What causes HIV to evolve so rapidly?

Answer 1

The human immunodeficiency virus (HIV) has a relatively simple structure: a compact sequence of genetic codecomposed of RNA molecules (ribonucleic acid), accompanied by a powerful enzyme (reverse transcriptase), all packaged within a protective shell that can attach itself to the wall of specific living cells in the human body. Once an HIV virus bonds to a cell of the right type, its RNA and reverse transcriptase enzyme can pass through the cell wall into the cell's interior.

There, the viral RNA is able to transcribe itself into a comparable strand of DNA (deoxyribonucleic acid), using molecules already present in the cell for raw materials, and helped by the reverse transcriptase enzyme. The RNAtranscriptionprocess is error prone, however, and here is where variation enters the picture. The new DNA version, including whatever changes have been introduced during copying, is moved into the cell's nucleus by the cell's own natural transport mechanisms. Once inside the nucleus, the alien DNA is incorporated into the cells own genes.

Now the cell's natural replication mechanisms take over. From the information stored in the alien DNA, multiple copies of the original viral RNA are manufactured, along with multiple copies of proteins and other molecules that are needed to build shells for the new viruses. These parts then begin to self-assemble inside the cell and bud from its walls to form complete new viruses, which travel through the body until they encounter more cells of the right type. Once they attach to those, the entire process is triggered all over again.

The power of evolution to drive this process is augmented by the huge numbers involved. Billions of new HIV viruses can be manufactured inside one human body in a single day. Each time a virus transcribes itself inside a new host T-cell, the chances are high that its copies will contain variations. While most of these variations will tend to render the next generation less fit to survive, a few will turn out to be beneficial, depending on the circumstances.

Those circumstances are produced by anti-viral drugs, which generally work by introducing molecules that interfere with one stage of the HIV transcription/replication process. Such interferences can greatly reduce the number of viral copies being produced inside an infected person at any one time, but the drugs can never completely eliminate the presence of the virus, or stop the replication process entirely. This is because the genes that manufacture the new viruses are embedded deep inside cells that the body cannot afford to lose. These cells, known as T-cells, are essential to the function of the human immune system. Drugs that might attack and kill T-cells would simply be doing what HIV itself already does. Once HIV begins to use a T cell to manufacture copies, the replication process eventually destroys that cell. And once enough T-cells are being destroyed on a daily basis that the body can no longer keep up with the loss by manufacturing new ones, the body's immune system begins to fail. The body then falls prey to opportunistic infections of many kinds, resulting in the varied symptoms of full-blown AIDS.

Answer 2

Viruses have either a single strand of DNA or RNA and since they don't have any sort of "spell check" the mistakes they make are not removed. This causes rapid mutation.