Viral Infection Mechanism

Part 1: The virus

The animation begins inside a pipette containing a sample of a virus. Here we see a schematic of the virus with its receptors, envelope, protein shell, and viral DNA. As the camera zooms out, we will see a drop of the solution being deposited on a petri dish containing cultured of human cells. Click on "play" to see the next part of the animation.

Part 2: The cell monolayer

As the camera zooms into the petri dish, we can see that it is covered with a thin layer of cultured cells susceptible to infection by the virus. The cell membrane, cytoplasm, and nucleus of a single cell are labeled.

Part 3: The cell receptors

Increased magnification reveals proteins on the surface of the cell. These are receptors that are used by cells to communicate with each other, as well as for other cellular functions. The virus takes advantage of these proteins to bind to the membrane of the cell. In the next part of the animation, we will see how receptors on the surface of the viral envelope bind to the cell's own receptors to allow fusion to occur.

Part 4: Fusion with the cell's plasma membrane

The fusion of the cell membrane with the viral envelope allows the viral protein shell and genome to be released into the cytoplasm of the cell. The viral receptor proteins remain on the surface of the infected cell's membrane and will later be incorporated into the envelopes of newly formed viruses. The contents of the virus are delivered into the cell's cytoplasm, where the viral protein shell is disassembled. This step frees the viral genome. It enters the cell nucleus, where it is replicated. Only a few proteins remain associated with the viral genome to permit its passage through the nuclear membrane.

Part 5: Replication of the viral genome

Once inside the host cell, different types of viruses have different mechanisms for replicating their genomes. DNA viruses generally replicate their DNA in the nucleus of the host cell, while RNA viruses generally replicate their RNA in the cytoplasm. Here, the example shows a DNA virus whose genome enters the host cell's nucleus and replicates by taking advantage of the host cell's DNA replication machinery. Newly synthesized viral DNA becomes the genome of progeny viruses, but it is also used to make mRNAs that code for viral proteins. The mRNAs enter the cytoplasm and use the host cell's protein synthesis machinery to make viral proteins. The replicated viral genome leaves the host cell's nucleus and reenters the cytoplasm, where it is packaged into the protein shell made from newly synthesized viral proteins.

Part 6: Viral budding

The newly formed viruses now return to the host cell's plasma membrane, which contains thousands of copies of newly synthesized viral receptor proteins. Viral budding is the process by which the virus acquires a new envelope from the plasma membrane of the host cell. Now the host cell is covered with thousands of copies of the original virus ready to be released and infect more cells.

The last part of the animation shows the progeny viruses infecting other cells in the petri dish. As each cell dies, it turns black. Over the course of several hours, dying cells spread out in a circle across the surface of the dish.

Viral Infection Background

Unlike bacteria, viruses are acellular (not cellular). A virus particle contains only one type of nucleic acid, either RNA or DNA, which is sometimes surrounded by a protein coat. Sometimes the coat is further encased in a membrane called an envelope. Viruses can only reproduce by using the cellular machinery of other organisms. Viral reproduction involves the replication of a virus's genetic material, either RNA or DNA, and the synthesis of proteins that either make up the virus (structural proteins) or that are needed to perform certain functions (such as replication).

In the process of transcription, the genetic information encoded in the viral DNA is copied or transcribed into a complementary base sequence of RNA. The virus then uses the cellular machinery to synthesize specific proteins from the RNA through the process of translation.

For an RNA virus, the genetic material is already in a form that can be translated into protein.

Viral Infection Animation tips

The animations in this section have a wide variety of classroom applications. Use the tips below to get started but look for more specific teaching tips in the near future. Please tell us how you are using the animations in your classroom by sending e-mail to grantswww@hhmi.org.

1. Use the animations to make abstract scientific ideas visible and concrete.

2. Explain important scientific principles through the animations. For example, the biological clocks animations can be used to demonstrate the fundamentals of transcription and translation.

3. Make sure that students learn the material by repeating sections of the animations as often as you think necessary to reinforce underlying scientific principles. You can start, restart, and play back sections of the animations.

4. Urge students to use the animations in accordance with their own learning styles. Students who are more visually oriented can watch the animations first and read the text later, while others might prefer to read the explanations first and then view the graphics.

5. Incorporate the animations into Web-based learning modules that you create to supplement your classroom curricula.

6. Encourage students to incorporate the animations into their own Web-based projects.

Viral Infection Credits

Director: Dennis Liu, Ph.D.

Scientific Direction: Donald E. Ganem, M.D.

Scientific Content: Laura Bonetta, Ph.D.

Animators: Eric Keller, Satoshi Amagai, Ph.D.