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The HIV Life Cycle

Understanding HIV Replication

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Updated July 19, 2007

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. This article will explain the process of HIV replication, will review the HIV life cycle step-by-step and will explain what HIV medications do to interrupt the process.

A Graphic Representation of the HIV Life Cycle

Introduction of HIV

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.

HIV Infection Routes and Risks

Viral Attachment

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.

This is What Viral Attachment Looks Like

Viral attachment is blocked by the class of drugs called entry inhibitors. Blocking this stage prevents HIV from using the T-cell. If allowed to attach, HIV uses the cell for the next steps in reproduction.

Entry Inhibitor Fact Sheets

Viral Fusion & Penetration

Once attached to the cell, HIV injects proteins of its own into the cellular fluids (cytoplasm) of the T-cell. This causes a fusion of the cell membrane to the outer envelope of the HIV.

This is What Viral Fusion & Penetration Looks Like

In order to block this step the class of drugs called entry or fusion inhibitors block this fusion between cell and HIV.

Fusion Inhibitor Fact Sheet

The Uncoating

In order to use its genetic material (RNA) for reproduction, the protective coating surrounding the RNA must be dissolved. Without this step, conversion of RNA to DNA (the building blocks of new HIV copies) can't take place, and reproduction is halted.

This is What Uncoating Looks Like

Reverse Transcription

Once in the cell, the single stranded RNA of the HIV must be converted to the double stranded DNA. It accomplishes this with the help of the enzyme reverse transcriptase. Reverse transcriptase uses building blocks from the T-cell to help change the HIV RNA to DNA. The DNA contains the genetic information needed for HIV reproduction.

This is What Reverse Transcriptase Looks Like

Drugs called reverse transcriptase inhibitors block HIV's reverse transcriptase from using these building blocks. Nucleoside and nucleotide analog reverse transcriptase inhibitors (NRTIs) and contain faulty imitations of the proteins found in a T-cell's cytoplasm. Instead of incorporating a protein into the growing chain of DNA, the imitation building blocks in NRTIs are inserted, which prevents the double strand of DNA from becoming fully formed.Non-nucleoside reverse transcriptase inhibitors block reverse transcription by attaching to the enzyme in a way that prevents it from functioning.

NRTI Medication Fact Sheets

More NRTI Fact Sheets

NNRTI Fact Sheets

Integration

To use the cell to reproduce, it must integrate the newly formed DNA into the cell nucleus. While the process is not fully understood, it is thought to be aided by transport proteins supplied by HIV.

This is What Integration Looks Like

Viral Latency

Once integration has occurred, HIV must wait for more protein building blocks to be formed by the cells or in other words, HIV is waiting for materials it needs to complete the reproductive process.

Final Assembly

Now that all the materials are available, they must be separated (cleavage) and assembled into new HIV. This process is possible because of the enzyme protease. This enzyme separates the parts allowing them to be reassembled into new HIV.

This is What Assembly Looks Like

Drugs called such as Kaletra, Crixivan, and Viracept bind to the protease enzyme and prevent it from separating, or cleaving, the subunits.

Protease Inhibitor Fact Sheets

Budding

The final step of the viral life cycle is called budding. With its genetic material tucked away and a new outer coat made from the host CD4 cell's membrane, the newly formed HIV pinches off and enters into circulation, ready to start the whole process again.

This is What Budding Look Like

The HIV Life Cycle - From Start to Finish

This is obviously a very simple look at the HIV life cycle. But it gives you an idea of what an intricate process HIV replication is. Our understanding of the life cycle continues to grow and from that knowledge new drugs targetting new stages in the life cycle are being developed. Giving HIV+ people medication options makes understanding the HIV life cycle a must.

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