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VirusesI can reproduce
inside a living cell, yet I am dormant (inactive) outside a cell
Q: Am I living or non-living?
Q: Am I a eukaryotic or prokaryotic cell?
What do you think?
What is a Virus?Viruses are
considered to be non-living organisms – however they do affect the lives of other organisms
Why non-living? Because they rely
on other organisms for survival and reproduction
Latin word for virus is poison!
Virus CharacteristicsThey are not cellular (no cytoplasm, no
membrane-bound organelles, no cell membranes!)
So – what is their structure?1. An inner core – which contains strands of
either DNA or RNA (but not both)2. An outer protective protein coat – called the
capsid, which surrounds the genetic material The capsid gives the virus its shape The shape and size of the capsid is different for
each virus and is one way viruses can be classified
3. An outer lipid envelope made from the host cell’s cell membrane (not all may have this).
Virus Characteristics Viruses are simpler than any unicellular
prokaryote Smaller in size than bacteria (5-400 nm) Only reproduce by infecting living cells
Viruses in HistoryViruses have been
written about since the 10th century.
Many Muslim doctors of the Middle Ages described viral diseases and how to best keep them from spreading.
Viruses in HistoryThe first virus identified
was in tobacco plants, the tobacco mosaic virus (TMV).
The first human virus found was that which caused Yellow Fever.
In 1915 first time scientists realized that bacteria could become infected by a virus!
By 1940, scientists began to grow and study viruses to better understand them.
Classifying VirusesViruses are classified based on the shape and
size of their capsid
Cylindrical viruses (e.g. tobacco mosaic)
Head and tail viruses (e.g. T4 bacteriophage)
Spherical viruses (e.g. HIV)
Polyhedral viruses (look like small crystals) (e.g. polio)
Bacteriophages = a category of viruses that infect and
destroy bacterial cells
Reproduction in VirusesViruses do not reproduce by cell division
(makes sense since they are not cellular)So - what do they do?
Viruses undergo replication within a host cell (i.e. to make multiple copies of themselves)
Viruses can do this in both prokaryotic and eukaryotic cells; depends on virus type
Generally a virus will have a host range – the limited number of host species, tissues, or cells that a virus or other parasite can infect
Replication cyclesA. Lytic Cycle – the replication process in
viruses in which the virus’s genetic material uses the copying machinery of the host cell to make new viruses.
Lytic Cycle Stages Stage 1: Attachment –
phage attaches to host cell wall at specific receptor sites
Stage 2: Entry – the virus injects its genetic material (RNA or DNA) into the host cell Relases enzyme to
break down cell wall Some animal viruses
enter the host cell by phagocytosis
Some plant viruses are injected into cells by insects
Lytic Cycle Stages Stage 3: Replication –
the host cell makes more viral DNA or RNA and proteins (in nucleus or cytoplasm)
Stage 4: Assembly – new viral particles are assembles
Stage 5: Lysis and Release – the host cell breaks open (lysis) and releases new viral particles
considered to be a virulent phage
Replication CyclesB. Lysogenic Cycle or
Provirus– the replication process in viruses, in which the viral DNA enters the host cell’s and becomes intergrated into the hosts DNA
Does not kill the host cell right away – therefore called temperate phage
Dormant state is called lysogeny
Lysogenic Cycle cont. Virus DNA re-enters
lytic cycle when something triggers it into taking over the host’s DNA (control center)
Now the virus becomes virulent again
Viruses and DiseaseViruses cause some
of the most common illnesses such as the common cold, the flu, cold sores, etc…
Viruses also cause some of the deadliest diseases known to man, Ebola, Avian Flu, AIDS, etc…
The virulence of a virus is its ability to cause disease.
Viruses and DiseaseAntibiotics are not
effective against viruses (only bacterial infections)
Vaccines are used to prevent viral diseases from spreading (e.g. polio)
There are now some antiviral drugs – which aim to inhibit viral development.
DNA VirusesThese usually infect
a host through a chance encounter with the virus.
These include some of the more well known viruses such as the herpes virus and any of the pox viruses such as smallpox or chicken pox.
RNA VirusesUse RNA as their
genetic material or use an RNA intermediate to replicate.
These include the Rubella virus which causes the German Measles.
Reverse transcripting virusesThese convert RNA
into DNA and incorporate it into a host cell.
Often called retroviruses because they work backwards.
HIV is the most well known of these types of viruses.
The Origin of the VirusThe origin of the modern virus is unclear. No
fossil history exists (that we know of!)Three hypotheses exist (text pg. 338)
Applications of Viruses Viruses have been used to
study the basic mechanisms of molecular biology such as DNA replication, protein synthesis, etc…
Viruses have also been used by geneticists to genetically modify organisms.
Virotherapy uses viruses to treat bacterial diseases and some forms of cancer.
Viruses are also being used as biological insecticides.
Applications of VirusesViruses are
currently being used in nanotechnology acting as tools to stop or start sequences needed in host cells.
Viruses have also been explored as a viable biological weapon. Scary thought!
End on a cuddlier, happier note…
