Making Copies of an HIV Particle
Many biologists do not consider viruses to be “living” organisms, because they cannot carry out many of the functions that define life. For example, viruses cannot use food; nor are they able or make copies of themselves (“reproduce”) without invading a living cell and redirecting the cell’s internal mechanisms to make new virus copies. Outside cells, viruses exist as genetic material (DNA or RNA) surrounded by a protective coat of protein, called a capsid. HIV’s capsid contains two strands of RNA.
Some viruses also wrap themselves in a modified form of the cell membranes from which they emerge. This modified membrane, called an envelope, is studded with proteins that enable the virus to latch onto and infect other cells. HIV and the influenza (flu) virus are examples of viruses that are surrounded by an envelope. The complete, assembled viral package—consisting of the genetic material, capsid and envelope (when present)—is referred to as a “virus particle” (or virion) to distinguish it from the virus components present inside host cells.
Use the student-constructed models as a basis for a class discussion about the structure and function of the HIV particle. For example, ask, What is contained inside the particle? [capsid and genetic material] What does the capsid do? [contain and protect genetic material] Why might some virus particles also have an envelope? [provides a way to dock with certain kinds of cells and fuse with the cell membrane]
- Discuss the main parts of the HIV particle, and their functions.
- Glycoprotein gp120: Identifies and initially docks with host cell.
- Glycoprotein gp41: Completes docking and assists in fusion with host cell.
- Viral envelope: Two-layer lipid membrane
- Capsid: Protein shell
- Matrix: Protein shell
- Viral RNA: Two identical strands of genetic material
- Reverse transcriptase: Uses viral RNA as a template to produce DNA once the particle is inside a cell.
Project microscopic images of the HIV particle and have students compare the outsides of their models to the images. Mention that the double circles on the exterior of the envelope on their models represent the glycoprotein spikes needed by the virus particle to attach to the CD4+ white blood cells.
Have students remove their capsid models from the inside of the viral envelope. Ask them to examine the inside of the capsid. Point out the RNA strands and discuss their function: to transmit genetic information to the host cell. Describe the RNA strands as an instruction manual that directs the cell to make virus components. Also mention the reverse transcriptase enzyme and its function, which is to transform the genetic information on the RNA strands into DNA, the genetic code within each host cell.
Keywords: AIDS | DNA | HIV | HIV/AIDS | RNA | SEM TEM | T-cell | blood | capsid | disease | epidemic | epidemiologist | epidemiology | illness | immune system | infection | microbe | microbiology | microscope | pandemic | pathogen | replication cycle | retrovirus | viral assembly | virion | virus | white blood cell
- Vogt, G., and Moreno, N. (2012) The Science of HIV/AIDS Teacher’s Guide. Baylor College of Medicine: Houston. ISBN: 978-1-888997-62-0
- Illustration by G.L. Vogt and M.S. Young © Baylor College of Medicine.
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Grant Number: 5R25RR018605