Viruses

Essential Question

How do viruses use a host cell to copy themselves?

Many viruses cause disease, but a virus is not a cell. It cannot grow, use energy, or make copies of itself on its own. In this lesson, you’ll see what a virus is made of, how it takes over a host cell to make copies, and why scientists debate whether viruses count as living.

What Is a Virus?

• A virus is genetic material inside a protein coat
• Genetic material: DNA or RNA
• Capsid: the protein coat
• Some viruses also have an outer envelope

A virus is a tiny particle made of genetic material surrounded by a protein coat called a capsid. The genetic material is DNA in some viruses and RNA in others. Some viruses, such as influenza, are also surrounded by an envelope made of membrane, with proteins sticking out of it. A bacteriophage, or phage, is a virus that infects bacteria. Many phages look like a head on a tail, and they use the tail and tail fibers to attach to a bacterium. A virus has no cytoplasm, organelles, or ribosomes, so it is not a cell.

Viruses Are Tiny

• Typical viruses are about 100 nm across
• A bacterium is about 20 times as long as one
• A typical animal cell is hundreds of times wider
• Most viruses are too small for a light microscope

The graph compares the sizes of some viruses and cells. A nanometer (nm) is one billionth of a meter. Each tick on the axis is 10 times larger than the one before, so equal steps on this log scale stand for equal multiples. Influenza viruses and coronaviruses are about 100 nm across, a phage is about 200 nm long, and a bacterium such as E. coli is about 2,000 nm (2 µm) long. A human red blood cell is about 7,000 nm wide. Most viruses are too small to see with a light microscope, so scientists use electron microscopes to see them. These sizes are approximate, because viruses and cells vary.

Viruses Need a Host Cell

• A virus has genes but cannot use them alone
• It enters a host cell and uses the host’s machinery
• Host ribosomes make the viral proteins
• The host supplies energy (ATP) and building blocks

A virus has genes but no way to use them by itself. It has no ribosomes to make proteins and cannot make its own ATP. After it enters a host cell, the cell’s ribosomes read the viral genes and make viral proteins, and the cell’s nucleotides (and often its enzymes) are used to copy the viral genetic material. In this way, the viral genetic material directs the host cell to build the proteins the virus needs. The new genomes and proteins then assemble into new virus particles. A virus can infect only cells that have the right receptors on their surface, which is why many viruses infect only certain species or certain cell types.

The Lytic Cycle

• The virus takes over the host cell and destroys it
• Steps: attachment, entry, copying, assembly, release
• Release by lysis: the host cell bursts
• One infected cell can release many new viruses

In the lytic cycle, a virus uses the host cell to make new viruses and then destroys the cell. In step 1, the virus attaches to receptors on the host cell. In step 2, its genetic material enters the cell. In step 3, the host’s machinery copies the viral genetic material and makes viral proteins. In step 4, new viruses assemble. In step 5, the cell bursts, which is called lysis, and releases the new viruses to infect other cells. The drawing shows a bacteriophage and a bacterium. Many viruses that infect animal cells follow the same steps, although they often enter the cell whole, and some leave by budding through the cell membrane instead of bursting the cell.

The Lysogenic Cycle

• The viral DNA joins the host chromosome
• The host cell lives and divides normally
• The viral DNA is copied with the host DNA
• A trigger can switch the virus to the lytic cycle

Some viruses can also use the lysogenic cycle. The viral DNA inserts into the host chromosome, where it is called a prophage in bacteria. The host cell is not destroyed. Each time the host cell divides, it copies the viral DNA along with its own DNA, so every daughter cell carries the virus. The virus can stay hidden this way for many generations. Later, a trigger, such as stress on the cell, can cause the viral DNA to leave the chromosome and start the lytic cycle.

Are Viruses Alive?

