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)

    Use of the reverse transcriptase enzyme is a unique characteristic of retroviruses such as HIV. After the virus releases its genetic material into a host cell, reverse transcriptase converts the viral RNA into a complementary piece of DNA, a process is known as ‘reverse transcription’.

   Reverse transcriptase works by looking in turn at each of the nucleoside building blocks that make up the viral RNA and using them as a template to assemble a complementary DNA chain using nucleotide building blocks present in the cell.

    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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