b.sc.(micro) i em unit 3.5 viruses

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COURSE :B.SC. (MICRO)SUBJECT : ELEMENTARY

MICROBIOLOGYUNIT 3.5

Viruses

Viruses:

• are noncellular infectious agents

Virology:

• study of viruses

Virologists:

• scientists who study viruses

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General features of VirusesViruses are infectious agents with both living

and nonliving characteristics.

1. Living characteristics of viruses

a. They reproduce at a fantastic rate, but only

in living host cells.

b. They can mutate.

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…General features of Viruses2. Nonliving characteristics of viruses

They are acellular, that is, they contain no

cytoplasm or cellular organelles.

They carry out no metabolism on their own

and must replicate using the host cell's

metabolic machinery.

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Virus particles contains either DNA or RNA (not both)

Nucleic Acid is surrounded or coated by a protein shell (capsid)

Some viruses possess a membrane-like envelope surrounding the particle

…General features of Viruses

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…General Properties of viruses

Consists of 1 molecule of DNA or RNA enclosed in coat of protein

May have additional layers

Cannot reproduce independent of living cells nor carry out cell division as procaryotes and eucaryotes do

An intact viral particle is called a virion.

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The Size and Morphology of Selected Viruses

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Generalized Structure of Viruses

Viral components Nucleic acids

Capsid

Envelope

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…Generalized Structure of Viruses

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The Structure of Viruses

Virion size range is ~10-400 nm

virions : nucleocapsid

Some viruses : only of a nucleocapsid, others have additional components

Envelopes

virions having envelopes = enveloped viruses

virions lacking envelopes = naked viruses

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Introduction – Structure – Replication – Virology – Medicine - Review

A VIRUS is either DNA or RNA, that is protected by a protein coat called a CAPSID.

DNA CAPSID

VIRUS11

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How Do Viruses Differ From Living Organisms?

Viruses are not living organisms because they are incapable of carrying out all life processes.

Viruses are not made of cells

can not reproduce on their own

do not grow or undergo division

do not transform energy

lack machinery for protein synthesis

AcellularInfectious

Agent:

H1N1 Influenza

Virus

Images: Eukaryotic living organism named Kayla, T. Port; H1N1 Influenza Virus, Public Health Image Library (PHIL) #11702 From the Virtual Microbiology Classroom on ScienceProfOnline.com

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http://www.scienceprofonline.org/microbiology/what-is-a-virus.html

http://www.scienceprofonline.org/chemistry/what-are-proteins-amino-acids-peptide-bonds.html

http://phil.cdc.gov/phil/home.asp

http://www.scienceprofonline.com/virtual-micro-main.html

http://www.scienceprofonline.com/

What Are Viruses Made Of?

Nucleic acid, proteins, and sometimes, lipids.

Nucleic acid surrounded by a protective protein coat, called a Capsid.

An outer membranous layer, called an Envelope. made of lipid and protein, surrounds the capsid in some viruses.

Image: Virus Structure, Drs. Foster & Smith PetEducation.com From the Virtual Microbiology Classroom on ScienceProfOnline.com13

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http://www.scienceprofonline.com/chemistry/nucleotides-nucleic-acids-atp-rna-dna.html

http://www.scienceprofonline.com/chemistry/what-is-a-lipid-organic-chemistry-fats-phospholipids-waxes-steroids.html


http://www.peteducation.com/



Protein coat provides protection for viral nucleic acid and means of attachment to host’s cells.

Composed of protein subunits called capsomeres.

Some capsids composed of single

type of capsomere; others

composed of multiple types.

Image: Tobacco mosaic virus, US Gov; Tobacco Mosaic Virus Structure, Y tambe, Wiki

The Viral Capsid


From the Virtual Microbiology Classroom on ScienceProfOnline.com14

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http://en.wikipedia.org/wiki/File:TobaccoMosaicVirus.jpg

http://en.wikipedia.org/wiki/File:Tobacco_mosaic_virus_structure.png



The Viral Envelope

Acquired from host cell during viral replication or release; envelope is portion of membrane system of host.

Envelope’s proteins and glycoproteins often play role in host recognition.


Image: Virus Structure, Drs. Foster & Smith PetEducation.com From the Virtual Microbiology Classroom on ScienceProfOnline.com15

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http://www.peteducation.com/



Helical viruses resemble long rods that may be rigid or flexible.

The viral nucleic acid is found within a hollow, cylindrical capsid that has a helical.

viruses - cause rabies and Ebola hemorrhagic fever are helical viruses.

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Polyhedral Viruses Many animal, plant, and bacterial viruses –polyhedral

viruses.

The shape of capsid -icosahedron, a regular polyhedron with 20 triangular faces and 12 corners .

capsomeres of each face -equilateral triangle.

example –adenovirus, poliovirus.

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Complex Viruses

bacterial viruses – complicated structures : called complex viruses.

One example -bacteriophage.

Some bacteriophages : capsids to which additional structures are attached.

capsid (head): polyhedral and the tail sheath is helical.

head - the nucleic acid.

example - poxviruses, which do not contain

clearly identifiable capsids but have several coats around the nucleic acid.

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tail :hollow tube through which the nucleic acid passes during infection

T4 -largest phage.

T4 tail - surrounded by a contractile sheath, which contracts during infection of the bacterium.

end of the tail, phages : base plate and one or more tail fibers attached to it.

The base plate and tail fibers - involved in the

binding of the phage to the bacterial cell.

Not all phages have base plates and tail fibers.

