Question

Compare replication strategies of parvoviruses with those of papillomaviruses?

Answer 1

Ams ; parvovirus ( single stranded DNA)  

( papillomavirus - double stranded DNA )

Parvovirus replication -  

Before the start of any replication, parvoviruses must first enter the cell. An attachment to host receptors initiates clathrin-mediated endocytosis of the virion into the host cell. The virion consequently penetrates into the cytoplasm via permeabilization of host endosomal membrane, and reaches the nucleus (where uncoating occurs) via microtubular transport.

In order to replicate their genetic material, viruses have to recruit and assemble replication factors at an origin of replication. The non-structural replication proteins (NS and Rep) of the parvoviruses represent fundamental viral proteins that recognize the replication origins within the terminal repeats.

Since the parvoviral genome is a single-stranded DNA, no special polymerases are necessary for replication, thus even naked viral nucleic acid is infectious. This single copy of DNA is initially converted to a duplex replication intermediate by cellular proteins.

Transcription of newly formed double-stranded DNA gives rise to viral mRNAs when host cell enters S-phase (or synthesis phase) of the cell cycle and are subsequently translated to produce viral proteins. It generates an ensemble of mature transcripts, as a result of both co and posttranscriptional events.

Replication proceeds through cycles of terminal resolution and hairpin-primed strand displacement synthesis via a rolling hairpin mechanism. During this process, individual genomes are excised and their telomeres regenerated by the introduction of single-strand nicks into replication origins generated at either end of each genome.

Newly synthesized single-stranded DNA can either be converted to double-stranded DNA and serve once again as a template for transcription or replication, or it can be encapsidated to form new virions that are released by cell lysis. It must be noted that Parvoviridae can replicate autonomously only in actively cycling cells; otherwise, co-infection with other viruses (such as adenovirus or herpesvirus) is essential

Papillomavirus replication ;

The first is by HPV DNA replicating only once per S phase, much like the cellular DNA. This form of replication is termed as ordered replication. The alternative is that HPV DNA replicates randomly, whereby some molecules replicate a few times per S phase, some replicate once, and some do not replicate at all.

HPV DNA replication during its life cycle occurs in three separate phases . After viral entry into the cell nucleus and the activation of viral gene expression, the viral genome copy number increases to several hundred copies per cell during the initial phase of genome amplification