VIRUS REPLICATION

Ajaydesouza February 25, 2023

 VIRUS REPLICATION


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PLANT VIRUS REPLICATION














PLANT VIRUS REPLICATION



STAGES OF VIRUS REPLICATION

  1. ATTACHMENT OF VIRUS

  2. PENETRATION OF HOST

  3. UNCOATING

  4. TRANSCRIPTION

  5. TRANSLATION

  6. ASSEMBLY

  7. RELEASE OF VIRION



ATTACHMENT (ADSORPTION)

  • This is the first steps in virus infection in which interaction of virus with a specific receptor site on the surface of host cell occurs

  • Viral protein on the capsid or phospholipid envelope interacts with specific receptors on the host cellular surface. This specificity determines the host range (tropism) of virus



PENETRATION

  • The virus enters the plant cell by wounds created artificially, by vector, or naturally when an infected pollen grain deposits itself inside an ovule

  • The process of attachment to a specific receptor can induce conformational changes in viral capsid protein or the lipid envelop those results in fusion of viral and cellular membranes.





Mode of entry of plant viruses

Mode of entry

Virus name

Mechanical wounds

TMV, PVX, BCMV

Wounds made by vectors

PLRV, WTV, TLRL

Seeds or vegetative propagation

CMV, BCMV, Pea stripe virus







UNCOATING

  • Once within the host, the virus sheds its capsid or coat and becomes naked.

  • Uncoating is defined as release of viral genome from capsid and is accessible to enzyme required to translate, transcribe and replicate it



TRANSCRIPTION

  • The process of transcription involves copying information from a strand of DNA into a new messenger RNA molecule (mRNA). DNA preserves genetic material in the cells' nucleus securely and permanently as a template or reference.

  • Transcription of viral genome is usually the next step in all virus except in those viruses whose genome acts directly as mRNA (Eg. Picorna virus)

  • RNA viruses that carry minus (-) stranded RNA first transcribe their RNA to plus (+) stranded RNA that functions as mRNA (transcriptase)

  • The transcription is catalyzed by viral RNA polymerase released during uncoating

  • The genetic information replicated from DNA is carried by messenger RNA (mRNA) in the form of a sequence of three base “code words” that each designate a certain amino acid.



Host cell strategies

Nucleic acid conversion

Enzymes for replication

RNA to RNA

RNA dependent RNA polymerases

RNA to DNA

RNA dependent DNA polymerases (Reverse Transcriptase)

Host cell DNA to RNA

DNA dependent RNA polymerases





TRANSLATION

  • A process by which viral mRNA is translated into viral protein, using the host cellular machinery (amino acid, ribosomes, t-RNA of host).

  • The ribosome, a structure that is a factory for the creation of proteins, is where translation takes place.







REPLICATION OF GENOME

  • Replication is the process of creating new RNA molecules that are identical to the template RNA.

  • The virus specific enzyme that replicates proteins all (+) ss-RNA viruses that encode one or more proteins are replicated by an enzyme called RNA polymerase (RNA dependent).

  • After nucleic acid replication many copy of progeny nucleic acids formed.

  • The viruses begin to synthesis mRNA utilizing host cell machinery for the massive synthesis of viral RNA particles that come.



ASSEMBLY

  • Once the virus's nucleic acid and protein subunits have been created, the virus’s nucleic acid arranges the protein subunits around it to form virions, the entire virus particle.

  • With RNA viruses (ss-RNA or ds-RNA), the entire process occurs in the plant cell's cytoplasm.

  • The virions of the ss-DNA virus are created in the nucleus and released into the cytoplasm of the host cell.



RELEASE

  • The mechanism of virus release vary with types of virus.

  • The naked viruses are generally released by cell lysis.

  • The enveloped viruses are released by budding through special area of host cell membrane during which virion acquire a portion of host cell membrane

  • In some animal and plant virus host cells are not killed the virus release through special channels

  • The formed virus particles are released into various plant sections via physiological plant processes.



ds-DNA VIRUS REPLICATION

  • The cell nucleus is where viral ds-DNA first enters and gets twisted and supercoiled to produce a mini chromosome.

  • This double-stranded viral DNA, which resembles a mini chromosome, is then translated into two single-stranded RNAs and carried into the cytoplasm.

  • In this process, one tiny strand is translated into the virus coat protein, and the other big RNA strand is encapsulated by coat protein subunits and utilized as a template for reverse transcription to create the entire virion ds-DNA

ds-DNA VIRUS REPLICATION

  • Virion attaches to the cell via receptor binding

  • Virion enter cell in a vesicle (Eukarya)

  • Partially uncoated virion reaches nucleus (Eukarya)

  • DNA enters nucleus / is uncoated

  • DNA available in nucleus / cell

  • Transcription of “early” mRNAs

  • Transcription of “early” proteins

  • Interaction of “early” proteins with dsDNA

  • DNA replication

  • Secondary transcription

  • Translation of structural proteins

  • Partial assembly of protein with DNA (in nucleus)

  • Export from nucleus (or) final assembly

  • Export from the cell



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