Virology

Virology Virology is the branch of microbiology, which deals with the study of viruses General Properties: • Viruses are the smallest (20-300 nm in diameter), obligate intracellular infectious agents Consist of either RNA or DNA surrounded by a protective protein shell • Protein shell may be surrounded by an envelope containing lipid or protein • Multiplication occurs only within host cells, involving separation of either their genomes or nucleocapsid from their protective shells • They do not have cellular organization • Their genome is an element of nucleic acid, either DNA or RNA • Their genome is enclosed in a protein shell known as capsid, which may be surrounded by lipid-containing membrane called envelope • The entire infectious unit is called ''Virion”- an extra cellular infectious virus particle • Viruses are sensitive to heat-inactivated by heating at 56°-60°C in 30 minutes; exceptions are Hepatitis B virus and polio virus • Viruses are resistant to cold and can survive -70°C, pH range 5-9, but viruses are destroyed by extreme acidity or alkalinity • Viruses are sensitive to oxidizing agents such as hydrogen peroxide, potassium permangnate and organic iodophores • Enveloped viruses containing lipids are sensitive to ether • Beta-propiolactone is actively virucidal • Glutaraldehyde is effective against viruses and has an important role in the disinfection of apparatus used in renal dialysis units • Chlorination of water kills most viruses but hepatitis A and polio viruses are relatively resistant to chlorination, especially when present with organic or faecal material • Viruses are sensitive to sunlight, ultraviolet rays and ionizing radiations • Viruses are resistant to antibiotics and chemotherapeutic agents • Viroids are plant pathogens, which do not exhibit any extracellular dormant phase. They contain single-stranded RNA molecules without protein coat (capsids). They are resistant to heat but sensitive to nuclease. They do not cause human infections Symmetry of Viruses Two types of symmetries-helical and icosahedral (or cubic or polyhedral) are witnessed. The helical nucleocapsids measure 10-20 nm in diameter whereas the icosahedral capsids show considerable diversity. Icosahedral Symmetry An icosahedron is a platonic solid of geometry and consists of 12 vertices, and 20 faces each an equilateral triangle. The capsomers are arranged precisely according to icosahedral patterns characterized by 5:3:2- fold rotational symmetry. Seven such patterns are found among animal viruses. The icosahedral pattern gives a strong structure to enclose a maximum volume in capsid. Helical Symmetry Helical viruses resemble long rods that may be rigid or flexible. This is best exemplified by tobacco mosaic virus in which a nucleic acid of the shape of a filament is surrounded by protein molecules arranged helically so as to yield a structure with a single rotational axis. Rabies virus is an important example of this type of symmetry.

Icosahedral Symmetry

Helical Symmetry

Complex Viruses Some viruses have a very complicated structure and are referred to as complex viruses. Examples of complex viruses are poxviruses, which do not contain clearly identifiable capsids, but have several coats around the nucleic acid.

Complex Symmetry Structure and Chemical Nature A virus has two major components: 1. Nucleic acid-core or genome 2. Protein sheath-capsid - The nucleic acid core is surrounded by a protein coat known as capsid - The protein nucleic acid complex-the capsid, together with the enclosed nucleic acid is known as the nucleocapsid Viral Proteins Viral proteins can be broadly divided into two groups; Structural proteins and Non-structural proteins (functional proteins). Structural proteins are the proteins present in the coat of the virus whereas the enzymes and haemagglutinins are known as non-structural proteins. Haemagglutinins These proteins are present in certain viruses such as picornaviruses, togaviruses, reoviruses, myxoviruses, adenoviruses and papovaviruses and agglutinate erythrocytes of various animal species. This property is endowed by the glycoproteins present on envelope. Enzymes Four groups of enzymes have been detected in the viruses: a. Neuraminidase b. RNA polymerase c. Reverse transcriptase d. Enzymes of cellular origin Viral lipids are present in the viruses, originate from the cell membrane of the host and hence their

composition shall be same as that of the cell membrane. Lipids constitute about 35% of the dry weight of enveloped viruses. Viral Carbohydrates are present in viruses as parts of nucleic acids, glycoproteins on the surface, glycoproteins in the core (as in poxviruses) and in the outer capsid proteins of rotaviruses. Classification: Viruses are classified on the basis of physiochemical and biological properties into families, that are split into genera and finally into species. The important criteria employed are: 1. Type of nucleic acid of the genome and strategy of viral replication 2. Morphological characters such as size, shape, structure and presence of envelope Classification criteria of viruses

Viral Replication 1. Adsorption 2. Entry into host cells 3. Uncoating 4. Biosynthesis

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Receptormediated By endocytocis / Translocation By lysosomal enzymes of host cell Using host machinery

First regulatory early protein / enzymatic proteins are synthesized. Then structural or late proteins are synthesized. Early enzymatic proteins stimulate both replication of virus genome as well as translation of mRNA to form structural proteins i.e. capsid.

Viral Genome DNA/RNA

Replication

Multiple genome copies

mRNA

Translation

Late structural protiens

Early enzymatic protiens

Assembly

Virions

5. Assembly In Nucleus - In DNA viruses IN Cytoplasm - In RNA viruses 6. Release - By lysis - Reverse phagocytosis

Capsid

Virus envelope is derived from host cell membrane cytoptasmic / nuclear during the process of budding out. But the glycoprotein peplomers are virus encoded. These get impregnated in cytoplasmic membrane in advance over the site of future release of virus.

Viruses are classified on the basis of their morphology: Family Genome Envelope 1.

2.

DNA Viruses Herpes viridae Adeno viridae Hepadna viridae Pox viridae Papova viridae Parvo viridae RNA Viruses Picorna viridae Rhabdo viridae Orthomyxo viridae Paramyxo viridae Calici viridae Astro viridae Corona viridae Toga viridae Flavi viridae Bunya viridae Filo viridae Arena viridae Reo viridae Retro viridae Delta virus (floating genus)

Nucleocapsid Symmetry

DsDNA DsDNA DsDNA

Present Absent Present

Icosahedral Icosahedral Unknown

DsDNA DsDNA SsDNA

Present Absent Absent

Complex Icosahedral Icosahedral

SsRNA (+) sense SsRNA (-) sense SsRNA (-)

Absent Present Present

Icosahedral Helical Helical

SsRNA (-)

Present

Helical

SsRNA (-) SsRNA (+) SsRNA (+) SsRNA (+) SsRNA (+) SsRNA (-) SsRNA (-) SsRNA (-) SsRNA (+) SsRNA (+) SsRNA (-)

Absent Present Present Present Present Present Present Present Absent Present Present

Icosahedral Icosahedral Helical Icosahedral Icosahedral Helical Helical Unknown Icosahedral Unknown Unknown