GENERAL PROPERTIES OF VIRUS – the living

chemicals!!

WHAT ARE THESE? Unicellular organisms

Prokaryotes

Eukaryotes

Do not possess cellular organization

Contain only one type of genetic material DNA or RNA (Never both)

Obligate intracellular parasites

Lack enzymes for Protein & Nucleic acid synthesis – Depend on the Host

Mechanism of Multiplication is complex

Not affected by Antibiotics

WHAT ARE THESE?• Viruses can be crystallized like chemicals• Since they contain infectious nucleic acid

Living Chemicals!!• Diseases caused range from common cold to

Rabies and AIDS• Some viruses can even cause Cancers –

Oncogenic viruses

Morphology

• Extracellular infectious particle Virion

• Small in size – Ultramicroscopic

• Filterable – Can pass through filters in which bacteria find it

difficult to pass

• Virus particles seen after appropriate staining Elementary Bodies

• Largest virus 300nm (Size of smallest Bacteria)

• Smallest virus 20nm ( Size of largest protein molecule )

How do we Measure??• Historically !– Pass through colloidal membrane of graded

porosity Gradocol membranes • Later on!– Ultracentrifuge – size estimated depending on rate

of sedimentation• Now!– Electron Microscopy – both size and shape can be

studied

Shape of Virus• Nucleic acid• Protein coat Nucleocapsid

• Capsid Protects nucleic acid from action of nucleases

• Composed of capsomeres polypeptide molecules• Capsid attach to Host cell and introduce viral

genome into host cell

Symmetry • Icosahedral Symmetry – Cubical

• Helical symmetry

• Icosahedral polygon with 12

vertices (corners), 20 facets (Sides)

– Each facet is in shape of a

equilateral triangle

– Capsomeres in icosahedral

symmetry are of two types • Pentagonal capsomeres at the

vertices

• Hexagonal capsomeres in the facets

Symmetry

• Helical symmetry– Nucleic acid and capsomeres are

wound together to form a Helical tube

• Some viruses have Complex

Symmetry Pox virus

Envelope / Outer covering !• Envelope

– Enveloped virus

– Non enveloped ( Naked Virus)

• Derived from host cell while budding

• Made up of lipoproteins

• Lipid from host + protein coded by virus

• Protein subunits projecting from the surface Peplomers

• Virus may carry more than one type of peplomers E.g – Influenza virus

– Triangular spike – Haemagglutinin spike

– Mushroom shaped structure – Neuraminidase

• Envelope : Chemical , Antigenic & Biological properties

Chemical Properties• Only one type of genetic material

• Viruses are the only living forms where the genetic information is

carried solely by RNA.

• Can be extracted using chemicals and can initiate infection in host cells

• Viral protein in capsid protection of nucleic acid , Antigen specific

• Lipids in envelope derived from host cell

• Viruses do not have enzymes for producing viral components or for

producing energy – Depend on host cell enzymes

Resistance!• Mostly heat labile

– Destroyed in seconds @56°C

– Destroyed in minutes @37°C

– Destroyed in days @4°C

• Stable at lower temperatures

• For long term storage store at – 70°C

• Better method – Lyophilization/ freeze drying

• Can be stored for years & reconstituted when needed by adding water

• Some viruses do not wihstand freeze drying – Polio Virus

Resistance!• Resistance to acids – Vary

• Susceptible to alkaline conditions

• Inactivated by Sunlight, UV rays and ionizing radiation More resistant to chemical

disinfectants – Some of them act as preservatives for virus !

• Killed by oxidizing agents

– Hydrogen peroxide

– Potassium permagnate

– hypochlorites

• Formaldehyde & BPL are actively virucidal

• Ether, chloroform (Lipid solvents) active on enveloped viruses

Viral hemagglutination • Observed first in influenza

• Viruses agglutinate RBC’s of different species

• Hemagglutination is d/t heamagglutinin spikes

• Neuroaminidase acts as a receptor destroying enzyme. (RDE)

