General Properties of Viruses PDF

GENERAL PROPERTIES OF VIRUSES Dr. Asmaa Magouz Department of Virology  A. PHYSICAL PROPERTIES OF VIRUSES 1.SIZE OF THE VIRUS ❑ Viruses are the smallest infectious agents. ❑ Viruses are smaller in size than bacteria and can not be seen under ordinary microscope. ❑...

GENERAL PROPERTIES OF VIRUSES Dr. Asmaa Magouz Department of Virology  A. PHYSICAL PROPERTIES OF VIRUSES 1.SIZE OF THE VIRUS ❑ Viruses are the smallest infectious agents. ❑ Viruses are smaller in size than bacteria and can not be seen under ordinary microscope. ❑ The largest virus is half the size of the smallest bacteria, and the smallest virus is about the size of a large protein molecule. 3 ❑ The size of viruses is measured by nanometers (nm). ❑ Angstrom (A) is used for measuring very small structures as viral subunits. 1 meter ‗1000 millimeters (mm)  1mm ‗ 1000 micrometer(µm)  1µm ‗ 1000 nanometer (nm)  1nm ‗ 10 Angstrom (A)  Viruses vary from 20-300 nm in diameter. ❑ The smallest animal virus is FMD (20 - 28 nm). ❑ The largest one is Pox virus (300 X 250 nm). Poxvirus FMDV Methods for estimating the size of virus: 1.Electron microscope 2.Ultrafilteration 2.Ultracentrifugation 1.Electron microscope (EM) A) Negative staining technique ❖ The virus sample is treated with phosphotungistate. ❖ Phosphotungistate is an electron dense material which is able to penetrate between the virus particles but not cover the virus particles. ❖ So the viral particles will appear translucent while the background is opaque or black. B) Positive staining technique It includes staining of the viral nucleic acid and viral proteins and so the details can be seen more clearly. 2-Ultrafilteration ❖ By using cellulose acetate membrane filters of different pore sizes. ❖ The average pore diameters of two membranes, one that allows the virus to pass freely and the other that retain it completely. ❖ The virus size ‗ the average pore diameter X 0.64. 2-Ultracentrifugation ❑ Bacteria are heavier than virus and so they can be sedimented by ordinary centrifuge, while viruses need ultracentrifugation for sedimentation (50.000- 100.000g). ❑The centrifugation force causes the virus to migrate through the solution. ❑The rate at which the virus migrates through solution depends on the size of the virus (heavier viruses migrate more rapidly). 2.SHAPE OF VIRUS  Under EM there are different forms of viruses:  Brick shape as Poxvirus  Bullet shape as Rabiesvirus  Crown shape as Coronaviruses  Large spheres as Adenovirus  Thread form as Orthomyxoviruses  Spermatic form as Bacteriophages  Hexagonal shape as Herpesvirus Influenza Virus ( Thread shape or Spherical) Coronavirus (Crown shape) Poxvirus (Break shape) Bacteriophage (Spermatic Shape) Rabies virus (Bullet shape) Adenovirus (Large Spheres) Herpes Virus (Hexagonal in shape) 3.MOLECULAR WEIGHT  Althoughviruses are very small in size , each virus has a definite molecular weight.  Therate at which the virus particles migrate through liquids is their molecular weight and its represented by Dalton. B. CHEMICAL PROPERTIES OF VIRUSES 1-CHEMICAL STRUCTURE  The virus consists of viral genome (nucleic acid) enclosed with a protective coat (capsid).  In some viruses the nucleocapsid is surrounded by a lipoprotein envelope.  1- Viral nucleic acid  2- Viral capsid  3- Viral envelope  1-VIRAL NUCLEIC ACID  All viruses contain only one type of nucleic acid either DNA or RNA never both.  The DNA molecule consists of 2 complementary strands of polynucleotides running in opposite directions to form a double helix.  The 2 strands are arranged in the way that one of them lies in a 5—3 direction , while the other runs in 3—5 direction. The 2 strands of the double helix are held together by hydrogen bonds ❖ A and T are held with 2 hydrogen bonds ❖ G and C are held with 3 hydrogen bonds  The number of G units always equal number of C units, and A number equal T number.  Theratio (A +T)/(G +C) vary from one virus group to another and is important in virus characterization.  In RNA molecule the same is applied , except that uracil replace the thymine.  DNA viral genome  AllDNA viruses are ds (double stranded) except Parvoviruses and Circoviruses are single stranded.  