Viral disease

Learning objective:

• To be able to describe the structural features of a virus.

What diseases do viruses What diseases do viruses cause?cause?

• Diseases caused by viruses include Diseases caused by viruses include chickenpox, smallpox, common cold, chickenpox, smallpox, common cold, influenza, measles, mumps, rabies, influenza, measles, mumps, rabies, polio, yellow fever.polio, yellow fever.

Viruses• Viruses are the smallest and simplest of the Viruses are the smallest and simplest of the

microbes. microbes. • They are acellular (not made of cells) They are acellular (not made of cells) • Viruses are obligate parasites who can only Viruses are obligate parasites who can only

reproduce inside host cells which get reproduce inside host cells which get damaged in the process, leading to disease. damaged in the process, leading to disease.

• Viruses are thought to have arisen from Viruses are thought to have arisen from lengths of DNA that became separated from lengths of DNA that became separated from their cells.their cells.

Living or not?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 Outside the host cell viruses are inert and called virons.and called virons.

• By taking over the host cells metabolic By taking over the host cells metabolic machinery they are able to replicate.machinery they are able to replicate.

• Reproduction is the only common Reproduction is the only common characteristic that viruses have with characteristic that viruses have with other living organismsother living organisms

1. Why can viruses be classified as living 1. Why can viruses be classified as living and non living?and non living?

2. Explain why all viruses are considered 2. Explain why all viruses are considered to be parasites.to be parasites.

Size does matter..

• Viruses are smaller than bacteria – Viruses are smaller than bacteria – about 20 – 400 nmabout 20 – 400 nm

Morphology Morphology

Viruses consist of:Viruses consist of:• A core of nucleic acid which can be DNA A core of nucleic acid which can be DNA

or RNAor RNA• A protein coat or capsidA protein coat or capsid• There is no protoplasm or cytoplasm.There is no protoplasm or cytoplasm.• The capsid is a protein coat on the very The capsid is a protein coat on the very

outside to protect the genetic material. outside to protect the genetic material. • It is made up of protein units called It is made up of protein units called

capsomeres which link together to form a capsomeres which link together to form a very geometrical shape. very geometrical shape.

Function of capsid:Function of capsid:

• Protects the nucleic acid when the virus Protects the nucleic acid when the virus is not in a host cell.is not in a host cell.

• Helps the virus to gain entry into a host Helps the virus to gain entry into a host cell and introduce the viral nucleic acid. cell and introduce the viral nucleic acid.

• Viruses may also possess an envelope Viruses may also possess an envelope and that would have come from a and that would have come from a previous host; it’s not their own. previous host; it’s not their own.

Function of envelope:Function of envelope:

• Binds to the host cell membrane.Binds to the host cell membrane.• It helps the viro-particles to fuse with a It helps the viro-particles to fuse with a

new cell., (a disguise) e.g. HIVnew cell., (a disguise) e.g. HIV

Structure Structure

• Viruses have distinct structures Viruses have distinct structures and they and they identify the cells which they attack by identify the cells which they attack by recognising specific cell surface receptors. recognising specific cell surface receptors.

• Viruses will usually only infect one species. Viruses will usually only infect one species. • Viruses which infect bacterial cells are called Viruses which infect bacterial cells are called

bacteriophages, those that infect animals bacteriophages, those that infect animals and plants are called animal and plant and plants are called animal and plant viruses.viruses.

• A complete virus particle is called a virion. A complete virus particle is called a virion. • A virion is the dormant form of a virus that is A virion is the dormant form of a virus that is

transmitted between cells. transmitted between cells. • Virions are too small to see with a light Virions are too small to see with a light

microscope and were first seen in the 1930s microscope and were first seen in the 1930s using the electron microscope. using the electron microscope.

• Once inside a host cell virions dismantle into Once inside a host cell virions dismantle into their separate parts, and the virus can be their separate parts, and the virus can be reproduced.reproduced.

