A prophage is a bacteriophage genome integrated into a bacterial chromosome in a dormant lysogenic state. In Microbiology, it can later be induced to leave the host DNA and start the lytic cycle.
A prophage is the viral DNA of a bacteriophage after it has inserted itself into a bacterium's chromosome. In this lysogenic state, the virus is not immediately making new phages or bursting the cell. Instead, it is copied along with the bacterial DNA whenever the host cell divides.
This matters because the prophage is not just “inactive” in a passive sense. It is a viral genome with a plan B. As long as the host cell is healthy enough, the prophage can stay integrated and quietly persist through many generations of bacteria. That is why lysogeny can spread a viral genome through a bacterial population without immediate cell death.
The integration step is usually carried out by a viral enzyme called integrase, which helps the phage DNA recombine with a specific site in the bacterial chromosome. Once inserted, the prophage can remain stable for a long time. Some phages also produce repressors that keep lytic genes turned off, so the viral genome does not start building new particles right away.
A key feature of a prophage is that it can be induced. Stress such as UV light, certain antibiotics, or poor nutrient conditions can damage the host or disrupt repression. When that happens, the prophage excises from the chromosome, switches out of lysogeny, and enters the lytic cycle, where phage DNA is copied, new viral proteins are made, and the bacterial cell eventually lyses.
Microbiology classes often connect prophages to bacterial genetics and virulence. A prophage can carry genes that change the host's traits, including toxins or resistance-like traits that make the bacterium more successful in a specific environment. This is one reason phages are not just killers of bacteria, they can also reshape bacterial behavior when they sit in the genome as prophages.
Prophage is one of the cleanest examples of how viruses and bacteria can interact in a way that changes both the infection cycle and the host cell itself. If you are tracing the lysogenic cycle, the prophage is the turning point that explains how a phage genome can persist without killing the cell right away.
It also shows up in bacterial evolution. Because the prophage is copied every time the bacterium divides, the viral DNA can spread through a population and sometimes add new traits, like toxin genes. That is a big reason microbiologists pay attention to phage conversion, where a normally harmless bacterium can become more virulent after acquiring prophage DNA.
This concept also connects to why certain conditions can trigger outbreaks of phage replication. When a lab question gives you UV exposure, antibiotics, or cell stress, the prophage is often the step that changes from dormant to active. So the term helps you explain cause and effect, not just name a virus stage.
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view galleryBacteriophage
A prophage is the genome of a bacteriophage after it has entered the bacterial chromosome. If you see the term bacteriophage, think of the whole virus particle and its life cycle. Prophage refers to one specific state in that cycle, when the phage DNA is integrated and not making new virions yet.
Lysogenic Cycle
The lysogenic cycle is the life cycle that includes the prophage stage. In this pathway, the phage genome stays hidden inside the host DNA instead of immediately destroying the cell. A question about lysogeny often asks you to identify when the viral genome is dormant, and that is the prophage.
Lytic Cycle
The lytic cycle is what can happen after a prophage is induced. Once the phage DNA excises from the bacterial chromosome, it starts making viral parts, assembling new phages, and eventually lysing the cell. So the prophage is the dormant starting point before the switch into active replication.
burst size
Burst size is the number of new phages released from one infected cell after lysis. A prophage by itself does not produce a burst size yet, because it is dormant. But once induction happens and the lytic cycle begins, burst size becomes one way to measure how successful that viral replication was.
A quiz or short-answer question might show a bacterial cell with viral DNA sitting inside its chromosome and ask you to name the stage or explain what happens next. You need to identify that as a prophage and connect it to lysogeny, not lysis. In lab-based questions, you might be given a stress condition such as UV exposure and asked why phage particles suddenly appear later. The correct move is to trace induction, excision, lytic replication, and cell lysis in order. If a prompt asks how a bacterium gained a toxin gene from a phage, prophage integration is usually the mechanism you name.
A bacteriophage is the entire virus that infects bacteria, while a prophage is the bacteriophage genome after it has integrated into the host chromosome. Think of bacteriophage as the virus itself and prophage as one stage in its life cycle. If the question is about a free virus particle, use bacteriophage. If it is about dormant viral DNA inside the bacterial genome, use prophage.
A prophage is bacteriophage DNA inserted into a bacterial chromosome in the lysogenic state.
It stays dormant while the host cell divides, so the viral genome gets copied along with the bacterial DNA.
Environmental stress can induce a prophage to excise from the chromosome and enter the lytic cycle.
Prophages can change bacterial traits, including virulence, by carrying genes for toxins or other useful factors.
When you see prophage in Microbiology, think dormant viral DNA inside bacteria, not a free-floating virus particle.
A prophage is bacteriophage DNA that has been integrated into a bacterial chromosome. In this dormant state, it is copied when the host cell replicates, instead of immediately making new phages. It can later be induced to leave the chromosome and enter the lytic cycle.
A bacteriophage is the virus that infects bacteria, including the full viral particle outside the cell. A prophage is the phage genome after it has inserted into bacterial DNA. So bacteriophage refers to the virus in general, while prophage refers to the integrated, dormant form.
Stress conditions such as UV radiation, some antibiotics, or changes in nutrient availability can trigger induction. When that happens, the prophage excises from the bacterial chromosome and starts the lytic cycle. The host cell then produces new phages and usually lyses.
Prophages can change the traits of their bacterial hosts. Some carry toxin genes or other virulence factors, which can make a bacterium more harmful or better suited to survive in a given environment. They can also provide protection against infection by other phages.