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They are small: measured in nanometers.
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
That container is typically made of protein.
Viruses require cells as hosts. They have no metabolism on their own.
Viruses infect every form of life.
For instance, a virus that targets bacteria is called a bacteriophage.
- Genetic material, either DNA or RNA
- A "capsid" = a container made of protein
- An optional phospholipid envelope with glycoproteins on the outside surface
There are three:
- Helical i.e. a spiral like shape
- Polyhedral up to 20 sides
- Complex = with head & tail elements
Viruses have 2 kinds of metabolism
- Lytic, where they take over a cell
- Lysogenic, where their DNA/RNA is integrated into the host's
Viruses types by envelope
- Naked i.e. no envelope, easy for the immune system to kill.
The lytic life cycle: memorize as AESAR
- Entry & degradation of host's DNA
- Synthesis of new viruses
- duplication of virus' genetic material
- creation of new virus parts
- Assembly of parts into new viruses
- Release of new viruses
Burst size = the number of viruses
that a virus can produce before the host cell bursts.
The T4 phage makes about 200.
The lysogenic life cycle: memorize as AEIM
- Entry (no degradation of host DNA)
- Integration using integrase enzyme
- Host+virus DNA duplicated
A virus that uses the lysogenic life cycle is termed either a provirus or prophage.
What causes various proviruses to shift to the lytic cycle
is not completely understood, but in some cases stress is the stimulus.
These are newly discovered particles of RNA that are known to attack
plants, at this point. They lack both capsid and envelope, so they
are bare RNA.
This theory states that 15% of cancers are caused by viruses.
One example is HPV, which causes cervical cancer i.e. cancer of the cervix in women.
The virus is lysogenic and it inserts a destructive gene
into the tumor suppressor T53 gene. This allows a mutated oncogene
to cause a cell to become cancerous.
Well known lytic viruses
- Common cold
- AIDS mode of HIV
- Tobacco mosaic virus
Well known lysogenic viruses (proviruses)
- Hepatitis B
- Varicella-Zoster (chicken pox)
There are a few kinds of Herpes
- Herpes simplex = highly contagious, you can get it
from sharing a bottle, utensil, chapstick; touching a doorknob.
Effects are sores at point of contact e.g. mouth.
- Genital Herpes = you get it from sexual intercourse.
Effects are sores on genitals: painful.
- If you get it as a child, you become immune.
- If you get it as an adult, you will endure painful shingles.
There are 250 variants of HIV, therefore a vaccine is hard to create.
Some variants are strong, others weak.
HIV is an enveloped RNA virus that is unusual in that it also contains two useful enzymes
inside the capsid:
HIV's surface glycoprotein gp120 latches onto the CD4 receptor
of the human immune system's T4 cells.
When HIV becomes lytic, it destroys the T cells and
renders the immune system unable to defend a person
against even the common cold.
- HIV reverse transcriptase: converts RNA from HIV to cDNA
- HIV integrase: inserts the cDNA into the hosts DNA.
HIV viruses are created by budding, in which the cell membrane
of the host is used to create the envelope
of the new viruses.
HIV-resistant people have a defective T-cell receptor.
Prions are proteins that attack other proteins,
converting them into something like itself.
These then combine to form structures.
Thus they are not viruses.
Examples of prion diseases include:
- Mad Cow disease (BSE)
- Creutzfeld-Jacob disease
- Do it yourself
- Drama and Film
- Education & science
- Politics and propagandists
- Video Editing