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Introduction to Virology

Arun   Geetha Viswanathan
Arun   Geetha Viswanathan
Arun Geetha ViswanathanPhD Student at Rajiv Gandhi Centre for Biotechnology

General Characters and Classification of Viruses. Includes ICTV classification and Baltimore classification of viruses. A brief explanation of the Viral structure and Lifecycle.

Introduction to Virology

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Virology
General Characters and
Classification of Viruses
A piece of bad news
wrapped up in
protein.
- Peter Medawar, British
immunologist and Nobel laurate
Attribution-NonCommercial-ShareAlike 4.0
International (CC BY-NC-SA 4.0)
What are Viruses?
• They have no cell nucleus, organelles, or cytoplasm.
• Viruses show property of living things only inside a living cell
• Out side the cell they behave like a nonliving thing
• They are called obligate intracellular parasites
• virus particles contain only one kind of nucleic acid—either DNA
or RNA but never both
• Because they can't reproduce by themselves (without a host),
viruses are not considered living.
• they're very small, much smaller than the cells of living things,
and are basically just packages of nucleic acid and protein.
A virus is an tiny, infectious particle that can reproduce only
by infecting a host cell. Viruses "commandeer" the host cell
and use its resources to make more viruses, basically
reprogramming it to become a virus factory.
The structure of a virus
Viruses vary a ton in their sizes, shapes, and life cycles.
Viruses do, however, have a few key features in common. These include:
• A protective protein shell, or capsid
• A nucleic acid genome made of DNA or RNA, tucked inside of the capsid
• A layer of membrane called the envelope (some but not all viruses)
Virus capsids
• The capsid, or protein shell, of a virus is made up of many protein molecules.
• The proteins join to make units called capsomers, which together make up the
capsid.
• Capsid proteins are always encoded by the virus genome, meaning that it’s the
virus (not the host cell) that provides instructions for making them.
The capsids of some viruses are relatively simple and are made from multiple copies of a
single protein.
• Canine parvovirus, a very small virus that infects dogs, has a capsid made from 60
copies of the same capsid protein.
• The capsid is organized into 12 capsomers, each of which is made from 5 capsid
proteins.
• The capsids of other viruses are more complex and consist of multiple copies of
several different proteins
Virus capsids
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Introduction to Virology

