ter 13 Evolution and Diversity among the Microbes sno B The

ter 13. Evolution and Diversity among the Microbes sno B. The HIV Virus HIV belongs to a unique category of viruses called the retroviruses which have the ability to code for the enzyme which converts viral RNA to DNA. One reason that HIV is so difficult to treat is because it mutates at an incredible rate What feature of the virus is responsible for its frequent mutation? Explain what must happen with the HIV virus for it to trigger the development of AIDs in its host. The life cycle of the HIV virus can be seen in Figure 13-29. Label the appropriate events of each stage in the diagram. Host cell Viral DNA Host nucleus New viral New viral

Solution

HIV virus

1. The virus codes for reverse transcriptase enzyme, which converts RNA to DNA.

2. HIV virus undergoes spontaneous mutations. Reverse transcriptase enzyme in HIV does not have any proof reading activity. As a result mutations occur during viral replication resulting in mutation of the virus at an incredible rate.

3. HIV triggers AIDS by reducing the count of CD4+ T-cells. HIV infects these CD4+ T-cells and kills them. CD4= T-cells are also known as T-helper cells ply a crucial role in activating other cells of the immune system during infection. When the count of CD4+ cells falls below minimum, the person becomes vulnerable to opportunistic infections that results in AIDS. Cell to cell spread of HIV is respnsible for the massive death of CD4+ T-cells.

4. The events of HIV life cycle are as follows:

1. Entry of the virus in to the host cell by binding and fusion followed by release of RNA and reverse transcription of its RNA to DNA by reverse transcriptase.

2. Integration of the newly formed viral DNA in to the host chromosome.

3. Replication of the virus to form viral RNA and viral proteins.

4. Assembly of the virus and release of the virus from the cell by budding.