Introduction to Viral Replication and Biosynthesis:
Replication, in terms of virus, can be defined as the propagation of virus such that the number of virus particles increase in number i.e., viral multiplication. Since viruses are obligate intracellular parasites, all the stages of replication occur in the host cell, inducing a living host cell to synthesize all the essential components needed to make more virions. These components must then be assembled into new virions that are released from the cell.
General Steps of Viral Biosynthesis:
Once a host has been infected, new copies of the viral genome must be made and virus-specific proteins must be synthesized in order for the virus to replicate.
In order for new virus to be assembled, both new viral genomes and other virion components must be produced. This varies greatly depending on the family (and Baltimore Class) of virus.
Biosynthesis in viruses consist sequentially of following steps.
Early gene expression
The initial step in the viral replication process involves the activation and expression of the virus’s early-stage genes. Cellular transcription factors and cellular RNA polymerase II are involved in the transcription of these genes. In early promoters, these proteins attach to the viral DNA and encourage the synthesis of early pre-mRNAs. After undergoing processing in the nucleus, the early RNAs are sent to the cytoplasm for translation, resulting in the earliest proteins. Through a process called alternative splicing, a single original transcript can frequently give rise to multiple distinct mature mRNAs. Different proteins, such as the mouse Polyoma virus, are subsequently encoded by these distinct mRNAs.
In the viral life cycle, early proteins usually have many functions. They are necessary for the virus’s genetic material to replicate. Small DNA viruses often require one or a few viral gene products in addition to cellular DNA polymerases and other enzymes for DNA replication.
Only once viral DNA replication has started can the late genes be activated. Through the activation of pathways that cause cell entrance into S phase, early proteins also contribute to changes in host-cell metabolism.
Cellular DNA synthesis only occurs during the S phase of the cell cycle, and cellular replication enzymes are only present during S phase.
Viral DNA replication
Viral DNA is duplicated once the cells are stimulated to enter S phase and the early genes of the virus are expressed. This process takes place within the nucleus of the host cell, where it leads to the creation of new viral genomes. In cells undergoing lytic infection, the nucleus can generate hundreds to thousands of these new viral genomes.
Late gene expression
After viral DNA replication has begun, the late genes are transcribed and translated to give rise to late proteins. Late genes encode the structural proteins of the virus, including capsid proteins and, for enveloped viruses, the matrix and envelope proteins.
Although both late and early viral proteins are made in the cytoplasm, they are frequently returned to the nucleus, where nucleocapsid assembly and viral replication take place.
Biosynthesis in dsDNA viruses
Almost all DNA viruses have genomes that are similar to the host cell i.e., they are composed of double stranded DNA, and are therefore able to utilize host enzymes to express viral genes and replicate viral DNA. The majority of DNA viruses carry out replication within the host cell’s nucleus, as this is where the necessary cellular machinery for replication and transcription is found. Following infection, the viral nucleocapsid is typically transported into the nucleus, where it undergoes uncoating. The DNA maybe either linear or circular. All dsDNA viruses, with the exception of Poxvirus, replicate in the nucleus and use host cell DNA-dependent RNA polymerase to produce their mRNA. They produce their capsid and other cytoplasmic proteins using the host cell’s enzymes.
Subsequently, the proteins enter nucleus and assemble with newly synthesized DNA to form virions and then these virions are transported along with ER to host cell membrane for release. The genome of all DNA viruses are dsDNA except Parvoviruses. The Poxvirus replicate in cytoplasm where they do not have access to the host cell RNA polymerase. Therefore, they carry their own polymerase within the viral particle. Three kinds of viral polypeptides are produced: early α proteins, early β proteins, and late γ proteins. Structural proteins are late proteins, while enzymes are early proteins.
Biosynthesis in dsDNA viruses (Cytoplasmic replication)
Initial transcription takes place in core of virion. Proteins produced release genome from virion core.
Example: Pox virus: Pox viruses are the best studied NCLD viruses. They enter host cell by receptor mediated endocytosis. The central core escapes from endosome and enters cytoplasm. Core contains viral DNA dependent RNA polymerase. The polymerase synthesizes early mRNA, one of which directs the production of enzyme that completes virus uncoating. DNA polymerase and other enzymes required for DNA replication are also synthesized early in life cycle. About the time DNA replication starts, the transcription of late genes is initiated. Many late proteins are structural proteins used in capsid formation.
Biosynthesis in single stranded (ss) DNA virus
They use host cellular DNA polymerase to double-strand the viral genome. Viral proteins are translated from mRNAs, which are created using the proper DNA strand as a template (by the host cell RNA polymerase).
Example: Parvovirus: The virus enters by receptor mediated endocytosis. The way nucleocapsid is released from endosome is still being studied. Once it escapes endosome, it is thought to be transported to the nucleus by host cell’s microtubules. Since the Parvovirus does not code for any enzyme, it has to depend on host cell enzymes for all biosynthetic activities. The nucleus is where DNA synthesis takes place, and because DNA is negatively stranded, it acts as a template for the production of mRNA.
The polypeptides needed for DNA replication mechanism replication are encoded by certain RNA products. Since the ends of genome is palindromic and they can fold back on themselves. Formation of hairpin at the 3′ end of genome occurs which provides the primer for DNA replication, which is recognized by Host Polymerase and thus replication occurs.