Transcript Viral disease

Viral disease
Learning objective:
• To be able to describe the structural
features of a virus.
What diseases do viruses
cause?
• Diseases caused by viruses include
chickenpox, smallpox, common cold,
influenza, measles, mumps, rabies,
polio, yellow fever.
Viruses
• Viruses are the smallest and simplest of the
microbes.
• They are acellular (not made of cells)
• Viruses are obligate parasites who can only
reproduce inside host cells which get
damaged in the process, leading to disease.
• Viruses are thought to have arisen from
lengths of DNA that became separated from
their cells.
Living or not?
• Viruses lack the mitochondria necessary to
derive energy and they cannot reproduce on
their own.
• They are dependent on their host cells and
are only classed as living organisms when
they infect host cells.
• After reproducing, viruses cause their host's
destruction. Viruses are described as
obligate intracellular parasites.
• Outside the host cell viruses are inert
and called virons.
• By taking over the host cells metabolic
machinery they are able to replicate.
• Reproduction is the only common
characteristic that viruses have with
other living organisms
1. Why can viruses be classified as living
and non living?
2. Explain why all viruses are considered
to be parasites.
Size does matter..
• Viruses are smaller than bacteria –
about 20 – 400 nm
Morphology
Viruses consist of:
• A core of nucleic acid which can be DNA
or RNA
• A protein coat or capsid
• There is no protoplasm or cytoplasm.
• The capsid is a protein coat on the very
outside to protect the genetic material.
• It is made up of protein units called
capsomeres which link together to form a
very geometrical shape.
Function of capsid:
• Protects the nucleic acid when the virus
is not in a host cell.
• Helps the virus to gain entry into a host
cell and introduce the viral nucleic acid.
• Viruses may also possess an envelope
and that would have come from a
previous host; it’s not their own.
Function of envelope:
• Binds to the host cell membrane.
• It helps the viro-particles to fuse with a
new cell., (a disguise) e.g. HIV
Structure
• Viruses have distinct structures and they
identify the cells which they attack by
recognising specific cell surface receptors.
• Viruses will usually only infect one species.
• Viruses which infect bacterial cells are called
bacteriophages, those that infect animals
and plants are called animal and plant
viruses.
• A complete virus particle is called a virion.
• A virion is the dormant form of a virus that is
transmitted between cells.
• Virions are too small to see with a light
microscope and were first seen in the 1930s
using the electron microscope.
• Once inside a host cell virions dismantle into
their separate parts, and the virus can be
reproduced.
All virions contain
• Nucleic acid, which can be DNA or RNA, and single
or double-stranded.
• Viruses are classified according to the type of
nucleic acid they contain.
• The nucleic acid typically codes for 5-100 proteins
(by comparison, the bacterium E. coli has about 4000
genes).
• A protein coat called a capsid, made of subunits
called capsomeres.
• If the capsid proteins are closely bound to the
nucleic acid, then the combination is called a
nucleocapsid.
• Because capsids are composed of many repeating
subunits, they tend to have simple geometrical
shapes, such as helix or icosahedron (20 triangular
faces).
Some virions have very simple structures
containing nothing else, but many virions
have morecomplex structures, including:
– Enzymes, required to replicate the viral
nucleic acid or incorporate it into a host.
– A lipid envelope, not made by the virus
itself, but derived from a host cell
membrane.
– Matrix proteins to attach the capsid to the
envelope.
– Glycoproteins to allow the virus to attach
to host cells.
Some examples of virion
structures
• Discovered in 1852 by Ivanovsky when
investigating tobacco mosaic disease in
tobacco plants.
• He found that sap from a diseased plant
could still cause infection even after being
put through a bacterial filter so he concluded
there was a disease-causing organism
smaller even than a bacterium.
• Named from the Greek word meaning
poisonous fluid.
3 types of capsid
• helical e.g. TMV
• icosahedral e.g. HIV
• complex which has a polyhedral head
and a helical tail e.g. T2
Tobacco mosaic virus (TMV)
• A coil of RNA surrounded by a helical
capsid.
Adenovirus
• A single stranded DNA surrounded by an
icosahedral capsid.
Bacteriophage
• Bacteriophages
are viruses that
infect bacteria
• An example is the
T2 virus which
has complex
structures that
combine
icosahedral and
helical capsids.
HIV
Protein coat
Nucleic acid strand
The human immunodeficiency
virus (HIV) is an enveloped
retrovirus.
It comprises 2 copies of
single-stranded RNA together
with some enzymes,
surrounded by an icosahedral
capsid, which is in turn
surrounded by a sphere of
matrix proteins attached to a
lipid envelope.
VIRAL DISEASE
• What is a virus?
• What characteristics of living things do
viruses show?
• Do you consider viruses to be alive?
• Give reasons to justify your answer.
• What does obligate intracellular
parasite mean?
• Roughly what size are viruses?