Viruses: Videos & Practice Problems

Viruses are non-living parasitic particles composed of a protein coat (capsid) that encases genetic material (DNA or RNA). They reproduce by infecting host cells through two main life cycles: the lytic cycle, which destroys the host to release new viral particles, and the lysogenic cycle, where viral DNA integrates into the host genome. Viroids are infectious RNA molecules affecting plants, while prions are misfolded proteins causing diseases like mad cow disease. Scientists utilize viruses in research, particularly retroviruses, for gene manipulation and studying cell biology.

Viruses

Viruses, viroids, and prions represent fascinating entities within cell biology, with viruses being particularly intriguing due to their unique characteristics. Viruses are small parasitic particles that depend on host organisms for their life cycle, lacking the cellular structures and metabolic processes typical of living organisms. They are composed of a minimal number of proteins, typically ranging from 4 to 210, and possess a structure that includes a capsid—a protein coat that encases their genetic material, which can be either DNA or RNA. Some viruses also have an envelope, a phospholipid bilayer similar to the plasma membrane of cells, that surrounds the capsid.

The capsid proteins of viruses can take on two primary shapes: helical, resembling a spiral, and icosahedral, characterized by 20 identical triangular faces. Viruses are named after the diseases they cause, affecting a variety of hosts, including plants, animals, and bacteria. Importantly, viruses are not classified as living organisms because they cannot reproduce independently; they must hijack the cellular machinery of a host to replicate.

Viruses typically exhibit host specificity, meaning they infect particular organisms. For example, bacteriophages are viruses that specifically target bacteria. The life cycle of a virus can be categorized into two main types: the lytic cycle and the lysogenic cycle. The lytic cycle is responsible for the production of new viral particles. It involves several key steps: the virus binds to a host cell via specific receptors, penetrates the cell membrane, replicates its genetic material, assembles new viral particles, and finally releases these particles, often destroying the host cell in the process. This release can occur through lysis, where the cell is ruptured, or budding, where enveloped viruses exit the cell without causing immediate death.

In contrast, the lysogenic cycle involves the integration of viral genetic material into the host's genome. Initially, the virus infects the host cell through the lytic cycle, but instead of producing new viruses, it integrates its DNA into the host's DNA, forming what is known as a prophage (or provirus). This integrated viral DNA can remain dormant, replicating alongside the host's DNA during cell division. Eventually, environmental triggers, such as DNA damage, can reactivate the viral DNA, prompting it to enter the lytic cycle and produce new viral particles. This integration can have significant consequences, potentially leading to diseases such as cancer by disrupting normal cellular functions.

Understanding these cycles is crucial for comprehending how viruses propagate and the implications they have for health and disease. The lytic cycle focuses on the active production of new viruses, while the lysogenic cycle emphasizes the integration and dormancy of viral genetic material within the host genome.

Viruses for Research

Viruses play a crucial role in scientific research, particularly in the fields of cell biology and cancer studies. Since viruses are too small to be seen with the naked eye or even with traditional light microscopes, scientists employ specialized assays to quantify viral presence in samples. One of the primary methods used is the plaque assay, which allows researchers to calculate the concentration of viruses in a given sample.

In a plaque assay, cells are infected with viruses, leading to the integration of viral DNA into the host cell's genome. This integration causes the infected cells to proliferate rapidly, resulting in a layered growth pattern. When stained, these rapidly growing cells form visible structures known as plaques. By counting these plaques and correlating them with the volume of the viral sample (measured in microliters or milliliters), scientists can determine the viral load in the sample through mathematical calculations.

Another significant tool in viral research is the use of retroviruses. Retroviruses are enveloped RNA viruses that can integrate their genetic material into the host cell's chromosome. This characteristic is particularly valuable for scientists, as it allows for the manipulation of gene expression within specific cell types. The process begins with reverse transcription, where the viral RNA is converted into DNA. This newly formed DNA, consisting of two identical strands, can then integrate into the host chromosome, forming what is known as a provirus.

Once integrated, the viral gene can be expressed when the host cell replicates and transcribes that segment of its genome. Researchers can exploit this mechanism by removing the viral genome and replacing it with desired genes, enabling the expression of specific genes in targeted cell types. This innovative use of retroviruses provides a powerful method for studying gene function and regulation in cell biology.

