al. 16). The attachment of the two receptors usually occurs through non-covalent interactions. In
some instances, the phages may also attach to the pili, flagella or capsules. In animal cells, the
receptors are highly specific for a given virus. Nevertheless, some viruses may use different
molecules as receptors.
Thereafter, the viruses enter into the host cell. Various viruses use different mechanisms
of entering into the cell. Some fuse with the cell’s plasma membrane. A good example of this is
the HIVgp41 which interacts with a cellular chemokine receptor to induce fusion (Domingo, et
al. 19). Other viruses enter the cell through endocytosis. In both cases, the capsid, containing the
nucleic acid and viral enzymes, joins the cytoplasm. The tail of the phage at this juncture is
usually contractile. Following the attachment, the tail core is pushed through when the
contractile rings are compressed. The DNA is then injected into the host cell. This action is
catalyzed by lysozymes as well as 144 molecules of ATP (Wagner and Hewlett 34). It is crucial
to note that enveloped viruses enter the cell by fusing their membrane with the host membrane
while others enter by receptor-mediated endocytosis.
The next step usually involves targeting the site of viral replication. Biologically, host
DNA viruses are known to replicate in the nucleus. On the other hand, RNA viruses replicate in
the cytoplasm. In some instances, some viruses integrate their double-stranded DNA into the
host cell's chromosomes. Others copy their RNA into a double-stranded DNA, which is in turn
integrated into the host cell's genome.
The fourth stage involves the uncoating of the virus. Once the virus nucleocapsid has
been injected into the host cell, the nucleic acid has to be released. This occurs in a process
known as uncoating. The capsid is composed of protein subunits. The nucleic acids usually
dissociate from the subunits. Subsequently, the capsid disintegrates hence liberating the nucleic