Viruses

Viruses

Middle School

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Learning Objectives

• Define a virus and understand why scientists consider it to be non-living.

• Compare the structure of a virus to the structure of a living cell.

• Describe the lytic and lysogenic cycles, which are how viruses reproduce.

• Explain how viruses cause disease and how they can be useful in medicine.

Key Vocabulary

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Virus

A tiny, nonliving particle that contains DNA and needs a host cell to reproduce itself.

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Capsid

The protein coat that encloses the virus's genetic material and protects its important genome.

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Latency

The dormant state of a virus inside a host cell before it becomes active and virulent.

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Virion

A single, individual virus particle which is the infectious form of the virus outside a host.

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What is a Virus?

• A virus is a tiny, non-living particle, much smaller than a cell.

• It contains genetic material, like DNA or RNA, inside a protein coat.

• Most scientists consider viruses non-living as they cannot reproduce on their own.

• Dmitri Ivanovski first discovered viruses in 1892 in tobacco plants.

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Multiple Choice

Why are viruses considered non-living by most scientists?

• They can reproduce independently without a host.

• They are a type of prokaryotic cell.

• They lack their own metabolism and cannot reproduce without a host.

• They have a plasma membrane, cytoplasm, and ribosomes.

Classifying Viruses by Shape

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Helical Viruses

• ​The capsid forms a hollow, spiral tube made of protein.

• ​​This shape looks very similar to a coiled spring.

• ​The genetic material is found coiled inside the tube.

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Icosahedral Viruses

• ​The capsid is made of many flat, triangular faces.

• ​​This creates a roughly spherical or ball-like shape.

• ​This structure provides a very strong, protective protein shell.

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Complex Viruses

• ​These viruses are not purely helical or icosahedral.

• ​​They have extra parts like a protein tail or fibers.

• ​A bacteriophage is a common example of this type.

Multiple Choice

What are the two fundamental components that make up all viruses?

• Nucleic acid (DNA or RNA) and a capsid

• Ribosomes and mitochondria

• An envelope and a cytoplasm

• A nucleus and a cell membrane

Viral Reproduction Cycles

• The lytic cycle is a fast process where the virus replicates quickly.

• The host cell bursts, releasing many new viruses at once.

• The lysogenic cycle is a longer process where the virus stays dormant.

Multiple Choice

What is the key difference between the lytic and lysogenic cycles?

• The lytic cycle is a longer cycle where the virus remains dormant.

• In the lysogenic cycle, the viral DNA integrates into the host DNA and remains dormant.

• The lysogenic cycle results in the immediate bursting of the host cell.

• Only the lytic cycle requires a host cell to reproduce.

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Role and Impact of Viruses

• Viruses are parasites that cause diseases like chicken-pox, polio, flu, and HIV.

• Some viruses called bacteriophages specifically infect and destroy bacteria.

• Viral diseases cannot be treated with antibiotics; vaccines help build the body’s immunity.

• Retroviruses like HIV copy their RNA into DNA inside of a host cell.

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Multiple Choice

Choose the correct statement which refes to infecting a host cell.

• Bacteriophages are viruses that cause the common cold in humans.

• A vaccine typically contains a strong, live virus to fight infection.

• Antibiotics are the most effective treatment for viral infections.

• HIV is a retrovirus that copies its RNA into the DNA of host cells.

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Viruses vs. Cells

• Viruses are considered non-living and must have a host cell to reproduce.

• Viral infections like influenza and HIV do not respond to antibiotics.

• Cells are living organisms with their own metabolism for energy and reproduction.

• Bacterial infections like strep throat can be killed with antibiotics.

Multiple Choice

What is a fundamental difference between a virus and a bacterial cell?

• Viruses can reproduce on their own, while bacteria require a host.

• Bacterial infections can be treated with antibiotics, while viral infections cannot.

• Only bacterial cells contain nucleic acids.

• Viruses are living organisms, while bacteria are non-living.

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Common Misconceptions About Viruses

Misconception	Correction
Viruses are alive.	Viruses are not living as they need a host cell to reproduce.
Antibiotics can cure viral infections.	Antibiotics work on bacteria. Vaccines help prevent, but do not cure, viral infections.
Viruses are just tiny cells.	Viruses are not cells. They lack parts like a cytoplasm or cell membrane.
All viruses are immediately harmful.	Some viruses can stay inactive in the body for long periods.

Multiple Choice

The diagram shows an enveloped virus. What is the outermost layer of this virus?

• Capsid

• Lipid envelope

• Genetic material (RNA or DNA)

• Glycoprotein spikes

Multiple Choice

Why does a person infected with a virus in its lysogenic cycle not show symptoms of illness right away?

• Because the viral DNA is dormant within the host cell's own DNA.

• Because antibiotics are effectively fighting the infection.

• Because the virus is actively destroying host cells in the lytic cycle.

• Because the virus is immediately destroyed by the immune system.

Multiple Choice

A new virus causes severe symptoms to appear just 48 hours after infection. What can you predict about its life cycle, and what is the best public health strategy to combat it?

• It likely has a lytic cycle; the best strategy is to develop a vaccine for prevention.

• It likely has a lysogenic cycle; the best strategy is to treat with antibiotics.

• It likely has a lytic cycle; the best strategy is to treat with antibiotics.

• It likely has a lysogenic cycle; the best strategy is to develop a vaccine for prevention.

Multiple Choice

Analyze the relationship between a retrovirus like HIV and its host cell. Why are HIV infections so difficult to cure completely?

• Because HIV is a type of bacteria that multiplies too quickly to stop.

• Because HIV has an exceptionally strong capsid that cannot be broken down.

• Because HIV integrates its genetic code into the host's DNA, making it a permanent part of the infected cells.

• Because HIV only exists outside of cells, making it a difficult target for drugs.

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Summary

• Viruses are non-living and need a host cell to make more copies.

• A virus has genetic material (DNA or RNA) inside a protein coat called a capsid.

• Viruses copy themselves using the fast lytic cycle or the slow lysogenic cycle.

• Vaccines can prevent viral diseases, but antibiotics don't work on them.

• Retroviruses, like HIV, are special viruses that copy their RNA into DNA.

• Scientists use viruses in research and for medical tools like gene therapy.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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