The genome of all DNA viruses is a single molecule (non segmented).  The genome of DNA viruses can be either linear or circular.  The (G +C) content vary from 35% -74%  The molecular weight vary from 1X106 Daltons in small viruses to 200X106 Dalton in larger viruses.  RNA viral genome  All RNA viruses are ss (single stranded) except Reoviruses and Birnaviruses which are ds RNA.  ss RNA are either positive strand or negative strand.  All RNA viruses have linear genome, non is circular.  Viral RNA is either single molecule or segmented Ex. 8 segments in Influenza A virus Ex. 3 segments in Bunyaviridae Ex. 2 segments in Birnaviridae  Molecular weight vary from 2x106 to 15X106 Dalton. RNA VIRAL GENOMES ARE CLASSIFIED ACCORDING TO THEIR SENSE INTO: 1-Positive sense (+ sense RNA) (+ strand) ❑ RNA acts as mRNA because it has nucleotide sequences identical to those of the mRNA ❑ It is directly translated into proteins by the cell ribosomes without the need of transcription. ❑ the genome itself is infectious. ❑ Ex. Caliciviridae, Picornaviridae, Flaviviridae , Togaviridae, Coronaviridae, Retroviridae. ❑ Call Pico and Flavi To Come Rightway 2-Negative sense (-sense RNA) (- strand): ❑ RNA has nucleotide sequences complementary to those of the mRNA. ❑ This negative strand must be first converted into mRNA by transcriptase (polymerase) enzyme before translation into proteins. ❑ These viruses either carry this enzyme or have genes for this enzyme (the host cell synthesize it). ❑ This genome is not infectious by itself. ❑ Ex. Bunyaviridae , Paramyxoviridae, Orthomyxoviridae, Filoviridae, Rabdoviridae ❑ Bring Polymerase Or Fail Replication 3- Ambisense ❑ Ambisense viruses contain at least one ambisense RNA segment which is in part of positive and in part of negative polarity. ❑ Ex ;Arenaviridae  2.VIRAL CAPSID  It is the protein coat enclosing the nucleic acid.  The capsid together with the enclosed nucleic acid is called nucleocapsid.  Most viruses have one capsid but Reoviruses have 2 capsids.  The capsid is composed of morphological units called capsomeres.  These capsomers are held together with specific hydrophobic bonds forming regular arrangements.  The arrangement of these protomers determine the shape of the virus , while their number determine the size of the virus.  SYMMETRY  The capsomers are arranged in definite relationship to each other within the virion to give the capsid one of 3 types of symmetry:  1-Helical  2-Icosahedral  3-Complex 1-Helical symmetry ( Tubular capsid) ❑The capsomers are arranged spirally around the genome to make a hollow cylinder. ❑All animal viruses with helical nucleocapsid are enveloped.  2-Icosahedral symmetry (Cubical capsid):  The capsomers are arranged around central axis to form cubes. Icosahedral capsid  3-Complex symmetry ❑ Some viruses don’t have a clear identifiable capsid symmetry. ❑ They are neither helical nor icosahedral but have several coats around the genome. ❑ Ex; Poxvirus FUNCTION OF CAPSID  1-Protect the enclosed nucleic acid from cellular nucleases.  2-Participate in the attachment of the virus to the susceptible cell.  3-Responsible for the structural symmetry of the virus.  3-VIRAL ENVELOPE  Theenvelopes are acquired during maturation of virus by the budding process.  Budding can be through: ✓ host cell membrane ✓ Plasma membrane ✓ Nuclear membrane ✓ Endoplasmic reticulum ✓ Golgi apparatus Viral Budding  Theessential structure of the envelope is a phospholipid bilayer which is embedded by specific membrane proteins called matrix protein (they provide rigidity).  Thelipids of the viral envelope are derived from the host cell , but the proteins are virus coded.  Thereare glycoprotein peplomers or spikes projected from the surface of the envelope which differ in shape and size:  May be rod like as Haemagglutinin (HA) in Orthomyxoviridae.  May be mushroom like as Neuraminidase (N) in Ortomyxoviridae.  May be club shaped as in Coronaviridae.  Some viruses have no peplomers (smooth surface). Coronavirus  Function of envelope:  1-Protects the nucleocapsid against adverse environmental conditions.  2-Attachement of the virus to host cell.  3- Carry antigenic determinants.