Viroids and Prions

In addition to viruses, two other types of infectious particles are prions and viroids, each with distinct characteristics and mechanisms of action. Viroids are small, circular infectious RNA molecules primarily found in plant cells. Unlike traditional RNA, viroids do not encode proteins; instead, they function by binding to host proteins, thereby inhibiting their normal activity. This interference can lead to various plant diseases, and viroids can be transmitted between damaged plant cells, highlighting their infectious nature.

On the other hand, prions are unique infectious agents composed of abnormally folded proteins, rather than nucleic acids like RNA. They are known to cause several serious diseases, including mad cow disease (bovine spongiform encephalopathy). One of the most concerning aspects of prions is their resistance to conventional methods of destruction, such as cooking, which typically inactivates many pathogens. This resilience makes prion-related diseases particularly dangerous, as they can persist even after meat has been cooked.

In summary, viroids and prions represent two distinct classes of infectious agents that differ significantly from viruses. Viroids primarily affect plants by disrupting protein function, while prions pose a serious risk to animal health due to their ability to induce misfolding in normal proteins, leading to severe neurological diseases.

Which of the following is true?

• A. Viruses, viroids, and prions are all living infectious agents
• B. Viroids have two main life cycles:the lytic and lysogenic cycle
• C. Viruses have two main shapes:icosahedron and circular
• D. Viruses, viroids, and prions are all non-living infectious agents

Answer: (D)

During which life cycle is the virus making more copies of itself?

• A. Lytic Cycle
• B. Lysogenic Cycle

Answer: (A)

Retroviruses are able to integrate their genome into the host cell?

• A. True
• B. False

Answer: (A)

Here’s what students ask on this topic:

What are the main structural components of a virus?

Viruses are composed of a few key structural components. The primary structure is the capsid, a protein coat that encases the virus's genetic material, which can be either DNA or RNA. Some viruses also have an envelope, a phospholipid bilayer similar to a cell's plasma membrane, surrounding the capsid. The capsid proteins can be helical or icosahedral in shape. Viruses lack organelles, cytoplasm, and other cellular structures, making them much simpler than living cells. They rely on host cells for reproduction and metabolism, which is why they are considered non-living entities.

How do viruses reproduce inside host cells?

Viruses reproduce through two main life cycles: the lytic cycle and the lysogenic cycle. In the lytic cycle, the virus attaches to a host cell, injects its genetic material, and hijacks the host's cellular machinery to produce new viral particles. These new particles are then released, often destroying the host cell in the process. In the lysogenic cycle, the viral DNA integrates into the host genome and replicates along with it. The virus can remain dormant for a period before eventually entering the lytic cycle to produce new viral particles.

What is the difference between a lytic and a lysogenic cycle?

The lytic cycle involves the active production of new viral particles, leading to the destruction of the host cell. The virus attaches to the host cell, injects its genetic material, and uses the host's machinery to replicate. The new viruses are then released, often lysing the host cell. In contrast, the lysogenic cycle involves the integration of viral DNA into the host genome, where it can remain dormant for an extended period. The viral DNA replicates along with the host's DNA until certain conditions trigger its reactivation, leading to the lytic cycle.

How are viruses used in scientific research?

Viruses are valuable tools in scientific research, particularly in cell biology and cancer studies. One common method is the plaque assay, which measures the amount of virus in a sample by infecting cells and counting the resulting plaques. Retroviruses are especially useful because they can integrate their RNA genome into the host cell's DNA through reverse transcription. Scientists can manipulate retroviruses to insert specific genes into host cells, allowing for controlled gene expression studies. This technique is crucial for understanding gene function and developing gene therapies.

What are viroids and prions, and how do they differ from viruses?

Viroids are small, circular RNA molecules that primarily infect plant cells. They do not encode proteins but interfere with host cell functions by binding to cellular proteins. Prions are misfolded proteins that cause infectious diseases like mad cow disease. Unlike viruses, prions do not contain nucleic acids. Both viroids and prions are simpler than viruses and cause disease through different mechanisms. Viroids disrupt cellular processes by binding to proteins, while prions induce normal proteins to misfold, leading to cellular damage.