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In this article we will discuss about the replication of viral DNA and RNA in animals.
Replication of Viral DNA:
Replication of DNA of viruses usually takes place in the nucleus of the host cell; poxviruses are the exceptions because their genome replicates in the cytoplasm of the host cell. Messenger RNA (mRNA) of all animal DNA viruses is transcribed from DNA by host enzymes, whereas that of poxviruses is synthesized by a viral polymerase.
For convenience, two examples (herpes-viruses and hepadnaviruses) of DNA replication are as follows:
1. Herpesviruses:
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Herpesviruses (e.g., herpes simplex virus) represent a large group of icosahedral, enveloped, dsDNA viruses. Its genome is a linear piece of DNA about 1,60,000 base pairs in size and contains at least 50-100 genes.
Immediately upon un-coating, a portion of viral DNA (i.e., early genes) is transcribed by host RNA polymerase to form mRNA that migrates into the cytoplasm and directs the synthesis of several early proteins, mainly regulatory proteins and the enzymes required for viral DNA replication.
The DNA circularizes and its replication begins with the help of virus-specific DNA polymerase in the nucleus of the cell within 4 hours after infection. This results in the synthesis of new viral DNA molecules. Consequently, the other portion of the viral DNA (i.e. late genes) is transcribed to form mRNAs that synthesize late proteins in the cytoplasm.
These late proteins migrate into the nucleus wherein they constitute the capsids. Mature viral DNA is incorporated inside the capsids and complete viral particles are released from the host cell. Fig. 14.6 shows the whole cycle of herpes-virus replication in a host cell.
Hepadnaviruses (e.g., hepatitis B virus) differ from other DNA viruses with respect to the replication of genetic material. Its dsDNA replicates using the enzymes reverse transcriptase (an RNA dependent DNA polymerase).
After release from the capsid the dsDNA of the animal virus moves into the nucleus wherein it transcribes using host RNA polymerase resulting in several mRNAs, including a large RNA molecule called pre genome.
The RNAs move to the cytoplasm and are translated to give rise to capsid proteins and a polymerase; the polymerase acts as DNA polymerase, reverse transcriptase, and RNase H. RNA transcriptase transcribes the RNA using a protein primer resulting in -DNA copy of the progenome +RNA.
Subsequently, the RNA pregenome associates with DNA polymerase and core protein to form an immature core particle. Now the ribonuclease H component (R Nase H) degrades almost all the RNA progenome. The remaining RNA fragment acts as a primer for DNA polymerase to copy the -DNA and form a dsDNA genome, which incorporates into core particle giving rise to the nucleocapsid.
Replication of Viral RNA:
Genome replication of RNA viruses is much more diverse than DNA viruses in animals.
Depending upon the mode of replication and transcription, and their relationship to the genetic material of host cell, most of the RNA viruses are categorized as of four types:
(1) +ssRNA viruses (e.g., poliovirus),
(2) -ssRNA viruses (e.g., mumps and measle viruses, influenza virus),
(3) dsRNA viruses (e.g., reoviruses), and
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(4) retroviruses (e.g., HIV, Rous sarcoma virus).
The genome of ssRNA viruses (both +ssRNA and -ssRNA), replicate in the host cell cytoplasm. These viruses, except retroviruses, use viral replicase enzyme which directs the synthesis of complementary strand of parental ssRNA resulting in a dsRNA called the replicative form.
The complementary strand is -ssRNA in case of +ssRNA parental genome and +ssRNA in case of-ssRNA parental genome. The complementary strand then serves as a template for the synthesis of new progeny ssRNA genome. For example, picornaviruses have a +ssRNA genome and synthesize an intermediate -ssRNA complimentary copy in order to produce new ssRNA genomes (Fig. 14.7).
The dsRNA genome of reoviruses differs significantly from ssRNA genome viruses in its replication (Fig. 14.8). The animal virus contains 10 to 13 different dsRNAs, each of them codes for an mRNA using RNA-dependent RNA polymerases (transcriptases).
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During the late phase of reproductive cycle of the animal virus, a copy of each mRNA associates with the other mRNAs and special proteins to form a large complex. The RNAs in this complex are then copied by the viral replicase enzyme to form a dsRNA that functions as viral genome and is incorporated into capsid giving rise to new progeny virus particle.
Retroviruses though contain +ssRNA genome like picornaviruses and reoviruses, they differ in replication from other RNA viruses in that they synthesize mRNA and replicate their genome by means of DNA intermediates.
