An Envelope Of An Animal Virus Is Acquired During Which Of The Following Steps?

The following points highlight the 6 chief stages involved in the replication of animal viruses. The stages are: 1. Adsorption 2. Penetration 3. United nations-Coating 4. Replication of Viral Genome 5. Synthesis and Associates of Virus Capsids 6. Release of New Virus.

Stage # one. Adsorption:

Adsorption to the host cell surface is the beginning pace in reproduction cycle of beast viruses. Adsorption of virion to the host jail cell surface takes place through a random collision of virion with a plasma membrane receptor site; the receptor is a protein, and frequently a glycoprotein. Brute viruses, like bacteriophages, possess attachment sides with the aid of which it attaches to the receptor site.

Besides of glycoprotein receptors, sometimes, a complex carbohydrate (e.1000., heparan sulfate) is the receptor, these receptors vary in their distribution pattern on plasma membrane and this distribution variation plays a central role in tissue and host specificity of animal viruses.

For instance, poliovirus receptors are plant only in human being nasopharynx, gut, and cells of spinal cord. While receptors of measles virus occur in most tissues.

Differences in nature of polio and measles tin exist explained through the dissimilarities in the distribution of receptor proteins of host cells to which viruses become adsorbed. In some naked viruses (e.chiliad. adenoviruses) the attachment sites are pocket-size fibres at the corners of icosahedron. In enveloped viruses (due east.1000. myxoviruses) the zipper sites are the spikes present on the surface of envelope.

For example, influenza virus has two types of spikes: H (haemagglutinin) spikes and North (neuraminidase) spikes. The H spikes attach to the host cell receptor site by recognising sialic acrid (N-acetyl neuraminic acid), the saccharide derivative of glucoprotein.

Influenza neuraminidase helps the virus in penetrating the nasal and respiratory tract secretions by degrading mucosal polysaccharides. All the same, the receptor sites besides vary from person to person.

Stage # 2. Penetration :

Beast viruses penetrate the host prison cell presently after adsorption.

Though the detailed mechanism of penetration in not clear, the post-obit three modes are the most favoured past the researchers:

1. Direct penetration:

Some naked animal viruses (e.g., picornaviruses, adenoviruses) use vesicle acidification that causes a major alter in capsid structure after adsorption to plasma membrane. This altered capsid contacts the vesicle membrane and either releases the viral genome into the cytoplasm through a membrane pore (picornaviruses) or ruptures the membrane to release the viral genome (adenovirus) into the cytoplasm (Fig. xiv.3).

2. Fusion with plasma membrane:

The envelop of enveloped virus (e.1000., paramyxoviruses) fuses directly with host plasma membrane. Fusion may involve special envelop fusion proteins that bind to plasma membrane proteins. Finally, the nucleocapsid enters the cytoplasmic matrix where un-coating is done (Fig. 14.four). A virus polymerase associated with the nucleocapsid, transcribes the virus RNA while the latter is still within the capsid.

3. Endocytosis:

Many of the enveloped viruses and sure not-enveloped viruses enter the host cell through engulfment by receptor-mediated endocytosis and form coated vesicles.

The virions attach to coated pits with the poly peptide clathrin and the pits and then compression off to form coated vesicles filled with viruses. These vesicles fuse with lysosomes later on the clathrin has been removed. Lysosomal enzymes aid in un-coating of virion inside the cytoplasm. (Fig. xiv.5).

Stage # three. Un-Blanket :

Un-blanket is the process of separation of viral genome from the poly peptide coat. Though the process of united nations-blanket is not fully understood, it is proclaimed that the lysosomal enzymes aid in animal virus un-coating by degrading the capsid and low endosomal pHs ofttimes trigger the process of united nations-blanket.

It has been reported in some cases that the viral envelop fuses with the lysosomal membrane and the partially degraded capsid along with viral genome (nucleocapsid) is released into the host cytoplasm. Once in the cytoplasm, viral genome may exist released from the capsid upon completion of united nations-coating or may function while notwithstanding attacked to capsid components.

Stage # 4. Replication of Viral Genome :

The replication process of DNA viruses differs from that of RNA viruses. Withal, in some DNA viruses the replication takes place in cytoplasm (east.g., poxviruses) and in some others in the nucleus of host (due east.thou., parvoviruses, papovaviruses, adenoviruses, herpes viruses).

Replication of viral genome in RNA viruses is more than or less the same as in DNA viruses except the mechanism of formation of mRNA among the different group.

Phase # 5. Synthesis and Assembly of Virus Capsids :

Certain late genes direct the synthesis of capsid proteins. The latter spontaneously self-get together to form the capsid. It appears that in example of icosahedral viruses the capsid poly peptide assembly get-go forms procapsid in which the viral genome is inserted by some unknown machinery. All the same, in case of enveloped viruses the capsid protein assembly is generally similar to that of naked viruses (poxvirus is exception).

The capsids of these viruses are assembled in the cell cytoplasm by a lengthy, circuitous procedure that begins with the enclosure of a portion of cytoplasmic matrix through construction of a new membrane. Now the newly synthesized viral DNA condenses, passes through the membrane, and moves to the centre of the immature virus.

Stage # 6. Release of New Virus :

Release of newly formed animal viruses from the host prison cell differs between naked and enveloped viruses. The naked animate being viruses are released virtually often by the lysis of the host jail cell. In enveloped viruses, however, the virus-encoded proteins are incorporated in the plasma membrane and so the nucleocapsid is simultaneously released; the envelop is formed by membrane-budding.

Source: https://www.biologydiscussion.com/viruses/replication-of-animal-viruses-6-main-stages/54919

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