The Viral Lifecycle Lab

The Viral Lifecycle Lab

A virus is not a cell. It has no ribosomes, no machinery for building proteins, and no way of copying itself. On its own, outside a host, it can do nothing at all. So it borrows. A virus gets inside a living host cell and uses that cell's machinery to make copies of itself, and there are exactly two ways it can go about it: the lytic pathway and the lysogenic pathway. Telling those two apart precisely is what this Higher-tier topic is testing, and they are a frequently confused pair.

Two pathways

Both pathways end with new viruses, and they get there in opposite ways. Every question on this topic turns on one of these four contrasts:

The lytic cycle

Put the stages of the lytic pathway into order, from a virus arriving to new viruses being released.

• The virus attaches to the surface of the host cell
• It injects its genetic material into the cell
• The cell's machinery is hijacked to make many copies of the viral parts
• The new viruses assemble inside the cell
• The cell bursts, and the new viruses are released to infect others

Which pathway?

A virus infects a bacterium. Ten generations later, every descendant bacterium carries the viral DNA, all of them are alive and dividing, and not one has burst. Which pathway is this?

• The lysogenic pathway: the viral DNA is in the host genome and is copied at every division
• The lytic pathway, since the virus has clearly reproduced a great deal
• Neither: this is bacterial reproduction with no virus involved
• Both pathways running in the same cells at once

Name what is happening

The switch

The lysogenic pathway is not the end of the story, and this is the part that is most often left out of exam answers. A trigger, such as stress, radiation or a chemical, can switch a dormant provirus into the lytic pathway. The viral DNA leaves the host genome, replication begins, and the cell is burst exactly as it would have been in the first place. So the two pathways are not two kinds of virus. They are two things the same virus can do, and a lysogenic infection can turn lytic years later. An answer that treats them as separate diseases has missed the point of the topic.

From dormant to deadly

A virus has been sitting quietly in a person's cells for years. Then they become seriously run down. Trace what can happen.

• While it was dormant, the infection caused no symptoms at all. Why not?
• The stress acts as a trigger. What changes?
• Symptoms now appear rapidly. Why do they arrive so suddenly, after years of nothing?
• Someone asks whether the person caught a new virus. What do you say?

Two pathways, in words

In the lytic pathway the virus replicates _____ and the host cell is destroyed, bursting to release the copies. In the lysogenic pathway the viral DNA is incorporated into the host _____, where it sits as a dormant _____ and is copied each time the cell divides. A _____ can later switch it into the lytic pathway.

Why the difference matters

It would be reasonable to ask why a specification cares which pathway a virus uses. The answer is that they present completely different problems. A lytic infection is loud. Cells are dying, symptoms appear, the immune system has virus particles to find, and a drug that interferes with viral replication has something to interfere with. A lysogenic infection is none of those things. There are no free virus particles to attack, no cells bursting to raise the alarm, and nothing replicating for a replication-blocking drug to stop. The viral DNA is sitting inside the host's own genome, being copied by the host's own machinery, and anything that destroyed it would have to destroy the host cell's DNA as well. That is why some viral infections can be cleared and others are carried for life.

Why the drug fails

A drug works by blocking viruses from replicating. Why is it of little use against a virus in the lysogenic pathway?

• Because the provirus is not replicating: it is part of the host cell's DNA and is copied by the cell itself
• Because lysogenic viruses are naturally resistant to all drugs
• Because the virus hides in a part of the body drugs cannot reach
• Because lysogenic viruses replicate too fast for a drug to keep up

Which are true?

Select ALL THREE statements that are TRUE.

• The lytic and lysogenic pathways are two things one virus can do, not two different viruses
• A pathway in which the host cell survives can spread the virus further than one that kills it
• Neither pathway can begin without a host cell
• The lytic pathway is the one that stays dormant for years
• The lysogenic pathway bursts the cell as soon as the DNA enters it
• Once a virus is lysogenic it can never become lytic

Which spreads further?

Which pathway is likely to have spread through more cells before anybody notices the infection, and why?

• Lysogenic, because the host cells survive and divide, copying the viral DNA each time, with no symptoms to reveal it
• Lytic, because each burst cell releases hundreds of new viruses at once
• Both the same, since both end with new viruses
• Neither spreads without symptoms appearing first

Explain the pair

Exam practice. In about 50 words, explain the difference between the lytic and lysogenic pathways. Include:

• what happens to the viral genetic material in each
• what happens to the host cell in each
• the fact that one can switch into the other, and what causes that

Lifecycle summary

A virus reproduces only inside a host cell, by one of two pathways. In the lytic pathway it makes copies at once and the cell bursts, a process called _____. In the _____ pathway the viral DNA is incorporated into the host _____ and stays dormant, copied each time the cell divides, until a _____ switches it into the lytic pathway.