Hepadnaviridae
Encyclopedia
Hepadnaviruses are a family of virus
es which can cause liver
infections in humans and animals. There are two recognized genera:
The name of the family comes from : hepa(for HEPAtic)-DNA-virus.
s of partially double-stranded, partially single stranded circular DNA
. The genome consists of two uneven strands of DNA. One has a negative-sense orientation, and the other, shorter, strand has a positive-sense orientation.
As it is a group 7 virus, replication involves an RNA
intermediate. Three main open reading frames are encoded (ORFs) and the virus has four known genes which encode the core protein, the virus polymerase
, surface antigens (preS1, preS2, and S) and the X protein. The X protein is thought to be non-structural; however, its function and significance are poorly understood.
intermediate (which they transcribe back into cDNA using reverse transcriptase
). The reverse transcriptase becomes covalently linked to a short 3- or 4-nucleotide primer. Most hepadnaviruses will only replicate in specific hosts, and this makes experiments using in vitro methods very difficult.
The virus binds to specific receptors on cells and the core particle enters the cell cytoplasm
. This is then translocated to the nucleus, where the partially double stranded DNA is 'repaired' by the cell to form a complete circle of DNA. It then undergoes transcription
by the host cell RNA polymerase and the transcript is translated by host cell ribosomes. New virus particles are formed, which acquire lipid from the endoplasmic reticulum of the host cell, and the genome is packaged within these particles, which then bud off from the cell.
Hepadnavirus-infected cells translate the protein known as the virus surface antigen many times until there is too much protein to coat the virions formed. These proteins then aggregate to form rod shapes, and it is this antigen, known as the Australian or hepatitis B surface antigen, which is released from the cell and which leads to a very strong immune response from the host.
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es which can cause liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
infections in humans and animals. There are two recognized genera:
- Genus OrthohepadnavirusOrthohepadnavirusOrthohepadnavirus is a genus of viruses belonging to the family Hepadnaviridae.- External links :* *...
; type species: Hepatitis B virusHepatitis B virusHepatitis B is an infectious illness caused by hepatitis B virus which infects the liver of hominoidea, including humans, and causes an inflammation called hepatitis. Originally known as "serum hepatitis", the disease has caused epidemics in parts of Asia and Africa, and it is endemic in China... - Genus AvihepadnavirusAvihepadnavirusAvihepadnavirus, is the second genus of the Hepadnaviridae family, the first genus being the Orthohepadnaviruses. The Avihepadnaviruses consist of viruses which exclusively infect birds. The type species of this genus is the duck hepatitis B virus...
; type species: Duck hepatitis B virusDuck hepatitis B virusDuck Hepatitis B virus, abbreviated DHBV, is part of the Avihepadnavirus genus of the Hepadnaviridae, and is the causal agent of duck hepatitis B....
The name of the family comes from : hepa(for HEPAtic)-DNA-virus.
Genome
Hepadnaviruses have very small genomeGenome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
s of partially double-stranded, partially single stranded circular DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
. The genome consists of two uneven strands of DNA. One has a negative-sense orientation, and the other, shorter, strand has a positive-sense orientation.
As it is a group 7 virus, replication involves an RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
intermediate. Three main open reading frames are encoded (ORFs) and the virus has four known genes which encode the core protein, the virus polymerase
Polymerase
A polymerase is an enzyme whose central function is associated with polymers of nucleic acids such as RNA and DNA.The primary function of a polymerase is the polymerization of new DNA or RNA against an existing DNA or RNA template in the processes of replication and transcription...
, surface antigens (preS1, preS2, and S) and the X protein. The X protein is thought to be non-structural; however, its function and significance are poorly understood.
Replication
Hepadnaviruses replicate through an RNARNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
intermediate (which they transcribe back into cDNA using reverse transcriptase
Reverse transcriptase
In the fields of molecular biology and biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse...
). The reverse transcriptase becomes covalently linked to a short 3- or 4-nucleotide primer. Most hepadnaviruses will only replicate in specific hosts, and this makes experiments using in vitro methods very difficult.
The virus binds to specific receptors on cells and the core particle enters the cell cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
. This is then translocated to the nucleus, where the partially double stranded DNA is 'repaired' by the cell to form a complete circle of DNA. It then undergoes transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
by the host cell RNA polymerase and the transcript is translated by host cell ribosomes. New virus particles are formed, which acquire lipid from the endoplasmic reticulum of the host cell, and the genome is packaged within these particles, which then bud off from the cell.
Hepadnavirus-infected cells translate the protein known as the virus surface antigen many times until there is too much protein to coat the virions formed. These proteins then aggregate to form rod shapes, and it is this antigen, known as the Australian or hepatitis B surface antigen, which is released from the cell and which leads to a very strong immune response from the host.