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Viruses may undergo two types of life cycles: the lytic cycle and the lysogenic cycle. In the lytic cycle, the virus introduces its genome into a host cell and initiates replication by hijacking the host's cellular machinery to make new copies of the virus. [12] In the lysogenic life cycle, the viral genome is incorporated into the host genome.
To enter the cells, proteins on the surface of the virus interact with proteins of the cell. Attachment, or adsorption, occurs between the viral particle and the host cell membrane. A hole forms in the cell membrane, then the virus particle or its genetic contents are released into the host cell, where replication of the viral genome may commence.
In the lytic cycle, the viral DNA exists as a separate free floating molecule within the bacterial cell, and replicates separately from the host bacterial DNA, whereas in the lysogenic cycle, the viral DNA is integrated into the host genome. This is the key difference between the lytic and lysogenic cycles.
A viral infection does not always cause disease. A viral infection simply involves viral replication in the host, but disease is the damage caused by viral multiplication. [5] An individual who has a viral infection but does not display disease symptoms is known as a carrier. [17] Mechanisms by which viruses cause damage and disease to host cells
The replication cycle of poliovirus is initiated by binding to the cell surface receptor CD155 (1). The virion forms a pore in the cell membrane through which viral RNA is released into the cytoplasm (2). Translation of the viral RNA occurs by an IRES-mediated mechanism (3). The polyprotein is cleaved, yielding mature viral proteins (4).
A simplified diagram of the hepatitis C virus replication cycle. Replication of HCV involves several steps. The virus replicates mainly in the hepatocytes of the liver, where it is estimated that daily each infected cell produces approximately fifty virions (virus particles) with a calculated total of one trillion virions generated.
Poxviridae replication cycle. Replication of the poxvirus involves several stages. [14] The replication can be diveded on early, intermadieate and to late phase. The virus first binds to a receptor on the host cell surface; the receptors for the poxvirus are thought to be glycosaminoglycans.
Replication cycle of vesicular stomatitis virus (VSV) Viral replication is cytoplasmic. The replication cycle is the same for most rhabdoviruses. All components required for early transcription and the nucleocapsid are released to the cytoplasm of the infected cell after the first steps of binding, penetration and uncoating take place. [9]