The Strategies of Virus Replication

 Life cycle of most viruses is designed to maximize the production of progeny virus particles in the cells. Case of many animal viruses, the time elapsed from infection to the generation of the first progeny ranges form several hours to a day. Burden of producing a large number of virus particles causes the infected cell to die. This lysis of the host cell is called the viral lytic cycle and is an immediate and inevitable consequence of viral reproduction

In the other viruses will refrain from killing the host cell. They can establish a long term infection of the cell which cell releases a steady stream of viral particles over an extended period of time. If this continuous production of virus particles does not compromise the health of the host cell and it can live on indefinitely devoting some of its resources to making virus

Details of a viral replication cycle are dictated by the type of nucleic acid carried into the host cell by the infecting virion. Most DNA viruses enter the nucleus where they parasitize of host cell's DNA replication apparatus. There are exceptions notably the smallpox DNA virus encodes its own DNA replication machinery and thus remains in the cytoplasm. Most RNA viruses replicate in the cytoplasm because the enzymes used to replicate viral RNA are virally encoded

Plus ss RNA Retroviruses in Living cells

In this viruses are lipid containing viruses whose genomes can act as mRNA.  Most notorious of these is HIV and this viruses resulting in AIDS.  Aside from HIV retroviruses are rather uncommon in humans, but prevalent in other mammals and birds.  Genomes of retroviruses are similar in structure and size to picornaviruses like polio virus and one might suppose that the replicative strategy of a retrovirus resembles that of poliovirus. Life cycle of a retrovirus is unique and unusual

Plus strand of RNA is not associated with ribosomes, It has all the attributes of mRNA.  Instead and virion RNA is used as a template to make a DNA copy of the viral genome.  Copying of RNA into DNA is foreign for the host cell and must be carried out by a viral enzyme that is packaged in virion 

The viral enzyme, called reverse transcriptase carries out this process. Terms reverse and retro imply a mechanism that is the opposite of that normally operating in all cells usual flow of information in a cell is from DNA to RNA is not from RNA to DNA. Initial product of reverse transcription is an RNA and DNA hybrid double helix

RNA portion of this hybrid is degraded and reverse transcriptase copies the remaining DNA strand into dsDNA. Processes take place in the cytoplasm. The viral dsDNA is synthesized, it is transported into the nucleus where it is inserted and covalently linked to Host chromosomal DNA. Viral DNA that is integrated into the host genome is called a provirus and it is indistinguishable from the host cell genes. The retrovirus has created a version of the viral genome that has all the attributes of a cellular gene found in the host.  The integrated provirus can be transcribed by the host cell into (+) RNA that is transported to the cytoplasm and used either as mRNA in viral protein synthesis or as the genome for new progeny viruses

Ecology of Poliovirus (Compared to Influenza Virus)

When we Consider about Ecology of Poliovirus, the (+) ssRNA genome of poliovirus, a single long RNA molecule, is unable to recombine with (+) ssRNA genomes from other related picornaviruses by the process of crossing-over. Crossing over requires a set of highly specialized enzymes that are not available to the viruses. This means that there are only a very small number of closely related poliovirus strains and immunity against one of these strains confers immunity against the others. Influenza virus presents a dramatically contrasting example.  Influenza virus genome is composed of different RNAs, each carrying different genes

There are two different types of flu virus simultaneously infect a single host cell, two viral genomes may recombine by simply exchanging RNAs.  Progeny virion released from this cell can have RNAs from each different infecting virus and thus a new and unique strain has been created.  This is devastating consequences on human health

Natural reservoir for many flu viruses is the shore bird population around the world, which includes migratory ducks and geese.  At least a dozen distinct strains of flu virus live continually in these birds.  In rural China and other parts of the world, ducks and pigs are kept in close proximity with one another and with humans.  Ducks become infected with a flu virus originating in the wild shore birds.  They can then pass the virus to a pig, which is also readily infected by human flu viruses.  In the infected cells of the pig, the two viral genomes can mix and RNAs from both viruses can be packaged into one virion.  In this way, a new recombinant flu virus, one quite different from pre existing strains may emerge and infect humans

Exposed humans would have not experienced this strain before, and thus there is no immunity to it.  Flu virus now spreads throughout the population creating a worldwide epidemic.  These pandemics create serious respiratory infections, but usually prove lethal to only a few, mostly the weak and elderly

Consider about Ecology of Retroviruses

When we Consider about Ecology of Retroviruses have peculiar ecology. Retroviruses genomes can become integrated into the host cell chromosome.  Once integrated, the viral genome (pro virus) may be transcribed or it may stay dormant and unsubscribed.  If dormant, the retrovirus can exist undetected for a long time

Retroviruses have serious implications for HIV infections. HIV pro virus can integrate into the chromosome of a host white blood cell (lymphocyte) and remain undetected for years.  Only when that lymphocyte is stimulated by some physiological signal will transcription of the pro virus be activated.  Suddenly then, virus particles can burst from the cell and infect other nearby cells.  This dormant state, termed viral latency, means that it is difficult, indeed virtually impossible, to eradicate an HIV infection from the body.  Retroviruses can be many cells that harbor silent pro viruses; each is indistinguishable from a normal uninfected cells.