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VIRAL ENVELOPES

Envelopes vary in:

Size

Morphology

Complexity

Composition

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ENVELOPE GLYCOPROTEINS• They are firmly embedded in the envelope

bilayer.

• This is facilitated by domains of host

membrane proteins called spanners.

• They can form spikes or other structures on the

outside of the virion.

• These can be used to attach to a host cell.

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Let’s look at the Defining Properties of Viruses

Viruses are parasites that invade cells Viruses have either DNA (Deoxyribose Nucleic

Acid) or RNA (Ribonucleic Acid) Viruses direct the synthesis of new virus within

a host cell. Newly made viruses infect other cells.

Introduction – Structure – Replication – Virology – Medicine - Review 23

Cell BiologyLet's review a little cell biology since viruses cannot multiply without the help of cells. The viruses enter the cell and forces the cell to make more viruses.

The cell has three main zones:


nucleus

membranecytoplasm

Cytoplasm – this is the “factory” where biochemical reactions occur.

Membrane – this double layer protects the cell and allows the cell to communicate with the outside environment.

Nucleus – this holds the DNA or genetic information about the cell.

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Genetic Material of Viruses

Show more variety in nature of their genomes than do cells.

Like cells, viral genome is a nucleic acid., but can be _____ or ____; never both.

Primary way scientists categorize and classify viruses.

Can be dsDNA, ssDNA, dsRNA, ssRNA.

May be linear and composed of several segments or single and circular.

Much smaller than genomes of cells.

Images: DNA & RNA Diagrams, BiologyCorner


From the Virtual Microbiology Classroom on ScienceProfOnline.com25

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http://www.scienceprofonline.com/chemistry/nucleotides-nucleic-acids-atp-rna-dna.html

http://www.biologycorner.com/bio1/DNA.html



DNA/RNA

DNA, deoxyribonucleic acid, is the double-stranded molecule that encodes genetic information (e.g. your hair color, height, etc.) in the nucleus of cells. The complete set of DNA in a cell is called the genome.

RNA, ribonucleic acid, is typically single stranded so that it can be read to make proteins.


DNA or RNA code for genes that defines who we are.

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GENOMIC PACKAGING

• Genome packaging has an important role in the

infection.

• Viral genomes are packaged in one of three

ways:

• Directly in the capsid-inner side of the

protein coat

• Enclosed in special proteins-nucleic acid

binding protein

• Enclosed in proteins from the host cell27

Bacteriophage


Bacteriophage is a virus that infects bacteria.

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Enveloped Viruses


Enveloped viruses are viruses which have a membrane coat surrounding the protein coat or capsid. These viruses are common in animal viruses, but are uncommon in plant viruses.

Herpes Simplex Virus.

A membrane (made of proteins) surrounds the capsid (also made of proteins) which surrounds the viral DNA.

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DISEASE VIRUSES

AIDS HIV

WartHerpes Simplex

Virus

Flu Influenza

Measles Morbillivirus .

Cancer Hepatitis B


Examples of some viral diseases:

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Who do viruses infect?

Viruses infect Bacteria These viruses are called bacteriophages

Viruses infect Plants One example is the Tobacco Mosaic Virus

Viruses infect Animals One example is the common cold

Viruses usually infect a specific host including:


Replication Phases

V - Release- Assembly of virus

DNA and protein coat into whole new viruses

- Leaving the cell

Phase I

Phase II

Phase IV Phase VPhase III


I, II, III - Viruses enter cell- Attachment to cell membrane

- Penetration inside cell

- Losing virus protein coat

IV - Replication

- Tricks cell into making more viral DNA

- Tricks cell into making viral protein coat

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The concept of a virus as an organism challenges the way we define life:

* Viruses do not breathe.

* Viruses do not metabolize.

* Viruses do not grow.

* However, they do reproduce.


How Viruses Multiply

Active viruses enter cells and immediately begin to multiply, leading to the quick death of the invaded cells.

- Viruses

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How Viruses Multiply

- Viruses

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THE INFECTION CYCLE

• The infection cycle was first worked out in

bacteriophages (bacterial viruses).

• Animal virus infections can be either lytic or

lysogenic.

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LYTIC VERSUS LYSOGENIC

INFECTION

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Reference:Images (1 to 4) : Microbiology by Gerard J. Tortora,

Christine L Case, and Berdell R. Funke

Image 5: Eukaryotic living organism named Kayla, T. Port; H1N1 Influenza Virus, Public Health Image Library (PHIL) #11702

Image 6: Virus Structure, Drs. Foster & Smith PetEducation.com

Image 7: Tobacco mosaic virus, US Gov; Tobacco Mosaic Virus Structure, Y tambe, Wiki

Image 8: Virus Structure, Drs. Foster & Smith PetEducation.com

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Image 9: http://www.google.co.in/imgres?imgurl=http%3A%2F%2Fwww.mcb.uct.ac.za%2Ftutorial%2Fvirovirion.jpg&imgrefurl=http%3A%2F%2Fwww.mcb.uct.ac.za%

Image 10: https://www.google.co.in/search?q=icosahedron&source=lnms&tbm=isch&sa=X&ei=6s-kVNuPJMugugSJk4GYDg&ved=0CAgQ_AUoAQ&biw=1366&bih=634

Image 11: Microbiology by Gerard J. Tortora, Christine L Case, and BerdellR. Funke

Image 12: DNA & RNA Diagrams, BiologyCornerImage 13:

http://www.cat.cc.md.us/courses/bio141/lecguide/unit2/viruses/adlyt.html

Images (14 to 17): Microbiology by Gerard J. Tortora, Christine L Case, and BerdellR. Funke

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