In 1985, scientists discovered the human immunodeficiency virus (HIV) and with it the question what is aids was answered. HIV is a virus that is transmitted from person to person through the exchange of body fluids such as blood, semen, breast milk and vaginal secretions. Sexual contact is the most common way to spread HIV/AIDS, but it can also be transmitted by sharing needles when injecting drugs, or during childbirth and breastfeeding. As HIV/AIDS reproduces, it damages the body's immune system and the body becomes susceptible to illness and infection.
Understanding the HIV life cycle and HIV replication has made it possible to develop the medications we use to treat HIV and AIDS. Knowing about HIV replication or how HIV makes copies of itself allows us to develop ways to block the process, and in turn slow HIV's attack on our immune system.
Obviously, before HIV infection can occur it must enter the body. Exposure to infected bodily fluids through sexual contact or sharing of neeedles is the primary way HIV enters the body. Infection through child birth and breastfeeding are also ways people become exposed to HIV.
Anatomy of an HIV virus
The outer coat of the virus is called the viral envelope or lipid membrane (see figure above). The viral envelope is composed of two layers of fat molecules (lipid means fat). HIV gets its outer envelope from its host. As newly formed HIV particles break through a host cell's surface in a process called "budding," they wrap themselves in fat molecules from the host's outer membrane (NIAID, 2001).
The complex proteins that protrude through the surface of the viral envelope are frequently called spikes (see figure above). These spikes are HIV's landing gear, attaching the virus to a host cell and fusing the two together. Each HIV has an average of 72 spikes. Each spike is made up of two parts: a stem and a cap.
Within the viral envelope of a mature HIV particle is a bullet-shaped core called the capsid (see figure above). The capsid surrounds two single strands of HIV's single-strand genetic material, ribonucleic acid (RNA). Each strand of RNA has a copy of the virus's genes. These genes contain the information that HIV uses to make new virus particles. HIV has only nine genes, in comparison to human cells, which have an average of 30,000-50,000 genes. The capsid also houses two molecules of HIV reverse transcriptase. Reverse transcriptase is an enzyme that allows the HIV's RNA to change into double-strand deoxyribonucleic acid (DNA), so that it can pass into the host cell's nucleus, commandeer the host cell, and begin reproducing itself (NIAID, 2001).
Infectious Process Defined:
Once in the body, HIV needs a host to help it reproduce. The host in the case of HIV is the T-cell or CD4 cell. HIV seeks out CD4 cells and must attach to them by way of a "lock and key" type system. Proteins on the surface of HIV attach to complimentary proteins on the CD4 cell much like the way a key fits into a lock.
The HIV infection starts with the attachment of HIV by way of the gp120 protein to the CD4 target cell.
After attachement, HIV releases genetic material into the CD4 cell.
Partial uncoating of the viral core occurs to expose the viral RNA. Once in the cell cytoplasm, the conversion of the viral RNA into double-stranded DNA commences as the viral reverse transcriptase becomes active.
Reverse transcriptase synthesizes a double-stranded DNA copy of the single-stranded viral RNA generating a provirus.
The viral DNA migrates to and enters the host cell nucleus and becomes integrated into the cell DNA with the help of the enzyme integrase.
Once inside the host cell nucleus, the RNA changes viral DNA into RNA.
The viral mRNA leaves the nucleus. The translation of the viral mRNA results in the synthesis of three polyproteins essential in the continue process of viral reporduction.
Proteins associate with the inner surface of the plasma membrane and interact with proteins present in the plasma membrane. As these proteins accumulate on the inner surface of the plasma membrane, they aggregate and commence assembly to form the virus. As assembly continues, the new HIV leaves the cell.
As the virus buds from the cell, it acquires a lipid coat, carrying the gp 120 and gp 41 proteins. The virus is extruded into extra-cellular space in this immature state.
During (or soon after) the budding of the new HIV particle from the host cell membrane, the viral proteinase in p160 becomes active, generating the mature form of HIV. It's at this final step the the cycle begins again to form more HIV.
Below is a short video of the process to help you better understand the process: