Download Viruses Nonliving Structure Reproduction

Viruses
Nonliving
Viruses are not cells. They do not have a cell membrane or other components
of living cells.
Living host cells are required for their reproduction. Outside of the host, they
act as nonliving chemicals.
They do not metabolize or respond to stimuli.
They have genetic material and can therefore mutate and evolve.
They form parasitic relationships with living organisms; the virus benefits at
the expense of the living organism.
Structure
Viruses contain an inner nucleic acid core (genetic material) and an outer
protein coat (capsid).
The virus may also contain some enzymes.
An outer envelope (membrane) that is derived from the cell membrane of the
host cell may surround the capsid.
The envelope contains viral glycoprotein spikes.
They are usually smaller than 200 nm.
The genetic material in some viruses is DNA; in others it is RNA.
Reproduction
Specificity
Typically, a virus can enter a cell when either a part of the capsid or the spikes
in the envelope match receptors in the host cell.
Some viruses have a narrow host range and can infect only one or a few
species. For example, the smallpox virus infects only humans. Viruses with a
broad host range can infect many different species. For example, the rabies
virus is able to infect several different species of mammals.
In multicellular organisms, viruses infect specific host cell types.
Examples:
Influenza infects cells lining the respiratory tracts.
Poliomyelitis virus infects nerve cells.
Tobacco mosaic virus infects tobacco leaves.
Protein Synthesis
In a normal cell, DNA is copied to make mRNA by a process
called transcription. The information stored in mRNA is used by ribosomes to
assemble proteins from amino acids (called translation). The normal sequence
is:
DNA Viruses
When viral genetic material enters a cell, it is replicated, transcribed (mRNA is
produced) and translated (proteins are produced from the mRNA) by the host
cell. By this process, the host cell uses the genetic instructions in the virus to
make more viruses.
RNA Viruses
In some RNA viruses, the viral RNA serves as mRNA after infection.
The RNA of some viruses serves as a template to synthesize more RNA within
the host cell. Some of the replicated RNA serves as mRNA and is used to
produce proteins and some of the replicated RNA is packaged in new viral
particles.
Retroviruses are RNA viruses that use RNA as their genetic material but the
host cell must synthesize a DNA copy of the RNA before it can be transcribed
or translated.
Bacteriophages
are viruses that infect bacteria.
have 2 kinds of life cycles: lytic and lysogenic.
Lytic Cycle
The virus attaches to the bacteria cell, a viral enzyme digests away a part of the
wall and its DNA enters the host cell.
The host DNA disintegrates.
The viral DNA is transcribed and translated, producing protein coats and
enzymes.
The viral DNA is replicated.
The protein coats and DNA are assembled into new viral particles.
Lysozyme (an enzyme that digests the cell wall) causes the host cell wall to
rupture. The bacterium dies in the process.
Lysogenic Cycle
The viral DNA becomes integrated into the bacterial DNA after infection. It is
replicated along with the host DNA when the host reproduces. The viral DNA
is referred to as a prophage.
Certain environmental factors (example- UV radiation or chemicals) will
trigger the prophage to begin a lytic cycle.
Animal Viruses
Most RNA viruses and some DNA viruses that infect animals have a
membranous outer envelope.
In some RNA viruses, the genetic material is translated to produce proteins in
the host cell. In other kinds of RNA viruses, mRNA is produced using the viral
RNA as a template. The enzymes needed for this process are carried in the
virus.
Retroviruses
In retroviruses, the RNA genetic material is copied to produce DNA by an
enzyme called reverse transcriptase.
The new DNA produced from the RNA template is called cDNA.
The cDNA becomes incorporated into the host DNA. It contains the
instructions to produce the proteins needed to make virus particles.
The viral DNA incorporated within the host DNA is called a provirus.
RNA produced by transcription can serve as mRNA for the production of viral
proteins or it can become viral genome.
New viruses escape the host cell by budding.
Retroviruses and Cancer
When new viral particles are eventually built, some of the host genes may
accidentally become incorporated into the viral genetic material. These genes
may cause cancer when they are later inserted into a host chromosome.
Genes that cause cancer are called oncogenes.
Some RNA Viruses
Some RNA viruses have no DNA stage. Messenger RNA is synthesized from
an RNA template.
Viral Diseases
Some examples of viral diseases are the common cold, influenza (flu), mumps,
measles, polio, rabies, infectious hepatitis, and AIDS.
Viruses are not living and therefore viral diseases cannot be treated with
antibiotics. Antiviral drugs such as AZT function by interfering with DNA
replication.
Viral genes (oncogenes) can cause some kinds of cancer.
Vaccines stimulate the immune system to defend against certain viruses. The
use of vaccines has limited or prevented the spread of some viral diseases. For
example, smallpox has been eliminated as a result of vaccination programs.
Viroids
Viroids are naked RNA molecules that are several hundred nucleotides long.
They replicate within the host cell.
Prions
Prions are proteins that cause diseases.
Prions are thought to be a form of a normal protein that is not folded correctly.
Prions do not reproduce. Contact between a prion and the normal form of the
protein can cause the normal protein to become a prion.