Notes

Virology

Virology is the study of viruses and virus-like agents. Viruses are sub-microscopic, infectious particles of genetic material (DNA or RNA) contained in a protein coat.

Virus-like agents resemble viruses, but are non-infectious because they contain no viral genetic material. The expression of viral structural proteins, such as envelope or capsid, can result in the self-assembly of virus like particles (VLPs)

Virology is considered to be a subfield of microbiology or of medicine. It focuses on the following aspects of viruses: their structure, classification and evolution, their ways to infect and exploit hostcells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy.

A submicroscopic infectious organism, now understood to be a non-cellular structure consisting of a core of DNA or RNA surrounded by a protein coat. It requires a living cell to replicate, and often causes disease in the host organism.

Difference between bacteria and virus

	Bacteria	Virus
Intracellular parasite	No	Yes
Plasma membrane	Yes	No
Binary fission	Yes	No
Pass through bacteriological filter	No	Yes
Possess both DNA and RNA	Yes	No
ATP-generating metabolism	Yes	No
Ribosomes	Yes	No
Sensitive to antibiotics	Yes	No
Sensitive to interferon	No	Yes

MORPHOLOGY AND GENERAL PROPERTIES OF VIRUS

CHARACTERISTICS

Viruses are minute & simplest infectious agents. (1/2 to 1/100 of a bacterial cell)

• They are obligate intracellular parasites and contain no enzyme necessary for energy metabolism.
• Lack enzymes necessary for protein and nucleic acid synthesis and so depend upon synthetic machinery of host cells.
• Contain only one of the nucleic acid either DNA or RNA, never both.
• Size ranges from 20 nm (Parvo Virus) to 400 nm (Pox Virus = smallest bacteria i.e., Mycoplasma) [1nm = 1/1000 µm].
• It is an acellular organism that lacks true nucleus, cytoplasm and cell organelles.
• It reproduces only inside the host cell by a complex process, NOT by binary fission.
• Outside the host cell they can be crystallized and behave like the chemicals.
• Filterable through bacterial filters.
• Unaffected by anti-bacterial agents.
• They infect unicellular organisms such as bacteria and algae as well as higher plants and animals.
• The medical importance of viruses lies in their capacity to cause wide varieties of diseases ranging from minor common cold to highly fatal AIDS, Dengue etc.
• Inactivated at 56 ⁰C but stable at low temp.
• Optimum pH: 5-9
• Inactivated by exposure to x-rays or UV rays.
• ONLY Envelop viruses are sensitive to ether.
• Formaldehyde inactivate the virus but do not damage viral protein so used in vaccine production

Morphology

Size

• Virus differ from bacteria by their ability to pass through filters that can hold back bacteria
• Size ranges from 20 – 400 nm.
• The pox viruses are the largest virus (400nm).
• All the viruses are visible under the electron microscope.

Shape

Spheres, rods, bullet or brick (complex structure of pre-sized geometric symmetry)

• Varies in different groups
• Most animal viruses are spherical
• Bullet shape – Rabies virus
• Brick shape – Pox Virus

Structure

• Virion- is a complete infectious virus particle.
• Consists of a NUCLEIC ACID CORE (either DNA or RNA) surrounded by protein coat called CAPSID.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Capsid

• Capsid is composed of a large no. of morphological units called
• The capsomers are arranged symmetrically around the core.
• Capsid and Nucleic Acid core together are known as NUCLEO-CAPSID.

Functions of Capsid

• It protects the core (nucleic acid)
• Gives shape to viral particles
• It helps virus to enter into the host cell (carries receptors in non-envelop virus)

Envelope

• In some viruses, neucleocapsid is covered by outer covering called ENVELOPE
• (If envelope is present called as enveloped virus, if absent called as non-enveloped or naked virus).
• Envelope is derived from the host cell membrane when the virus is released from the cell.
• Envelope made up of lipids and proteins membrane (bilayered lipoproteins).
• Outer projection (glycoproteins) arises from envelop called SPIKES (peplomers).
• Envelope confers antigenic, biological and chemical (ABC) properties to the viruses.
• Generally, the presence of envelope confers instability of viruses.
• They are more sensitive to heat, detergents (bile salts) and lipid solvents (e.g. alcohol, ether) than the non-enveloped (naked) viruses.

Symmetrical arrangement of capsid around the core

1. Helical Symmetry
2. Icosahedral Symmetry
3. Complex Structure





4. 1. Helical Symmetry
   • In this symmetry, the capsomeres are bound to the nucleic acid in such a way that nucleic acid winds spirally to form a helix.
   • Example
     • Tobacco Mosaic virus (TMV)
     • • Swine flu virus( H1N1)

2. Icosahedral Symmetry

• Capsomers are arranged in a polygonal manner with 12 vertices (corners) and 20 sides (facets), and 30 edges, each facet being an equilateral triangle.
• Example- Adenovirus

3. Complex structure

• Some viruses have a complex structure
  • Do not exhibit any symmetry but complex structure.
  • g. Pox viruses are brick shaped with ridges on the external surfaces.

Chemical constituent of virus

1. Proteins
2. Nucleic acid
3. Lipids
4. Carbohydrates

1. Proteins:

Early proteins- enzymes

Late proteins- structural proteins e.g. capsid

• Capsid consists of proteins.
• Function of proteins:
  • Provide PROTECTION to the viral nucleic acid against inactivating enzymes
  • Help in ATTACHMENT of viruses to host tissue surface during infection
  • Some proteins are in the form of enzymes which helps in REPLICATION of virus when virus entered the host cell
  • Determine the ANTIGENIC characteristics of virus.
  • Provide STRUCTURAL SYMMETRY to viral particle
  • Some surface proteins helps in IDENTIFICATION of virus, e.g., Haemagglutinants in Influenza virus.

1. Nucleic acid:

• Nucleic acid is either DNA or RNA.
• It contains the genetic information necessary for the replication of virus.
• May be
  • single or double stranded
  • circular or linear in shape
  • segmented or un-segmented
  • The amount of nucleic acid present may vary in different groups of viruses. Small viruses such as Parvo virus and Picorna virus contains 3-4 genes where as large viruses contains several hundred genes per virion

1. Lipids:

• Lipids are present in the envelope of virus.
  • As envelope is derived from the host cell membrane, it shows a lipid composition of the host cell.

1. Carbohydrates:

• Carbohydrates form a part of spikes present on envelope.
  • Act as binding site and help in attachment of virus to host cell receptors.
  • Antigenic in nature.

Resistance:

• Viruses are destroyed by heat except a few.
• They are stable at low temperatures. For long term storage, they are kept at -70°C. A better method for prolonged storage is lyophilisation or freeze-drying.
• Viruses are inactivated by sunlight, UV rays and ionising radiation. They are, in general, more resistant than bacteria to chemical disinfectants.
• Phenolic disinfectants have a weak action on viruses.