BioEd Online

Teacher Background

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.

Depending upon students' grade and maturity levels, the essay, "The Deadly Cycle," may be used as teacher background information or as a student reading assignment. It is especially effective when read aloud.

Content Advisory 

See the following resources for additional information about HIV/AIDS and advice for discussing HIV/AIDS with students.

• National Institute of Allergy and Infectious Diseases, National Institutes of Health (NIH), offers resources on understanding HIV/AIDS: niaid.nih.gov/topics/hivaids/ and aidsinfo.nih.gov.

• National Institute on Drug Abuse, NIH, offers facts about drug abuse and the link between it and HIV/AIDS: hiv.drugabuse.gov.

• The Centers for Disease Control and Prevention provides up-to-date information on HIV/AIDS prevention: cdc.gov/hiv/topics.

Objectives and Standards

Life Science

• Living systems at all levels of organization demonstrate the complementary nature of structure and function.

• Cells carry on many functions needed to sustain life.

• Disease is a breakdown in structures or functions of an organism. Some diseases are the result of damage by infection by other organisms.

• Every organism requires a set of instructions for specifying its traits. Heredity is the passage of these instructions from one generation to the next.

• Cells have particular instructions that underlie their functions. Every cell is surrounded by a membrane that separates it from the outside world.

• Cells use and store information to guide their functions. The genetic information stored in DNA is used to direct the synthesis of the thousands of proteins that each cell requires.

• In all organisms, the instructions for specifying the characteristics of the organism are carried in DNA [usually], a large polymer formed from subunits.

Materials and Setup

Teacher Materials (see Setup)

• Images of HIV particles for projection or display (see Setup)

• Slides or transparencies of the "HIV Virus Particle" and "HIV Replication" student sheets

• LCD or document projector, “smartboard” or overhead projector

Materials per Student 

• Assembled HIV particle model with capsid structure from the activity, “Modeling an HIV Particle”

• Copy of each student sheet (see Lesson pdf)

• Copy of essay (if age appropriate; see Lesson pdf)

Setup

1. Assemble images of HIV particles into a presentation for projection in your classroom. Images can be found on the following websites.

   • Centers for Disease Control and Prevention, Public Health Image Library (phil.cdc.gov) 

   • Journal of Nanobiotechnology (www.jnanobiotechnology.com/content/3/1/6)

2. You also may download related slides directly from BioEd Online (bioedonline.org). Conduct this as a whole-class activity.

Procedure and Extensions

1. Depending upon students' grade and maturity levels, have students read the essay, "The Deadly Cycle." Next, 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]

2. Discuss the main parts of the HIV particle, and their functions. Refer to the illustration on the student sheet, “HIV Virus Particle” to provide more detail.

3. 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.

4. 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. Depending on the ages of your students, you may want to examine the HIV life cycle in more detail. Use the “HIV Replication Cycle” sheet as a guide. Following are the steps involved in HIV infection of a cell.

1. Attachment and entry. The HIV virus bumps into a CD4+ white blood cell, attaches to it, and injects the capsid and its contents into the cell.

2. Reverse transcriptase. Once inside the cell, HIV genetic material (in the form of RNA) is converted into a form that is compatible with the cell’s genetic information (DNA). In cells, DNA usually is used to produce new RNA through a process called transcription. When RNA is used as a template to produce DNA, as is the case with HIV infection, the process is referred to as “reverse” transcription.

3. Integration. The newly formed viral DNA moves into the cell nucleus, where it is spliced into the cell’s human DNA.The HIV genetic material may remain dormant or inactive for many years. In this state, HIV is able to “hide” from the immune system and is unaffected by antiviral treatments.

4. Transcription and translation. The viral DNA becomes active and directs the cell’s machinery to produce the virus components: viral RNA, viral envelope and capsid. This activation can occur many years after initial infection with HIV, and is not yet completely understood.

5. Assembly and release. The viral particle is assembled, fuses to the cell membrane and is released by “budding” off the surface of the cell. During the budding process, the new particle wraps itself in part of the host cell’s membrane to create the viral envelope. The new virus particles now circulate within the body and are able to invade other cells.

Handouts and Downloads