CHAPTER ONE
INTRODUCTION
A virus is a small infectious agent having a simple acellular organization with a nucleic acid and aprotein coat.It lacks independent metabolism and replicate only within living hosts. A virus invades living cells and uses its chemical machinery to keep itself alive and replicate in the host. It may reproduce with fidelity or with errors (mutations); this ability to mutate is responsible for the ability of some viruses to change slightly in each infected person, making treatment difficult. Viruses cause many human infections and are also responsible for a number of rare diseases. The human diseases they cause may affect different organs or parts of the body. An inflammation of the liver is often referred to by a general term called Hepatitis, which is often caused by a variety of viruses which include hepatitis A, B, C, D and E. Of the viral causes of hepatitis, there are few of greater global importance than hepatitis B (Ganem et al, 2001).
The hepatitis B virus (HBV) belongs to the family Hepadnaviridae,which consists of hepatotropic DNA viruses. The family of Hepadnaviruses comprises members recovered from animal species including the woodluck hepatitis virus (WHV), the ground squirrel hepatitis virus (GSHV), and the duck HBV. Common features of all of these viruses are enveloped virions containing 3 to 3.3 kb of relaxed circular partially duplex DNA and have reverse transcriptase activities. Hepadnaviruses show narrow host ranges, growing only in species close to the natural host, like gibbons, African green monkeys, rhesus monkeys and woolly monkeys (Gitlin,1997).The Hepatitis virus belongs to the genus Hepadnavirus. It is a 42nm partially double stranded DNA virus, composed of a 27nm nucleic acid core (HBcAg), a DNA polymerase reverse transcriptase, surrounded by an outer lipoprotein coat (called envelope) containing the surface antigen (HBsAg)that play a major role in the diagnosis of HBV infection (Ganem et al; 2001; Gitlin,1997). The genome consists of a partially double-stranded circular DNA molecule of about 3200 base pairs in length with known sequence as well as genetic organization. Virion particles are identical to the virion ‘tails’- they vary in length and have a mean diameter of about 22nm. They sometimes display regular, non-helical transverse striations.
The viral DNA polymerase-reverse transcriptase is encoded by the polymerase gene [P] and is of central importance for viral replication. Different from all known mammalian DNA viruses, hepadnaviruses replicate via reverse transcription of a RNA intermediate (Summerset al.,1982), the pregenomic RNA, which is a strategy central to the life cycle of RNA retroviruses. Similarities and differences between retroviral and hepadnaviral replication have been defined (Nassal, 1999). Based on the unique replication cycle of HBV, antiviral therapeutic strategies aimed at the reverse transcription of HBV RNA or at HBV reverse transcriptase have been successfully used as antivirals to treat HBV infection (Feld, 2002).
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