After completing this lesson, you will be able to: 

·        describe the life cycle of HIV;

·        identify stages in the life cycle targeted by antiretroviral drugs; and

·        teach pupils the simplified form of the life cycle of HIV.

In this lesson, we shall describe the life cycle of HIV as a series of steps. Six steps are commonly seen. These are binding; reverse transcription, integration, transcription, translation, and viral assembly. Before we begin, let us review what we learned in lesson 1 about the structure of HIV. In that lesson, we learned that viruses consist of two major parts- an outer protective coat called a capsid which is made of protein; and an inside which consists of genetic material. The genetic material is either of two substances with rather long names. These names have been abbreviated as DNA and RNA. DNA stands for deoxyribonucleic acid while RNA stands for ribonucleic acid. It is also worth noting that the capsid may or may not have an outer envelope made of fat.

Representation of HIV                  Structure of the virus

In lesson 3,  we also learned that the body’s immune system is made up of white blood cells (otherwise known as lymphocytes). One type of lymphocyte is called T-lymphocytes or T-cells with surface receptors known as CD4+. Let us now examine the stages in the life cycle of HIV. As stated earlier, six stages will be described.

Step 1: Binding

HIV binds to a CD4+ surface receptor, it activates other proteins on the cell's surface, allowing the HIV envelope to fuse to the outside of the cell.

Step 2: Reverse Transcription

The virus infects the cell- a process called "reverse transcription" takes place. At the end of the process the cell makes a DNA copy of the virus's RNA. After the binding process, the inside of the virus which contains the RNA and important enzymes is released into the host cell. A viral enzyme called reverse transcriptase makes a DNA copy of the RNA. This new DNA is called "proviral DNA."

Step 3: Integration

The HIV DNA is then carried to the cell's nucleus, where the cell's DNA is kept. Then, another viral enzyme called integrase hides the proviral DNA into the cell's DNA. When the cell tries to make new proteins, it can accidentally make new HIVs. Integration can be blocked by integrase inhibitors, a new class of drugs that are in the earliest stage of research.

Step 4: Transcription

Once HIV's genetic material is inside the cell's nucleus, it directs the cell to produce new HIV. The strands of viral DNA in the nucleus separate, and special enzymes create a complementary strand of genetic material called messenger RNA or mRNA (instructions for making new HIV). Transcription can be blocked by antisense antivirals or transcription inhibitors (TIs), new classes of drugs that are in the earliest stage of research.

Step 5: Translation

The mRNA carries instructions for making new viral proteins from the nucleus to a kind of workshop in the cell. Each section of the mRNA corresponds to a protein building block for making a part of HIV. As each mRNA strand is processed, a corresponding string of proteins is made. This process continues until the mRNA strand has been transformed or "translated" into new viral proteins needed to make a new virus.

Step 6: Viral Assembly

Finally, a new virus is assembled. Long strings of proteins are cut up by a viral enzyme called protease into smaller proteins. These proteins serve a variety of functions; some become structural elements of new HIV, while others become enzymes, such as reverse transcriptase. Once the new viral particles are assembled, they bud off the host cell, and create a new virus. This virus is then able to infect new cells. Each infected cell can produce a lot of new viruses.

Resources: Chart showing six stages in the life cycle of HIV.

Procedure: Using the chart showing the life cycle of HIV, lead a class discussion to describe the different stages in the life cycle of HIV. As the lesson progresses, call on pupils to describe what they see in the chart, stage by stage. Explain at each stage what role(s) antiviral drugs play in slowing down the process. Pupils to stage a drama to illustrate the stages.

In this lesson, we learned about six stages in the life cycle of HIV as follows:

Step 1: Binding

HIV binds to a CD4+ surface receptor, it activates other proteins on the cell's surface, allowing the HIV envelope to fuse to the outside of the cell.

Step 2: Reverse Transcription

The infected cell makes a DNA copy of the virus's RNA.

Step 3: Integration

A viral enzyme called integrase hides the proviral DNA into the cell's DNA. Then, when the cell tries to make new proteins, it accidentally make new HIVs.

Step 4: Transcription

Once HIV's genetic material is inside the cell's nucleus, it directs the cell to produce new HIV.

The strands of viral DNA in the nucleus separate, and special enzymes create a complementary strand of genetic material called messenger RNA or mRNA (instructions for making new HIV).

Step 5: Translation

The mRNA carries instructions for making new viral proteins from the nucleus to a kind of workshop in the cell. As each mRNA strand is processed, a corresponding string of proteins is made. This process continues until the mRNA strand has been transformed or "translated" into new viral proteins needed to make a new virus.

Step 6: Viral Assembly

Finally, a new virus is assembled. Once the new viral particles are assembled, they bud off the host cell, and create a new virus. This virus is then able to infect new cells. Each infected cell can produce a lot of new viruses.