REPLICATION OF RNA

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REPLICATION OF RNA

  •  RNA stands for Ribonucleic acid; it is one of the most important nucleic acids in an organism Even though RNA serves as the genetic material in some organisms, DNA is considered to be the predominant genetic material in all organisms.
  •    RNA replication is the process by which new copies of genome-length RNAs are made
  •     RNA replication occurs in the cytoplasm and is carried out by the viral RNA polymerase.
    The full length plus strand is coated with nucleocapsid protein as it is made.     The full length plus strand is coated with nucleocapsid protein as it is made.
  •   mRNAs are not coated with this protein, which would interfere with the host protein translation machinery
  •    The new positive strand is copied into full length minus strand, which is also coated with nucleocapsid protein as it is made.
  •   RNA-dependent RNA polymerase (RdRP, RDR) or RNA replicase is an enzyme that catalyzes the replication of RNA from an RNA template. Specifically, it catalyses synthesis of the RNA strand complementary to a given RNA template.
  •   Replication also involves synthesis of viral messenger RNA (mRNA) from early genes viral protein synthesis, possible assembly of viral proteins, then viral genome replication mediated by early or regulatory protein expression.
  •    Viruses that replicate via RNA intermediates need an RNA-dependent RNA- polymerase to replicate their RNA  But animal cells do not seem to possess a suitable enzyme. Therefore, this type of animal RNA virus needs to code for an RNA-dependent RNA polymerase.
  • No viral proteins can be made until viral messenger RNA is available; thus, the nature of the RNA in the virion affects the strategy of the virus.
  • In plus-stranded RNA viruses, the virion (genomic) RNA is the same sense as mRNA and so functions as mRNA. This mRNA can be translated immediately upon infection of the host cell. Examples: poliovirus (picornavirus), Toga viruses, and Flavi viruses.

THE REPLICATION CYCLE OF POLIOVIRUS

The cellular life cycle of poliovirus, it is initiated by binding of a polio virion to the cell surface macromolecule CD155, which functions as the receptor

(1). Uncoating of the viral RNA is mediated by receptor-dependent destabilization of the virus capsid

(2). Cleavage of the viral protein VPg is performed by a cellular phosphodiesterase, and translation of the viral RNA occurs by a cap-independent (IRES-mediated) mechanism

(3). Proteolytic processing of the viral polyprotein yields mature structural and non-structural proteins

(4).The positive-sense RNA serves as template for complementary negative-strand synthesis, thereby producing a double-stranded RNA

(5). Initiation of many positive strands from a single negative strand produces the partially single-stranded replicative intermediate (RI)

(6).The newly synthesized positive-sense RNA molecules can serve as templates for translation or associate with capsid precursors to undergo encapsulations and induce the maturation cleavage of VP0

(7) Which ultimately generates progeny virions. Lysis of the infected cell results in release of infectious progeny virions

(8) RNA viruses are classified into distinct groups depending on their genome and mode of replication. Positive-sense ssRNA viruses (Group IV) have their genome directly utilized as if it were mRNA, with host ribosomes translating it into a single protein which is modified by host and viral proteins to form the various proteins needed for replication. One of these includes RNA dependent RNA polymerase (RNA replicase), which copies the viral RNA to form a double-stranded replicative form; in turn this directs the formation of new virions.

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