  • 1. Virology General Characters and Classification of Viruses A piece of bad news wrapped up in protein. - Peter Medawar, British immunologist and Nobel laurate Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)
  • 2. What are Viruses? • They have no cell nucleus, organelles, or cytoplasm. • Viruses show property of living things only inside a living cell • Out side the cell they behave like a nonliving thing • They are called obligate intracellular parasites • virus particles contain only one kind of nucleic acid—either DNA or RNA but never both • Because they can't reproduce by themselves (without a host), viruses are not considered living. • they're very small, much smaller than the cells of living things, and are basically just packages of nucleic acid and protein.
  • 3. A virus is an tiny, infectious particle that can reproduce only by infecting a host cell. Viruses "commandeer" the host cell and use its resources to make more viruses, basically reprogramming it to become a virus factory.
  • 4. The structure of a virus Viruses vary a ton in their sizes, shapes, and life cycles. Viruses do, however, have a few key features in common. These include: • A protective protein shell, or capsid • A nucleic acid genome made of DNA or RNA, tucked inside of the capsid • A layer of membrane called the envelope (some but not all viruses)
  • 5. Virus capsids • The capsid, or protein shell, of a virus is made up of many protein molecules. • The proteins join to make units called capsomers, which together make up the capsid. • Capsid proteins are always encoded by the virus genome, meaning that it’s the virus (not the host cell) that provides instructions for making them.
  • 6. The capsids of some viruses are relatively simple and are made from multiple copies of a single protein. • Canine parvovirus, a very small virus that infects dogs, has a capsid made from 60 copies of the same capsid protein. • The capsid is organized into 12 capsomers, each of which is made from 5 capsid proteins. • The capsids of other viruses are more complex and consist of multiple copies of several different proteins Virus capsids
  • 7. Capsids come in many forms 1. Icosahedral – Icosahedral capsids have twenty faces, and are named after the twenty-sided shape called an icosahedron. 2. Filamentous – Filamentous capsids are named after their linear, thin, thread-like appearance. They may also be called rod-shaped or helical. 3. Head-tail –These capsids are kind of a hybrid between the filamentous and icosahedral shapes. They basically consist of an icosahedral head attached to a filamentous tail. Virus capsids
  • 8. Virus envelopes In addition to the capsid, some viruses also have a lipid membrane known as an envelope. Virus envelopes can be external, surrounding the entire capsid, or internal, found beneath the capsid. • Typical bilayer • Viruses with only a capsid and no envelope are known as naked, or nonenveloped, viruses. • projections referred to as spikes may or may not extend from the viral envelope. • These surface projections are glycoproteins that serve to attach virions to specific receptor sites on host cell surfaces. • In certain viruses the possession of spikes causes various types of red blood cells to clump, or hemagglutinate— a property useful in viral identification
  • 9. Virus genomes • All viruses have genetic material (a genome) made of nucleic acid. • may use either RNA or DNA, both of which are types of nucleic acid. • double-stranded DNA, double-stranded RNA, single-stranded DNA, or single- stranded RNA • Viral genomes also come in various shapes, sizes, and varieties, though they are generally much smaller than the genomes of cellular organisms. • They typically range from 2,000 to several hundred thousand nucleotides in length (E. coli has a genome that's 4.6 million nucleotides long)
  • 10. What is a viral infection? • A viral infection means that many viruses are using your cells to make more copies of themselves. • The viral lifecycle is the set of steps in which a virus recognizes and enters a host cell, "reprograms" the host by providing instructions in the form of viral DNA or RNA, and uses the host's resources to make more virus particles. 1. Attachment. The virus recognizes and binds to a host cell via a receptor molecule on the cell surface 2. Entry. The virus or its genetic material enters the cell 3. Genome replication and gene expression. The viral genome is copied and its genes are expressed to make viral proteins 4. Assembly. New viral particles are assembled from the genome copies and viral proteins 5. Release. Completed viral particles exit the cell and can infect other cells.
  • 11. 1. Attachment. In attachment, a specific protein on the capsid of the virus physically "sticks" to a specific molecule on the membrane of the host cell. This molecule, called a receptor, is usually a protein. A virus recognizes its host cells based on the receptors they carry, and a cell without receptors for a virus can't be infected by that virus.
  • 12. 2. Entry One typical route for viral entry is fusion with the membrane, which is most common in viruses with envelopes. Viruses may also trick the cell into taking them in by a bulk transport process called endocytosis. Some even inject their DNA into the cell
  • 13. 3. Genome replication and gene expression • involves copying the viral genome and making more viral proteins, so that new virus particles can be assembled. • The materials for these processes (such as nucleotides) come from the host cell, not the virus. • Most of the "machinery" for replication and gene expression is also provided by the host cell. • For instance, the messenger RNAs (mRNAs) encoding viral genes are translated into viral proteins using the host cell's ribosomes. • However, certain steps, such as the copying of an RNA virus's genome, cannot be performed by host cell enzymes. In such cases, the viruses must encode their own enzymes.
  • 14. 3. Genome replication and gene expression • All viruses must encode capsid proteins, and enveloped viruses typically also encode envelope proteins (which often aid in host recognition). • Viruses may also encode proteins that manipulate the host genome (e.g., by blocking host defenses or driving expression of genes to benefit the virus), help with viral genome replication, or play a role in other parts of the viral lifecycle.
  • 15. 4. Assembly • newly synthesized capsid proteins come together to form capsomers, which interact with other capsomers to form the full-sized capsid. • Some viruses, like head-tail viruses, first assemble an “empty” capsid and then stuff the viral genome inside. • Other viruses build the capsid around the viral genome, as shown below.
  • 16. 5. Release • The last step in the virus lifecycle is the release of newly made viruses from the host cell. • Different types of viruses exit the cell by different routes: some make the host cell burst (a process called lysis), while others exit through the cell's own export pathways (exocytosis), and others yet bud from the plasma membrane, taking a patch of it with them as they go. • In some cases, the release of the new viruses kills the host cell. (For instance, a host cell that bursts will not survive.) In other cases, the exiting viruses leave the host cell intact so it can continue cranking out more virus particles.
  • 18. Classification of Viruses • Viruses are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. • Currently there are two main schemes used for the classification of viruses: 1. The International Committee on Taxonomy of Viruses (ICTV) system 2. The Baltimore classification system
  • 19. ICTV system of classification • The International Committee on Taxonomy of Viruses (ICTV) was established in 1966 to provide a single taxonomic scheme for viral classification and identification. • For most viruses, order and families are the highest taxonomic groups established so far by ICTV • Family names are typically derived from special characteristics of viruses within the family or from the name of an important member of the family eg. Picornaviridae, Hepadnaviridae, Herpesviridae • Viruses are assigned to certain genera within families • Latinized names are not given to viruses at the species level; but given common English designations
  • 20. ICTV system of classification • Viral classification starts at the level of order and follows as thus, with the taxon suffixes given in italics: • Order (-virales) • Family (-viridae) • Subfamily (-virinae) • Genus (-virus) • Species • So far, six orders have been established by the ICTV: the Caudovirales, Herpesvirales, Mononegavirales, Nidovirales, Picornavirales, and Tymovirales.
  • 21. ICTV system of classification These orders span viruses with varying host ranges. • Caudovirales are tailed dsDNA (group I) bacteriophages, • Herpesvirales contains large eukaryotic dsDNA viruses, • Mononegavirales includes non-segmented (-) strand ssRNA (Group V) plant and animal viruses, • Nidovirales is composed of (+) strand ssRNA (Group IV) viruses with vertebrate hosts, • Picornavirales contains small (+) strand ssRNA viruses that infect a variety of plant, insect, and animal hosts, and • Tymovirales contains monopartite ssRNA viruses that infect plants
  • 22. Baltimore classification • first defined in 1971 • a classification system that places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), Sense and method of replication • these groups are designated by Roman numerals and discriminate viruses depending on their mode of replication, and genome type.
  • 24. Baltimore classification • Viruses can be placed in one of the seven following groups: I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses) II: ssDNA viruses (+) sense DNA (e.g. Parvoviruses) III: dsRNA viruses (e.g. Reoviruses) IV: (+)ssRNA viruses (+) sense RNA (e.g. Picornaviruses Togaviruses) V: (−)ssRNA viruses (−) sense RNA (e.g. Orthomyxoviruses Rhabdoviruses) VI: ssRNA-RT viruses (+) sense RNA with DNA intermediate in life-cycle (e.g. Retroviruses) VII: dsDNA-RT viruses (e.g. Hepadnaviruses)
  • 30. Classification of virus on the basis of host range: 1. Bacteriophage: • Phage are virus infecting bacteria. Eg, λ phage, T2, T4, φ174, MV-11 2. Plant virus: • Those virus that infects plants. Eg. TMV, cauliflower mosaic virus 3. Animal virus: • Those virus that infects animals. Eg. Polio virus, Retro virus, Herpes virus, Adeno virus 4. Insect virus: • Virus that infects insects. Eg. Baculovirus, Sacbrood virus, Entomopox virus, Granulosis virus Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)