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BIOLOGY
Viruses
Figure 27.2 The three domains of life
Table 27.2 A Comparison of the Three Domains of Life
Viruses (Latin, poison)
• Are noncellular
• Size 20-200 nm
• Always have at least two parts:
--an outer capsid composed of
protein subunits
--an inner core of nucleic acid
DNA or RNA
• May be surrounded by membranous
envelope
• May also contain various proteins,
especially enzymes such as
polymerases
Viruses
• Are called obligate intracellular
parasites, which means they
cannot live outside of a living cell
• Each kind of virus usually infects
one kind of organism and/or one
kind of tissue,
– e.g. hepatitis virus only infects
liver cells
• Believed that viruses are derived
from the kind of cell they infect and
must have evolved after cells
• They can mutate and, therefore,
they evolve; e.g. flu viruses are
constantly mutating
Figure 18.x4
Hepatitis
1. Helical—spirals of many
protein units called
capsomeres
2. Icosahedron—has 20
triangular subunits
3. Bacteriophage—with
polyhedral head, tail
sheath, and tail fibers
May also have
membranous envelope
and glycoprotein spikes.
Viruses shapes:
Virus Classification
• Classification is
based on:
– Type of nucleic acid.
– Size and shape.
– Presence / absence
of outer envelope.
Viral Reproduction
• Lytic cycle may be divided into five stages:
– Attachment
– Penetration
– Biosynthesis
– Maturation
– Release
Viral Reproduction
Lysogenic Cycle
• Phage becomes a prophage that is
integrated into the host genome.
– Becomes latent, and later may reenter the
lytic cycle.
Reproduction of Animal Viruses
• After animal viruses enter the host cell,
uncoating releases viral DNA or RNA and
reproduction occurs.
– If viral release occurs by budding, the viral
particle acquires a membranous envelope.
– Retroviruses
• Contain reverse transcriptase which carries out
RNA  cDNA transcription.
Reproduction of HIV-1
Infectious Agents Smaller than a Virus
1. Viroids
• naked circular strands of RNA several
hundred nucleotides long that infect plants
• replicate in host cells using cellular
enzymes
• symptoms include abnormal development
and stunted growth in the plants
• one viroid disease has killed over 10 million
coconut palms in the Philippines
Infectious Agents Smaller than a Virus
Prions
• protein particles that can cause other proteins
in the cell to have a misshapen tertiary structure
• cause a number of degenerative brain
diseases, including scrapie in sheep, mad cow
disease, and Creutzfeldt-Jakob disease in
humans