Download Virus -Consists or a nucleic acid surrounded by a protein coat

Virus
Bacteriophages
-Consists or a nucleic acid surrounded by a
protein coat
-Smaller then ribosomes, the tiniest viruses are
about 20macross
-Genomes can be double- or single stranded DNA
or RNA
-The capsid is a protein stranded shell that
surrounds the genetic material
-Some viruses also have viral envelopes that
surround the capsid and aid the viruses in
infecting their host
-Phages: are viruses that infect bacterial cells
Viruses reproduce only in host cells
-Viruses have a limited host range. This means
that they can infect only a very limited variety of
hosts.
Ex: Human cold virus infects only cells of the
upper respiratory tract
-Viral reproduction occurs only in the host cells;
Two variations have been studied in viruses:
--Lytic Cycle
--Lysogenic Cycle
Lytic Cycle
-The lytic cycle ends in the death of the host cell
by rupturing it (lysis)
-In this cycle, the virus injects its DNA into a host
cell and takes over the host cell’s machinery to
synthesize new copies of the viral DNA as well as
the protein coats. This self-assemble, and the
host cell is lysed, releasing multiple copies of the
virus.
Lysogenic Cycle
-In the Lysogenic cycle the virus’s DNA becomes
incorporated into the host cell’s DNA and is
replicated along with the host cell’s genome
-The viral DNA is known as a prophage
-Under certain conditions the prophage will
enter into the lytic cycle
Retroviruses
-Are RNA viruses that use the enzyme reverse
transcriptase to transcribe DNA from an RNA
template
-The new DNA then permanently integrates into
a chromosome in the nucleus if an animal cell
-The host cell continually transcribes the viral
DNA into RNA that may be used to synthesize
viral proteins or may be released from the host
cell to infect more cells
Ex: HIV
RNA viruses have highly efficient replicative
capabilities that allow for rapid evolution and
acquisition of new phenotypes because RNA
viruses lack replication error-checking
mechanisms, and thus have higher rates of
mutation