The early evolution of the Hepatitis B, like that of all viruses, is difficult to establish.The divergence of orthohepadnavirus and avihepadnavirus occurred ~125,000 years ago (95% interval 78,297–313,500).
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The genotypes have a distinct geographical distribution and are used in tracing the evolution and transmission of the virus.
Differences between genotypes affect the disease severity, course and likelihood of complications, and response to treatment and possibly vaccination.
(virion), consists of an outer lipid envelope and an icosahedral nucleocapsid core composed of protein.
The hepatitis B virus is classified as the type species of the Orthohepadnavirus, which contains three other species: the Ground squirrel hepatitis virus, Woodchuck hepatitis virus, and the Woolly monkey hepatitis B virus.
The genus is classified as part of the Hepadnaviridae family, which contains two other genera, the Avihepadnavirus and a second which has yet to be assigned.
This family of viruses have not been assigned to a viral order.
and in a New World woolly monkeys suggesting an ancient origin for this virus in primates.
The nucleocapsid encloses the viral DNA and a DNA polymerase that has reverse transcriptase activity similar to retroviruses.
The outer envelope contains embedded proteins which are involved in viral binding of, and entry into, susceptible cells.
The virus is one of the smallest enveloped animal viruses with a virion diameter of 42 nm, but pleomorphic forms exist, including filamentous and spherical bodies lacking a core.
These particles are not infectious and are composed of the lipid and protein that forms part of the surface of the virion, which is called the surface antigen (HBs Ag), and is produced in excess during the life cycle of the virus.