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Viral Life Cycle

Virology: The Viral Lifestyle ©

 

Stage 1: Attachment

*   Attachment is termed as the specific binding of the attachment site of virus on its host cell receptor sites

*   Process is highly specific

*   Just like a lock and a key

 

Stage 2: Penetration

2 general mechanisms/methods

First Method

*   Direct fusion of viral envelope with host cell surface membrane

*   Virus envelope merges with host cell surface membrane

*   Delivers viral capsid into cytoplasm of host cell

Second Method

*   Endocytosis of Virus

*   Membrane deepens to form a vesicle, enclosing the material to be taken in

*   H+ protons are pumped into vesicle, lowers pH condition

*   Lysomal degradative enzymes work optimally at low pH condition

*   Vesicles fuse with lysosome; enzymes break down vesicle membrane

*   Releasing viral capsid into host cell

 

?STOP n THINK?

Endocytosis is a process that involves the internalization of the plasma membrane in the form of a vesicle, enclosing the material to be taken in.

 

Stage 3: Uncoating

*   Refers to all events that occur between entry of virion and expression of viral genome

*   Viruses have nuclear localization signals in their genome

*   Thus virus genome can make their way to the genome

*   By docking itself to a nucleospore

*   Microtubules attached to the capsid shrinks, drawing it closer to the nucleus

*   Simultaneously, disassembly of capsid occurs

*   DNA/RNA and associated proteins are then drawn into the nucleus

 

Stage 4: Transcription and Replication

First Step: The DNA of the virus gets transcribed into high molecular weight RNA molecules.

Second Step: Host cell modifies RNA by adding a 5’ cap and 3’ poly-Adenine tail, removing any introns. Introns are non-coding sequence.

Third Step: The modified RNA travels out of nucleus into cytoplasm and gets translated in the ribosomes to form primary structured proteins (linear shape). These proteins further fold (by disulphide bonds) and cleave (by phosphorylation) to become active. 

*   RNA viruses have problems using its RNA genome as a copy template

*   Requires the help of RNA polymerase

*   DNA viruses encounter problems in replicating, requires the presence of OH at 3’end

*   DNA viruses develop strategies like circularizing, elongating and reverse transcribing to overcome their shortcomings

 

Stage 5: Assembly

RNA icosahedral capsid

First step: nucleation reaction. Nucleation reaction refers to the interaction of RNA with a few capsid subunits to form an initiation complex.

Second step: dimers made up of individual subunits are added to form a “cap” on vertices.

Third step: additional dimers are added subsequently to the growing shell.

 

?STOP n THINK?

Dimers are biological entity that is made up of 2 monomers.

 

DNA icosahedral capsid

·       Condensation of genomic DNA with histones to produce nucleosomes.

·       Results in a compact particle around which capsid is built and histone is lost during formation of capsid.

 In short, the process of assembly involves viruses packaging their genomes in “gift wrappers”. This gift wrappers are the viral capsids.

 

Stage 6: Release

Naked Viruses

·       Suggested that there may be some disruption of lysosomes, where lysomal hydrolytic enzymes are released into the cytoplasm

·       Causes host cell to lyse or break apart

Enveloped Viruses

·       Strategy: Budding & Exocytosis

·       Viral glycoproteins and proteins are inserted into host membranes

·       Translation for viral protein membrane code occurs

·       Exocytosis: Nucleocapsid becomes enwrapped in membrane to form a vesicle

·       Vesicle pinched off from membrane

New baby virions then go on to infect more healthy neighbouring cells.

 

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