حقن الاجنة

Avirus is a set of genes, composed of either DNA or RNA, packaged in a proteincontaining
coat. The resulting particle is called a virion. Viruses that infect humans
are considered along with the general class of animal viruses; viruses that infect bacteria
are referred to as bacteriophages, or phages for short. Virus reproduction requires
that a virus particle infect a cell and program the cellular machinery to synthesize the
constituents required for the assembly of new virions. Thus, a virus is considered an
intracellular parasite. The infected host cell may produce hundreds to hundreds of
thousands of new virions and usually dies. Tissue damage as a result of cell death
accounts for the pathology of many viral diseases in humans. In some cases, the
infected cells survive, resulting in persistent virus production and a chronic infection
that can remain asymptomatic, produce a chronic disease state, or lead to relapse of an
infection.
In some circumstances, a virus fails to reproduce itself and instead enters a latent state
(called lysogeny in the case of bacteriophages), from which there is the potential for reactivation
at a later time. A possible consequence of the presence of viral genes in a latent
state is a new genotype for the cell. Some determinants of bacterial virulence and some
malignancies of animal cells are examples of the genetic effects of latent viruses. Apparently
vertebrates have had to coexist with viruses for a long time because they have
evolved the special nonspecific interferon system, which operates in conjunction with the
highly specific immune system to combat virus infections.
In the discussion to follow, the biological and genetic bases for these phenomena are
presented; three themes are emphasized.
1. Different viruses can have very different genetic structures, and this diversity is reflected
in their replicative strategies.
2. Because of their small size, viruses have achieved a very high degree of genetic
economy.
3. Viruses depend to a great extent on host cell functions and, therefore, are difficult to
combat medically. They do exhibit unique steps in their replicative cycles that are
potential targets for antiviral therapy