3. A virus is an obligate intracellular parasite containing genetic
material surrounded by protein
• Virus particles (virions) themselves do not grow or undergo
division.
• Viruses rely on host cells to provide the energy and materials
needed for replicating their genomes and synthesizing their
proteins.
• Most viruses are too small to be seen directly with a light
microscope.
4. Viruses infect all types of organisms
Animals
Plants
bacteria and archaea.
Viruses are not really living.
They are simply pieces of genetic code in a container that
chemically connect to a living cell, inject that genetic material
mechanically, and whose genes take over the cell's metabolic
processes.
5. Viral Genomes
All cells contain double-stranded DNA genomes.
Viruses have either DNA or RNA genomes.
Virus genomes can be classified according to
whether the nucleic acid in the virion is DNA or RNA
whether the nucleic acid is single- or double-stranded, linear,
or circular.
viruses whose genome consists of DNA follow the central
dogma of molecular biology (DNA→RNA→ protein)
RNA viruses are exceptions to this rule
6. General characteristics of viruses
Living characteristics of viruses
• Viruses belong to biology because they possess genes, replicate,
evolve, and are adapted to particular hosts, biotic habitats, and
ecological niches.
Nonliving characteristics of viruses
• They are acellular
• They carry out no metabolism on their own and must replicate
using the host cell's metabolic machinery.
• viruses possess either DNA or RNA but not both.
• viruses contain only one type of nucleic acid
• They are totally dependent on a host cell for replication
• Viral components must assemble into complete viruses (virions)
7. Reasons for studying viruses
1.Some viruses cause disease
Viruses are important agents of many human diseases, ranging
from the trivial (e.g. common colds) to the lethal (e.g. rabies),
viruses also play roles in the development of several types of
cancer.
As well as causing individuals to suffer, virus diseases can also
affect the well-being of societies.
Smallpox had a great impact in the past and AIDS is having a
great impact today.
8. 2. Some viruses are useful
• Phage typing of bacteria :a phenotypic method uses
bacteriophages for detecting & identifying single strain of bacteria.
• Sources of enzymes: A number of enzymes used in molecular
biology are virus enzymes.
• Pesticides: Some insect pests are controlled with baculoviruses.
• Anti-bacterial agents: used to treat some bacterial infections of
humans.
3. Virus studies have contributed to knowledge
• Much of the basic knowledge of molecular biology, cell biology
and cancer has been derived from studies with viruses.
9. • Laboratory cultivation of viruses
viruses lack metabolic machinery of their own and are totally
dependent on their host cell for replication, they cannot be
grown in synthetic culture media
Animal viruses are normally grown in animals, embryonated
eggs, or in cell cultures where in animal host cells are grown
in a synthetic medium and the viruses are then grown in these
cells.
Viruses are found wherever there is life
The origin of viruses is unclear because they do not form
fossils
They are on the borderline between life and non-life.
