Download Characteristics of Viruses

Viruses
Characteristics of
Viruses
 Non cellular structures – do not have
membranes or any cell organelles.
Consist of an outer protein coat (capsid) and a
strand of nucleic acid either DNA or RNA :
Characteristics of
Viruses
Do not carry out metabolic reactions on their
own – require the organelles and enzymes of a
host to carry out such reactions.
Come in a variety of shapes.
Living or Non-living???
Living
 have nucleic acid
– DNA or RNA.
Have a protein
coat.
Can replicate
(inside a living cell)
and pass on genetic
information.
Non – Living
Non – cellular
Cannot reproduce by
themselves
Do not have cell
organelles
Have only 1 type of
nucleic acid
Do not feed or respire
Shapes of Viruses
Viruses may have 3 distinct shapes:
1)Rod shaped e.g. tobacco mosaic virus
2) Round e.g. influenza or mumps
Shapes of Viruses
3) Complex e.g. the T phage ( a virus that infects
bacteria)
Virus Replication
Viruses enter the host cell and use the energy
and organelles of the host to produce new viruses
and for this reason are known as obligate
parasites.
Bacteriophage - is a virus that infects bacteria
and whose replication cycle takes approx. 30
minutes e.g. T-phage (this replication cycle is
similar to the way in which most viruses infect
humans)
Stages in virus
replication
Stages in virus
replication
Stage 1: Attachment
Virus attaches to the host cell – proteins on the
virus match up with receptor sites on the host
wall.
Stage 2: Entry
Virus forms a hole in host cell. The DNA of the
virus enters the host while the protein coat stays
outside. (In the case of animal cell these protein
coats enter and are digested)
Stages in virus
replication
Stage 3: Synthesis
Host DNA is made inactive. The viral DNA
uses the host’s organelles to produce new viral
DNA and proteins.
OR
Sometimes viral DNA joins with host DNA.
When host DNA is copied so is the viral DNA
and passed on to the daughter cells. Resulting
daughter cells may behave normally or produce
new chemicals e.g. scarlet fever.
Stages in virus
replication
Stage 4: Assembly
Using these new viral molecules new viruses are
formed inside the host cell.
Stage 5: Release
The host cell bursts (Lysis) to release new virus
particles and the process repeats.
Stages in virus
replication
Stages in virus
Replication
Retroviruses: contain RNA instead of DNA and
an enzyme that converts the virus RNA to DNA.
This DNA then makes new copies of the virus
RNA and new viruses inside the host cell e.g.
HIV virus.
Medical and Economic
Importance of Viruses
Must know 2 disadvantages and 1 beneficial example of viruses
Disadvantages of viruses:
1. Viruses cause human diseases e.g. colds, flu,
measles, rabies, AIDS etc.
2.Human diseases cause medical problems
resulting in economic losses such as medical
costs and lost time at work.
3.Viruses cause animal diseases e.g. foot and
mouth and rabies.
Medical and Economic
Importance of Viruses
Disadvantages of viruses:
4.Viruses cause plant diseases e.g. tobacco,
tomato and potato mosaic diseases.
Benefits of viruses:
1. Genetic Engineering – in genetic engineering
viruses are sometimes used to transfer genes
from one organism to another. These viruses
are known as vectors.
Medical and Economic
Importance of Viruses
Benefits of Viruses:
2.Control of Infections – there is hope that
bacteriophages may be used to control some
bacterial infections.
Control of Virus
Infections
• Virus are controlled by the body’s general
defence system (e.g. skin, stomach acid etc.) and
the specific defence system (e.g. production of
antibodies.)
•Vaccinations are available for some viral diseases
e.g. MMR
•Antibiotics have no affect on viruses.
•Some drugs have been produced to interfere
with viral replication e.g. interferon, acyclovir.