All virions contain• Nucleic acid, which can be DNA or RNA, and single Nucleic acid, which can be DNA or RNA, and single

or double-stranded. or double-stranded. • Viruses are classified according to the type of Viruses are classified according to the type of

nucleic acid they contain. nucleic acid they contain. • The nucleic acid typically codes for 5-100 proteins The nucleic acid typically codes for 5-100 proteins

(by comparison, the bacterium (by comparison, the bacterium E. coli E. coli has about has about 4000 genes).4000 genes).

• A protein coat called a capsid, made of subunits A protein coat called a capsid, made of subunits called capsomeres. called capsomeres.

• If the capsid proteins are closely bound to the If the capsid proteins are closely bound to the nucleic acid, then the combination is called a nucleic acid, then the combination is called a nucleocapsid. nucleocapsid.

• Because capsids are composed of many repeating Because capsids are composed of many repeating subunits, they tend to have simple geometrical subunits, they tend to have simple geometrical shapes, such as helix or icosahedron (20 triangular shapes, such as helix or icosahedron (20 triangular faces).faces).

Some virions have very simple structures Some virions have very simple structures containing nothing else, but many virions containing nothing else, but many virions have morecomplex structures, including:have morecomplex structures, including:– Enzymes, required to replicate the viral Enzymes, required to replicate the viral

nucleic acid or incorporate it into a host.nucleic acid or incorporate it into a host.– A lipid envelope, not made by the virus A lipid envelope, not made by the virus

itself, but derived from a host cell itself, but derived from a host cell membrane.membrane.

– Matrix proteins to attach the capsid to the Matrix proteins to attach the capsid to the envelope.envelope.

– Glycoproteins to allow the virus to attach Glycoproteins to allow the virus to attach to host cells.to host cells.

Some examples of virion Some examples of virion structuresstructures

• Discovered in 1852 by Ivanovsky when Discovered in 1852 by Ivanovsky when investigating tobacco mosaic disease in investigating tobacco mosaic disease in tobacco plants.tobacco plants.

• He found that sap from a diseased plant He found that sap from a diseased plant could still cause infection even after being could still cause infection even after being put through a bacterial filter so he concluded put through a bacterial filter so he concluded there was a disease-causing organism there was a disease-causing organism smaller even than a bacterium.smaller even than a bacterium.

• Named from the Greek word meaning Named from the Greek word meaning poisonous fluid.poisonous fluid.

3 types of capsid3 types of capsid

• helical e.g. TMVhelical e.g. TMV• icosahedral e.g. HIVicosahedral e.g. HIV• complex which has a polyhedral head complex which has a polyhedral head

and a helical tail e.g. T2and 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.

BacteriophageBacteriophage

• Bacteriophages Bacteriophages are viruses that are viruses that infect bacteriainfect bacteria

• An example is the An example is the T2 virus which T2 virus which has complex has complex structures that structures that combine combine icosahedral and icosahedral and helical capsids.helical capsids.

HIVHIVProtein coatProtein coat

Nucleic acid strandNucleic acid strand

The human immunodeficiencyThe human immunodeficiencyvirus (HIV) is an enveloped virus (HIV) is an enveloped retrovirus.retrovirus.It comprises 2 copies ofIt comprises 2 copies ofsingle-stranded RNA togethersingle-stranded RNA togetherwith some enzymes, with some enzymes, surrounded by an icosahedral surrounded by an icosahedral capsid, which is in turn capsid, which is in turn surrounded by a sphere ofsurrounded by a sphere ofmatrix proteins attached to a matrix proteins attached to a lipid envelope.lipid envelope.

VIRAL DISEASE

• What is a virus?What is a virus?

• What characteristics of living things do What characteristics of living things do viruses show?viruses show?

• Do you consider viruses to be alive?Do you consider viruses to be alive?

• Give reasons to justify your answer.Give reasons to justify your answer.

• What does obligate intracellular What does obligate intracellular parasite mean?parasite mean?

• Roughly what size are viruses?Roughly what size are viruses?