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approximately 1O.5kb. Virions contain three structural proteins, capsid, membraneand envelope (E) and infected cells have been known to contain seven non-structural(NS) proteins, NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5 (Rice, 1996).The evolution, dispersal patterns and epidemiological characteristics ofFlaviviruses are believed to have been determined through a combination ofconstrains imposed by the arthropod vector, the vertebrate hosts, the associatedecology and the influence of human commercial activity (Zanotto et aI., 1995). Theemergence and the expansion of DHF in the tropics have followed an increase inhuman and mosquito population densities brought about by urbanization andindustrialization (Zanotto et al., 1996). Previously, the trans-Atlantic dispersal ofYFV and possibly many other Flaviviruses was thought to have coincided with thetransportation of people and mosquitoes from Africa to the Americas on slave ships(Strode, 1951; Gould et al., 1997).2.2.2 MORPHOLOGY OF FLA VIVlRUSESFlavivirus virions are spherical in shape with a diameter of 40 to 60nm(Murphy, 1980). An electron-dense spherical nucleocapsid of 30nm in diameter issurrounded by a lipid bilayer. Particles typically have a rather smooth appearanceand regular surface projections are usually not apparent (Murphy, 1980), although7nm ring-shaped structures have been observed on the surface of DEN virusparticles (Smith et al., 1970). Mature virions sediment between 170 and 21OS, havea buoyant density of 1.19 to 1.23 glml, and are composed of 6% RNA, 66% protein,9% carbohydrate, and 17% lipid (Russel et al., 1980). The envelope proteins- E andM (membrane) are type 1 membrane proteins embedded by the lipid bilayer by c­terminal hydrophobic anchors. Released virions also contain variable amount of theun processed Membrane precursor membrane (prM) (Russel et. al., 1980).2.2.2.3 PHYSICAL AND CHEMICAL PROPERTIES OF FLAVIVlRUSESFlavivirus virions consist of a spherical ribonucleoprotein core surroundedby a lipoprotein envelope with surface projections. The projections seen in electronmicrographs are clarified by X-ray crystallography and represent molecules ofenvelope glycoproteins, which form rod -like structures anchored to the viralmembrane at their basal ends. Envelope lipids constitute approximately 17% of the7