Concept 1: A virus consists of a nucleic acid surrounded by a protein coat
Viruses are smaller and simpler than cells, lacking structures and metabolic
machinery.
Viruses rely on host cells for reproduction and metabolic activities.
Structure of Viruses
Viruses are infectious particles consisting of nucleic acid enclosed in a protein coat
(the capsid), and some have a membranous envelope derived from the host cell's
membrane.
Viral genomes can consist of different types of nucleic acid and can be organized as
a single linear or circular molecule or multiple molecules.
Viral envelopes are membranous outer layers that surround the capsids (a
protein shell around a virus genome). These envelopes are derived from the host
cell's membranes and contain host cell phospholipids and membrane proteins, as
well as viral proteins and glycoproteins.
Concept 2: Viruses replicate only in host cells
Viruses lack metabolic enzymes and protein-making machinery, relying on host cells
for replication. They are obligate intracellular parasites.
Host specificity of infection is determined by interactions between viral surface
proteins and receptor molecules on host cells.
General Features of Viral Replicative Cycles
Viral infection starts when a virus binds to a host cell and its genome enters the cell.
Different viruses use various mechanisms to enter host cells, such as injection,
endocytosis, or fusion.
Once inside, the viral genome takes control and reprograms the cell to replicate the
viral genome and produce viral proteins.
DNA viruses use the host cell's DNA polymerases, while RNA viruses use virally
encoded RNA polymerases to replicate their genomes.
The viral components self-assemble into new viruses after replication.
The release of viruses from infected cells can cause cellular damage and symptoms
associated with viral infections.
Released viruses can infect additional cells and spread the infection.
, Replicative Cycles of Phages
Phages are well-understood viruses that can replicate through the lytic cycle or the
lysogenic cycle.
The lytic cycle leads to the death of the host cell, while the lysogenic cycle allows
replication of the phage genome without killing the host.
A virulent phage replicates only through the lytic cycle, while temperate phages
can use both modes of replication.
In the lytic cycle, the phage's DNA enters the host cell, and the phage genes turn
the cell into a phage-producing factory, leading to cell lysis.
In the lysogenic cycle, the phage DNA integrates into the bacterial chromosome as
a prophage, and the phage genome remains mostly silent within the bacterium.
The switch from the lysogenic to the lytic mode can be triggered by environmental
signals.
Viruses are smaller and simpler than cells, lacking structures and metabolic
machinery.
Viruses rely on host cells for reproduction and metabolic activities.
Structure of Viruses
Viruses are infectious particles consisting of nucleic acid enclosed in a protein coat
(the capsid), and some have a membranous envelope derived from the host cell's
membrane.
Viral genomes can consist of different types of nucleic acid and can be organized as
a single linear or circular molecule or multiple molecules.
Viral envelopes are membranous outer layers that surround the capsids (a
protein shell around a virus genome). These envelopes are derived from the host
cell's membranes and contain host cell phospholipids and membrane proteins, as
well as viral proteins and glycoproteins.
Concept 2: Viruses replicate only in host cells
Viruses lack metabolic enzymes and protein-making machinery, relying on host cells
for replication. They are obligate intracellular parasites.
Host specificity of infection is determined by interactions between viral surface
proteins and receptor molecules on host cells.
General Features of Viral Replicative Cycles
Viral infection starts when a virus binds to a host cell and its genome enters the cell.
Different viruses use various mechanisms to enter host cells, such as injection,
endocytosis, or fusion.
Once inside, the viral genome takes control and reprograms the cell to replicate the
viral genome and produce viral proteins.
DNA viruses use the host cell's DNA polymerases, while RNA viruses use virally
encoded RNA polymerases to replicate their genomes.
The viral components self-assemble into new viruses after replication.
The release of viruses from infected cells can cause cellular damage and symptoms
associated with viral infections.
Released viruses can infect additional cells and spread the infection.
, Replicative Cycles of Phages
Phages are well-understood viruses that can replicate through the lytic cycle or the
lysogenic cycle.
The lytic cycle leads to the death of the host cell, while the lysogenic cycle allows
replication of the phage genome without killing the host.
A virulent phage replicates only through the lytic cycle, while temperate phages
can use both modes of replication.
In the lytic cycle, the phage's DNA enters the host cell, and the phage genes turn
the cell into a phage-producing factory, leading to cell lysis.
In the lysogenic cycle, the phage DNA integrates into the bacterial chromosome as
a prophage, and the phage genome remains mostly silent within the bacterium.
The switch from the lysogenic to the lytic mode can be triggered by environmental
signals.