• Destruction of receptor will lead to reversal of heamagglutination

release of virus from red cell surface : ELUTION

• Convenient method of detection and assay of influenza virus

• When red cells are added to serial dilutions of viral suspension, the highest

dilution that produces heamagglutination – Heamagglutination titre

Viral Hemagglutination No Agglutination Settle at bottom like a button

Agglutination Red cells spread in a shield like pattern

Inactive virus also lyse red cells : hence used to titrate

killed influenza vaccines

Lysis is inhibited by Antibody to virus

Heamagglutination inhibition : antiviral antibody

estimation

Elution seen in viruses which have neuroaminidase only

Viral multiplication

• Adsorption / Attachment• Penetration• Uncoating• Biosynthesis• Maturation• Release

Adsorption/ Attachment • Contact by random collision

• Cell surface contain specific receptors for attachment of virus

– Influenza – glycoprotein receptors on respiratory epithelium

– HIV – CD4 receptor on host cell, viral surface glycoprotein (gp120)

– Polio – lipoprotein receptor

Penetration • Bacteria have thick cell wall , viruses cannot penetrate so , only nucleic acid is introduced

• Animal cells do not have rigid cell walls so the whole virus gets in

• Viruses may also be engulfed fully like phagocytosis : VIROPEXIS

• Enveloped viruses may fuse with plasma membrane of host cell releasing the nucleic

material into host cell

UNCOATING • Stripping of outer layers and capsid

• Uncoating is d/t action of host lysosomal enzymes

• In some viruses (Eg.Pox Virus) uncoating happens in

2 steps

• Step 1 uncoating : Action of host lysosomal enzymes

• Step 2 uncoating : Action of viral uncoating enzymes

Biosynthesis • Viral nucleic acid

• Capsid protein

• Enzymes needed for viral synthesis, assembly and release

• Regulator proteins : shut down host mechanism and direct synthesis of viral

components

• DNA virus synthesize viral nuclear material in host cell nucleus

– Exceptions – pox virus

• RNA virus synthesize viral nuclear material in cytoplasm.

– Exceptions – orthomyxo and paramyxo virus

• Viral proteins are synthesized in cytoplasm.

Transcription of mRNA from nucleic acid

Translation of mRNA to early

proteins

Replication of viral nucleic

acid

Synthesis of late / structural

proteins

Shut down host mechanism

Classification based on replication mechanism

• Class I (Adenovirus, Herpes virus, Papova virus) dsDNA enters host

nucleus & uses host cell enzymes for transcription – infectious

• (hepadena virus) Partial dsDNA become dsDNA in host cytoplasm using

viral polymerase and then moves into host cell nucleus – non infectious

• Class II (Parvo virus) ssDNA moves into host nucles and converts into

dsDNA using host enzymes.

• Class III (Reovirus) dsRNA transcribed to mRNA by viral polymerase

• Class IV positive strand RNA RNA itself acts as mRNA

Classification based on replication mechanism

• Class V Negative strand RNA uses viral RNA polymerases to

produce mRNA

• Class VI (retroviridiae) ssRNA is converted to RNA:DNA

hybrid by viral reverse transcriptase.

From this hybrid DNA is synthesised to form provirus.

Provirus is integrated into host cell chromosome for further

replication. May cause neoplasia

Maturation • Nucleus : Herpes, Adeno

• Cytoplasm : Picorna, pox

• Non enveloped virus are now fully formed

• Enveloped virus: only nucleocapsid is formed.

• Envelope is derived from host cell during budding

Release • Bacteria – lysis of bacterium

• Animals – non lytic mechanism – budding

• Infected host cell may divide forming infected daughter cells

• Progeny virions affect nearing cells

• Some viruses like poliovirus lyse the cell when released.

Abnormal replicative cycles • Some virions may not be infective

• This is due to defective assembly incomplete virus : infection will have high

heamagglutinin titre but low infectivity (Von Magnus phenomenon)

• Some hos cells do not allow viral replication. Viral components may be synthesised

by assembly and maturation is defective aborive infection

• some viruses are genetically defective: cannot form fully effective progeny

defective viruses

• Yeild of progeny virus only with a help of “Helper Virus”

Eg – Rous sarcoma virus(RSV) cannot form viral envelope. So only when this virus infects

a cell that is already infected with avian leukosis virus RSV can form a healthy progeny

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END OF SESSION !!