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

Viruses have both living and non-living properties. They contain nucleic acids surrounded by a protein coat called a capsid, and some have an additional lipid envelope. Viruses come in various shapes and sizes, and infect bacteria, plants, and animals. They are able to replicate by entering host cells and using the host's cellular machinery to produce new virus particles, which then exit and infect new host cells.

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1 COURSE :B.SC. (MICRO) SUBJECT : ELEMENTARY MICROBIOLOGY UNIT 3.5
Viruses Viruses: • are noncellular infectious agents Virology: • study of viruses Virologists: • scientists who study viruses 2
General features of Viruses  Viruses 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. 3
…General features of Viruses 2. 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. 4
 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 5
…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. 6
The Size and Morphology of Selected Viruses 7 1
Generalized Structure of Viruses Viral components  Nucleic acids  Capsid  Envelope 8 2
…Generalized Structure of Viruses 9 3
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 10
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 VIRUS 11 4
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 Acellular Infectious 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 12 5
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.com 13 6
 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 What Are Viruses Made Of? From the Virtual Microbiology Classroom on ScienceProfOnline.com 14 7
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. What Are Viruses Made Of? Image: Virus Structure, Drs. Foster & Smith PetEducation.com From the Virtual Microbiology Classroom on ScienceProfOnline.com 15 8
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. 16 9
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. 17 10
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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. 19
 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. 20
VIRAL ENVELOPES Envelopes vary in:  Size  Morphology  Complexity  Composition 21
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. 22
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: Introduction – Structure – Replication – Virology – Medicine - Review nucleus membrane cytoplasm 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. 24
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 What Are Viruses Made Of? From the Virtual Microbiology Classroom on ScienceProfOnline.com 25 12
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. Introduction – Structure – Replication – Virology – Medicine - Review DNA or RNA code for genes that defines who we are. 26
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 cell 27
Bacteriophage Introduction – Structure – Replication – Virology – Medicine - Review Bacteriophage is a virus that infects bacteria. 28
Enveloped Viruses Introduction – Structure – Replication – Virology – Medicine - Review 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. 29
DISEASE VIRUSES AIDS HIV Wart Herpes Simplex Virus Flu Influenza Measles Morbillivirus . Cancer Hepatitis B Introduction – Structure – Replication – Virology – Medicine - Review Examples of some viral diseases: 30
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: Introduction – Structure – Replication – Virology – Medicine - Review 31
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 Introduction – Structure – Replication – Virology – Medicine - Review 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 32 13
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. Introduction – Structure – Replication – Virology – Medicine - Review 33
How Viruses Multiply Active viruses enter cells and immediately begin to multiply, leading to the quick death of the invaded cells. - Viruses 34 14
How Viruses Multiply - Viruses 35 15
THE INFECTION CYCLE • The infection cycle was first worked out in bacteriophages (bacterial viruses). • Animal virus infections can be either lytic or lysogenic. 36
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LYTIC VERSUS LYSOGENIC INFECTION 38 17
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 39
Image 9: http://www.google.co.in/imgres?imgurl=http%3A%2F%2Fwww.mcb.uc t.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=i sch&sa=X&ei=6s- kVNuPJMugugSJk4GYDg&ved=0CAgQ_AUoAQ&biw=1366&bih=634 Image 11: Microbiology by Gerard J. Tortora, Christine L Case, and Berdell R. Funke Image 12: DNA & RNA Diagrams, BiologyCorner Image 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 Berdell R. Funke 40