The Human Immunodeficiency Virus (HIV)
What criteria or characteristics would you use to group living things versus nonliving things? Scientists have grappled with this question for years, and the debate is still ongoing in the scientific community. Some characteristics of living things that scientists have agreed on include the ability to grow and develop, reproduce, maintain homeostasis, process energy (that is, obtain and use energy for various activities), and respond to the environment.
What about viruses? Would you consider them living or nonliving? Let’s look closely at the life cycle of the human immunodeficiency virus (HIV) – the virus that causes AIDS – to find out.
Written by Nadeene Riddick, HHMI
Background image by National Institute of Allergy and Infectious Diseases, U.S. National Institutes of Health (NIH)
What HIV looks like
Viruses come in many shapes and sizes. HIV viruses contain an RNA genome housed in a conical-shaped capsid. The virus’s outer layer is a lipid membrane that is studded with viral proteins known as envelope proteins. Watch this animation to learn more about the structure and size of HIV.
Video by Animation Lab; University of Utah; Utah, USA
What does an infected cell look like?
This scanning electron microscopy image shows an HIV-infected CD4 T cell (the cell is blue/green and the virus is yellow). CD4 T cells, sometimes called helper T cells, play an important role in activating the immune system when pathogens invade organisms, including humans.
Image by National Institute of Allergy and Infectious Diseases, NIH
How HIV enters a target cell
Envelope proteins on HIV particles attach to a specific protein receptor on a cell’s surface called CD4. After this initial attachment, the virus interacts with another protein, which allows it to fully enter the cell. Once the envelope protein and membrane proteins interact, the envelope protein folds back on itself, which forces the fusion of the T cell membrane and the viral membrane. This allows the viral capsid to enter the cell. Watch this animation to see how HIV enters a target cell.
Video by Animation Lab, University of Utah
How HIV reproduces
HIV is a retrovirus, which means it uses special viral proteins to make a DNA copy of its RNA genome and integrate it into the host cell. Once HIV infects a cell, it essentially hijacks the cell and turns it into a virus-producing factory. The cell, under control of the virus, begins making all the components needed to make new viruses (viral RNA genome and many types of viral proteins, such as envelope and gag, or group-specific antigen, proteins). Viral proteins gather and assemble at the cell membrane, along with viral RNA. This animation shows the early steps in the assembly and formation of a new virus particle.
Video by Animation Lab, University of Utah
How virus particles separate from the host cell
Once viral proteins and genome copies start gathering at the cell membrane, envelope viruses like HIV begin exiting their host cell through a process called budding. Then the budding particles pinch off from the cell membrane through a process called fission. Watch this animation to observe the fission process.
Video by Animation Lab, University of Utah
What happens when viral particles near the fission step of replication?
This transmission electron microscopy image shows HIV viral particles (yellow) near the end of the budding process; the cell they’ve infected is in blue.
Image by National Institute of Allergy and Infectious Diseases, NIH
What do the steps of budding look like?
This transmission electron microscopy image shows many HIV viral particles (red/orange) at various stages in their life cycle. Can you identify viruses early in the viral formation process? How about viruses in the budding or fission process? And how many newly released viruses can you observe?
Image by National Institute of Allergy and Infectious Diseases, NIH
What comes after replication?
After exiting the cell, new virus particles must mature before they can infect other cells. This scanning electron microscopy image depicts the surface of an infected T cell (blue/green) and the multitude of viruses (yellow) budding from the cell membrane. Nearby target cells, like CD4 T cells, will likely be infected by these new viruses – and the cycle will continue. However, if new viral particles leave the body and don’t infect another host, they will die without the support of a host cell.
Image by National Institute of Allergy and Infectious Diseases, NIH
Conclusion
So, back to the original question above: Consider all you’ve learned and observed about HIV’s life cycle in these images and animations. As you’ve seen, HIV replicates the way many living things do, their life cycle consumes energy, and they respond to their environment (the envelope proteins react to cell receptors, for example).
However, viruses like HIV require a living host for all aspects of their life cycle and they don’t have the ability to regulate or maintain homeostasis.
Based on this information, do you consider viruses to be living or nonliving things? The debate continues . . .
Image by National Institute of Allergy and Infectious Diseases, NIH
For suggestions on how to incorporate this journey into your teaching, see our “Implementation Suggestions.”