Stages of Replication

Stages of Replication 1) Attaching In the attachment phase, replication occursVirus by binding of viral proteins (or 'antireceptors' ) to cellular receptor molecules of target cells. receptors on the cell surface can be proteins (generally glycoproteins) or example, carbohydrate residuesPoxviridae attached or to glycoproteins ormore glycolipids. In some complex viruses (for the genus Herpesviridae) than one receptor isantigen, used. Annamely exampleCD4, is Human Immunodeficiency Virus-1 (HIV-1),ofthethemain receptor is the T-cell which is the binding site for attachment HIV-1 protein (antireceptor) gpthe120. So theyarehave anbacterial alternative route when penetrating into use cells.cellular Most bacteriophages, receptors in the cell wall, although certain phages components such as pili, flagella as the main adsorption site. 2) Penetration The entry ofreceptor virus particles into host cells normallyisoccurs someantime after the attachment of the virus to the is complete. Cell penetration generally energy-dependent process, i.e.of cells must be metabolically active for penetration to occur. The three main mechanisms penetration are: a.process The process of translocating all virus acrossandthespecific cytoplasmic membrane. is mediated by proteins present in theparticles viral capsid membrane receptors.This b.common The process of endocytosis of toviruses into intracellular vacuoles. At thisspecific stage, viral it is the most mechanism for viruses enter cells. This stage does not require proteins but relies onendocytosis the formation andefficient internalization of holes layeredupin the cellcollecting membrane. Receptormediated is an process for taking and extracellular macromolecules. c.either The directly process ofonfusion of the viral orenvelope (ifendocytosis the virus hasin one) with the vesicles. cell membrane, the cell surface through cytoplasmic Fusion requires the presence of a specific fusion protein in the viral envelope, for example hemagglutinin intheinfluenza A the viruses orandtransmembrane glycoproteins from retroviruses. This protein triggers fusion of cell virus membranes which produces the nucleocapsid and goes directlyandtopH-independent. the cytoplasm. Viruses will trigger membraneoccurs fusionalmost in two universally ways, namelyin pH-dependent The process of endocytosis animal the formation of clathrin-coated holes that result in engulfment of the vesiclescells by theandcell'requires s cytoplasm 3) Uncoating Uncoating describes thedegraded events that occur aftergenome the virusisenters the usually host cell.stillThein viral capsidof isa partially or completely and the viral exposed, the form complex ofdifficult nucleicto acids and proteins. Uncoating occurs simultaneously or into soontheafter entry making it study. In bacteriophages that insert their genome directly host cell, penetration and uncoating processes occur simultaneously. The final product ofuncoatingdepending on the structure of the viral genome or nucleocapsid. only In some viruses, the resulting structure is relatively simple, for example,Picornaviruseswhich has a small basic protein of around 23 amino acids. In some other viruses, the nucleocapsid is very complex, for exampleisinremoved the Poxviruses uncoating process which occurs inenter twothe stages. First, the outer membrane when the virus particles enter the cell and cytoplasm, followed by uncoating the and nucleocapsid the cytoplasm. In this case, the nucleus still contains a lot ofwith protein there are atentering least 10 different enzymes. 6) Ripening Maturation is one of the stagesstructural of the replication cycle in which viral particles become infectious. This process often involves changes to the new particle resulting from specific cleavage of viral proteins to form the mature product. Apart from that, this process also involves changes in protein conformation thatinoccur duringstructure the assembly process, for example instructural protein hydrophobic interactions. Changes protein often lead to substantial changes in theof nucleoproteins capsid. Sometimes aregenome also internal changes, for example condensation with there the viral and thisstructural event is often observed with an electron microscope. 7) Disengagement In viruses thathostundergo a lyticandcycle in most enveloped viruses,Cells shedding is with a simple process. The infected cell bursts releases new virus particles. infected viruses often rupture and become damaged because viral replication can disrupt normal cellular functions, for example, the expression ofwhich essential genes. Many viruses canof also encode proteins that trigger or suppress apoptosis, also results in the release viral particles. Enveloped viruses acquire theirbefore lipid membrane when they exit the host are cell taken membrane, or into intracellular vesicles release. The virion envelope proteins up during this process and used as viral particles before leaving the cell. This process is known as budding. As noted previously, assembly, maturation, and release are generally simultaneous processes for viruses released by budding.