Animal viruses differ from stages in mechanism of entering the host cell. This is due to differences in host cell i.e. one is prokaryotic and the other eukaryotic in nature.
It is accomplished into the following stages:
1. Attachment:
Animal viruses like bacteriophages posses the attachment sites with the help of which it attaches to the receptor sites present on host cell surface. The receptor sites are the proteins or glycoproteins present on membrane surface of the host cell.
The attachment sites of one group of viruses differ from the others. Distribution of these proteins plays a key role in tissue and host specificity of animal viruses. For example, poliovirus receptors are found only in human nasopharynx, gut and cells of spinal cord.
While receptors of measles virus are present in most tissues. Differences in nature of polio and measles can be explained through the dissimilarities in the distribution of receptor proteins of host cells to which viruses get adsorbed.
In some naked viruses (e.g. adenoviruses) the attachment sites are small fibres at the comers of icosahedron. In enveloped viruses (e.g. myxo-viruses) the attachment sites are the spikes present on the surface of envelope.
For example, influenza virus has two types of spikes: H (haemagglutinin) spikes and N (neuraminidase) spikes. The H spikes attach to the host cell receptor site and recognise siatic acid (N-acetyl neuraminic acid). Influenza neuraminidase helps the virus in penetrating the nasal and respiratory tract secretions by degrading the mucosal polysaccharides. However, the receptor sites also vary from person to person.
2. Penetration:
After the attachment, virus penetrates the host cell. In enveloped animal viruses, penetration occurs by endocytosis, a process of bringing nutrients into the cell.
If a virion attaches to a small out folding i.e. microvillus on plasma membrane of a host cell, it will unfold the virion into a fold or plasma membrane forming a vesicle. When the virion is enclosed within a vesicle, its envelope is disintegrated and the capsid is digested resulting in release of nucleic acid in cytoplasm.
The detailed mechanism of entry of virus in not clear. However, the following three modes of entry of viruses occur:
(a) Direct penetration:
Some naked viruses (e.g. poliovirus) undergo a major change in capsid structure after adsorption to plasma membrane. This change facilitates the release of nucleic acids into cytoplasm (Fig. 17.1A).
(b) Fusion with plasma membrane:
The envelop of enveloped viruses (e.g. paramyxoviruses) fuses directly with host plasma membrane and nucleocapsid is deposited in cytoplasmic matrix where uncoating is done (Fig. 17.1B). When virus is within the capsid, a virus polymerase attached to nucleocapsid transcribes the virus RNA.
(c) Endocytosis:
Most enveloped viruses enter the host cell through engulfment by receptor-mediated endocytosis and form coated vesicles. Virions attached to coated pits with the protein clathrin. Lysosomes help in uncoating of virion inside the cytoplasm. (Fig.17.1C).
3. Uncoating:
It is a process of separation of viral nucleic acid from the protein coat. This process is not fully understood. In some viruses uncoating is done by lysosomal enzymes of the host cell which degrade protein coat and make the nucleic acid free in cytoplasm. In poxviruses, the viral DNA synthesizes a specific protein after infection. Thus, it varies with virus groups.
4. Replication:
The replication process of DNA viruses differs from that of RNA viruses. However, in some DNA viruses multiplication occurs in cytoplasm (e.g. poxviruses) and in some others replication occurs in the nucleus of host (e.g. parvovirases, papovaviruses, adenoviruses, herpes viruses).
Multiplication of RNA viruses is more or less the same as in DNA viruses except the mechanism of formation of mRNA among the different group (Fig. 17.2).
5. Assembly and Release:
After replication of genetic material and synthesis of viral proteins assembly of viral particles occurs inside the host cell. Thereafter, they are released from the host cell.