one negative strand of (4) Assembly of new virions within the host cell
RNA) (5) Release of the new virions from the host cell
Viral species is a group of viruses sharing the same genetic Lysogenic Replication/ Lysogenesis
information and ecological niche. process in which a bacteriophage enters a bacterial cell, inserts
Viral species are designated by descriptive common into the DNA of the host, and remains inactive.
names. the phage is then replicated every time the host cell replicates its
Ex. Human immunodeficiency virus 1 (HIV-1) chromosome
later, the phage may leave the chromosome
Viral Replication the host cell remains alive
Virion – a complete, fully developed viral particle the phages are called temperate phages or lysogenic phages
Viral enzymes are almost entirely concerned with replicating or Viruses and Cancer
processing viral nucleic acid. Enzymes needed for protein 1908: Wilhelm Ellerman and Olaf Bang, virologists working in
synthesis, ribosomes, tRNA, and energy production are supplied Denmark, were first to demonstrate the relationship between cancers
by the host cell and are used for synthesizing viral proteins, and viruses by founding out that leukemia could be transferred to
including viral enzymes. healthy chickens by cell-free filtrates that contained viruses
1911: F. Peyton Rous found that a chicken sarcoma (cancer of Transformation is the changing of a normal cell into a cancerous
connective tissue ) can be similarly transmitted cell.
1972: A human cancer-causing virus was discovered and isolated by Prions
American bacteriologist Sarah Stewart an infectious protein
‘proteinaceous infectious particle’
(1) Most of the particles of some viruses infect cells but do not named by the American neurobiologist Stanley Prusiner who
induce cancer proposed that infectious proteins caused a neurological disease
(2) Cancer might not develop until long after viral infection in sheep called scrapie in 1982
(3) Cancers do not seem to be contagious, as viral diseases are the infectivity of scrapie-infected brain tissue is reduced by
treatment with proteases but not by radiation
Normal Cells into Tumor Cells include 9 animal diseases: Ex. ‘mad cow disease’ that emerged
Oncogenes are cancer-causing alterations to cellular DNA that in cattle in Great Britain in 1987
affect the parts of the genome all 9 animal diseases are neurological diseases called spongiform
Oncogenes were first identified in cancer-causing viruses and encephalopathies because large vacuoles develop in the brain
were thought to be a part of the normal viral genome if an abnormal prion protein (PrPSc) enters a cell, it changes a
American microbiologists J. Michael Bishop and Harold E. normal prion protein to PrP which now can change another
Varmus received the 1989 Nobel Prize in Medicine for proving normal PrP, resulting in an accumulation of the abnormal PrPSc
that the cancer-inducing genes carried by viruses are actually
derived from animal cells
Oncogenes can be activated to abnormal functioning by a variety of Viroids
agents: short pieces of naked RNA, only 300 – 400 nucleotides long,
Mutagenic chemicals with no protein coat
High-energy radiation infectious RNA
Viruses (oncogenic viruses or oncovirus) pathogens only of plants since they do not code for any proteins
annually, infections by viroids, such as potato spindle tuber
Both DNA- and RNA-containing viruses are capable of inducing viroid, result in losses of millions of dollars from crop damage
tumors in animals.
