The function of the immune system is to protect animals from foreign agents and infectious organisms. It responds in a specific way to pathogens and displays a long term memory of earlier contacts with the disease agents. The immune system consists of two functional components:

  1. Non-specific defence (innate or non adaptive immune system)
  2. Specific defence (adaptive immune system)

     Virusurile sunt entitati infectioase complexe si organizate, deoarece contin doua componente esentiale: una genetica (acidul nucleic) si una proteica . Uneori, proteinele nu sunt esentiale pentru exprimarea proprietatilor sistemului, pentru ca acidul nucleic in stare pura este infectios.

 Caracterul inalt organizat al virionului rezulta din faptul ca relatiile spatiale dintre genom si capsida sunt definite si constante si deriva din simetria la nivel molecular.

    Retrovirus contain viral RNA and several copies of reverse transcriptase (DNA polymerase). After infecting a cell, the reverse transcriptase is used to make the initial copies of viral DNA from viral RNA. Once a DNA strand has been synthesized, a complementary viral DNA strand is made. These double strand copies of viral DNA are inserted into the host-cell chromosome and host-cell RNA polymerase is used to make virus-related RNA. 

   These RNA strands serve as templates for making new copies of the viral chromosomal RNA and serve also as mRNA. mRNA is translated into viral proteins that are used to make the virus envelope. New viral particles are assembled, bud from the plasma membrane, and are released. An example of this process is illustrated in the replication of the retrovirus, HIV (human immunodeficiency virus).

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