• Viruses have genes, and their populations evolve
• But they are not cells and cannot reproduce alone
• Scientists classify them in different ways
• The answer depends on how life is defined

Biologists use a list of traits to decide whether something is alive, and viruses do not fit neatly. Like living things, viruses have genetic material, and their populations evolve by natural selection. Unlike living things, viruses are not made of cells, do not make their own ATP, and can reproduce only inside a host cell. Some scientists conclude from this that viruses are nonliving particles. Others describe them as being at the edge of life, or argue that a virus is alive while it is inside a host cell. This is a classification debate, not a settled question, because it depends on which traits you count as essential for life. The facts themselves are not in dispute: viruses carry genetic material and need a host cell to copy it.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Put the Steps in Order

The five drawings show steps of the lytic cycle of a bacteriophage, shown out of order. Put them in the correct order, name each step, and tell what the host cell provides in the copying step.

Show the solution

1. B: a phage is attached to the outside of the bacterium. This is attachment.
2. D: the phage head is empty and its DNA is inside the cell. This is entry.
3. E: many copies of the viral DNA and many viral proteins are inside the cell. This is copying and making proteins.
4. C: complete new phages are inside the cell. This is assembly.
5. A: the cell has burst and the new phages are released. This is release (lysis).
6. What the host provides in E: the ribosomes, ATP, and building blocks (nucleotides and amino acids) used to make viral DNA and viral proteins.
7. Answer: B, D, E, C, A: attachment, entry, copying, assembly, release.

Example 2: How Many Viruses Fit?

Use the sizes in the table. About how many influenza viruses would fit side by side across the width of a human red blood cell? How many times as long as a bacteriophage T4 is an E. coli bacterium?

Object	Size (nm)
Influenza virus	100
Bacteriophage T4 (length)	200
E. coli bacterium (length)	2,000
Human red blood cell (width)	7,000

Show the solution

1. Viruses across a red blood cell: 7,000 nm ÷ 100 nm = 70
2. Bacterium compared with a phage: 2,000 nm ÷ 200 nm = 10
3. Check: an E. coli bacterium is 2,000 ÷ 100 = 20 times as long as an influenza virus, and a phage is twice as long as an influenza virus, so 20 ÷ 2 = 10 fits.
4. Answer: About 70 influenza viruses fit across a red blood cell. An E. coli bacterium is 10 times as long as a T4 phage.

Example 3: A Virus Growth Curve

Scientists mixed bacteriophages with bacteria so that 1 million bacteria per mL were each infected by one phage. An infected bacterium counts as one infectious unit until it bursts. The graph shows the number of infectious units over time. Write a claim, evidence, and reasoning: when are new viruses made, and when are they released? Then find how many new viruses each infected cell released.

Show the solution

1. Claim: the new viruses are made inside the host cells during the first 20 minutes, and they are released between 20 and 40 minutes.
2. Evidence: the count stays at 1 million per mL from 0 to 20 minutes, then climbs to 12 million per mL at 30 minutes and 95 million per mL at 40 minutes.
3. Reasoning: a cell with new viruses inside still counts as one infectious unit, so the count stays flat while the viruses are being made. The count rises only when cells burst and release the viruses.
4. Viruses per cell: 100 million ÷ 1 million = 100 new viruses for each infected cell, using the final count at 50 minutes.
5. Answer: The viruses were made inside the cells for about 20 minutes and released between 20 and 40 minutes. Each infected cell released about 100 new viruses.

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Practice

Label the parts of a virus, put the steps of the lytic cycle in order, compare the lytic and lysogenic cycles, use a growth curve as evidence, and take part in the debate over whether viruses are alive. Try it on your own first, then check your work with the answer key.

• Viruses Practice Worksheet (PDF)
• Viruses Practice Answer Key (PDF)

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Lesson Slides

• 7.4 Viruses Slides (PDF)

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Big Ideas

• A virus is genetic material in a protein coat, not a cell
• A virus can copy itself only inside a host cell
• Lytic cycle: attach, enter, copy, assemble, release
• Lysogenic cycle: viral DNA joins the host chromosome and is copied with it
• Whether viruses are alive is a debate about how life is defined

Back to the essential question: a virus has genes but no ribosomes and no way to make its own ATP, so it must enter a host cell. The host’s machinery reads the viral genes, makes viral proteins, and copies the viral genetic material, and then the new viruses assemble and leave, often by bursting the cell. Whether this makes a virus alive depends on how life is defined, and scientists classify viruses in different ways. Next, you’ll turn to how cells get their energy, starting with photosynthesis.

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