Lesson 3: Viruses Flashcards
Viruses
A virus is a
non-cellular particle
made up of genetic
material and
protein that can
invade living cells
Viral Structure
All viruses consist of a core of nucleic acid (DNA or RNA) surrounded by a protective protein coat called a capsid.
Some viruses have an envelope that contains protein spikes (antigens) that are used to infect living cells.
Viral Specificity
Viruses tend to infect only one specific type of host, such as humans or potato plants.
However, there are some that infect a more general group of organisms such as mammals.
Ex. Rabies and Influenza
Viral Life Cycles
To reproduce, viruses must
invade, or infect a living host
cell. There are two general life
cycles of a virus
- Lytic Cycle
- Lysogenic Cycle
The Lytic Cycle 1
Special molecules on the viral tail fibers match the cell membrane proteins on the host cell.
Attachment of the virus and injection of its nucleic acid then takes place.
Usually the host cell cannot distinguish between host and viral DNA, and will
transcribe both.
The viral mRNA acts like a molecular wrecking crew, shutting down and
taking over the host cell.
The Lytic Cycle 2
Host DNA is destroyed but not
the viral DNA.
The viral DNA uses the cells
resources to construct hundreds
of virus particles.
This whole process may take as
little as 20 min.
Lytic infection follows when the
cell bursts and releases active
viruses to infect other host cells.
Lysogenic Cycle
After infection, the viral DNA inserts itself into the hosts DNA.
This combination is now called a prophage.
This may remain part of the host DNA for many generations.
The prophage may:
Block entry of other viruses to the host cell
May insert useful DNA
Prophage DNA may become active, remove itself from the host DNA and start to produce virus particles. The trigger for this change may be sudden temp. or nutrient changes.
Retroviruses
A special class of viruses that
contain RNA as their genetic
material.
Also contain the enzyme Reverse
Transcriptase that allows the
virus to “reverse transcribe” viral
RNA into DNA that is then
inserted into the host genome.
The Debate: Are Viruses Alive?
Since all viruses depend so completely for their existence on living cells, it is unlikely that they evolved first.
It is more likely that they developed from the genetic material of living cells
and have continued to evolve ever since.
Viruses are considered parasites (depend on another organism to sustain them and harm their host in the process)
Viruses are not considered living cells however they are clearly part of the system of life on Earth
