Bacteriophages are viruses that infect bacteria, and they are useful models for understanding how viruses reproduce. After attaching to a bacterial cell, a phage injects its genetic material and uses the host cell in one of two main ways. In the lytic cycle, the virus quickly makes many new virus particles and breaks the cell open.
In the lysogenic cycle, the viral DNA stays hidden inside the host genome and is copied whenever the bacterium divides.
The key difference between the two cycles is whether viral genes immediately take control or remain mostly inactive as a prophage. Environmental stress, DNA damage, or changes in host conditions can trigger a prophage to leave the bacterial chromosome and enter the lytic cycle. This switch is important because it can suddenly release many new viruses and can also move bacterial genes between cells.
Understanding these cycles helps explain viral replication, bacterial evolution, and the use of phages in biotechnology and medicine.
Understanding Biology: The Lytic and Lysogenic Cycles
The choice between these pathways depends on gene control. Some phages are virulent, meaning their genes are set up to reproduce soon after entry. Temperate phages have regulatory proteins that can keep most viral genes silent.
A repressor protein binds near viral genes and blocks their transcription. Transcription is the process of making RNA copies from DNA instructions. Without the needed RNA, the cell does not make phage proteins.
This control allows the viral DNA to persist without immediately destroying the cell. The virus is not inactive in every sense. Its genetic material must remain stable and be copied accurately as the bacterial cell reproduces.
Integration is a precise DNA event rather than simple mixing. Enzymes recognize short matching regions in phage DNA and bacterial DNA. They cut and join the DNA molecules at those sites.
The inserted viral DNA then behaves like one small part of the bacterial chromosome during DNA replication. When a bacterium splits into two daughter cells, each can receive the prophage. A large bacterial population may therefore carry the same viral genes while showing no visible sign of infection.
Some prophage genes can change bacterial traits. For example, certain bacterial toxins are encoded by genes originally brought by phages. This means a virus can affect disease even when it is not making new virus particles.
Induction occurs when the balance of gene control changes. DNA damage is especially important because bacteria activate repair systems when their chromosomes are threatened. In some lysogenic phages, this response leads to destruction of the phage repressor.
Once repression ends, viral genes can be transcribed. The phage DNA may be removed from the chromosome, copied many times, and packaged into protein coats. Late viral genes often produce enzymes that weaken the cell wall.
Other proteins make holes in the cell membrane. Water then moves into the damaged cell, pressure rises, and the cell ruptures. The number released varies with the phage type, the bacterial species, nutrients, and growth conditions.
Students often confuse a prophage with a complete phage particle. A prophage is viral DNA inside a bacterial chromosome. It is not a tiny virus waiting intact inside the cell.
It also helps to separate replication of DNA from assembly of virus particles. During lysogeny, viral DNA can be copied with host DNA without new phages being built. During lytic growth, many separate parts must be produced and assembled in the correct order.
These cycles matter in medicine and laboratories. Phage therapy works best with phages that reliably kill target bacteria, while temperate phages may transfer genes between bacteria. When studying diagrams, track the location of viral DNA, the state of viral gene activity, and whether complete phage particles are present.
Key Facts
- Attachment: a bacteriophage binds to specific receptors on the bacterial cell surface.
- Injection: the phage transfers viral DNA into the bacterial cell while the outer capsid usually remains outside.
- Lytic cycle: viral DNA is copied, viral proteins are made, new phages assemble, and the host cell lyses.
- Lysogenic cycle: viral DNA integrates into the bacterial chromosome as a prophage.
- Induction: stress such as UV light or DNA damage can cause a prophage to exit the chromosome and enter the lytic cycle.
- Burst size = number of phages released per lysed cell.
Vocabulary
- Bacteriophage
- A virus that infects bacteria and often has a protein capsid, tail fibers, and genetic material.
- Lytic cycle
- A viral replication pathway in which the virus rapidly produces new particles and destroys the host cell.
- Lysogenic cycle
- A viral pathway in which viral DNA becomes part of the host genome and is copied without immediately killing the cell.
- Prophage
- Viral DNA that has integrated into a bacterial chromosome during lysogeny.
- Induction
- The process in which a prophage is activated and switches from the lysogenic cycle to the lytic cycle.
Common Mistakes to Avoid
- Saying the lytic cycle is dormant, which is wrong because the lytic cycle involves active viral replication and host cell destruction.
- Forgetting that viral DNA is injected into the bacterium, which is wrong because the phage protein coat usually stays outside the cell.
- Thinking lysogeny means the virus is gone, which is wrong because the viral genome remains as a prophage and can be copied with the host DNA.
- Assuming the lysogenic cycle can never switch to the lytic cycle, which is wrong because stress signals such as UV damage can trigger induction.
Practice Questions
- 1 A bacterium contains one prophage and divides once every 20 minutes. If no induction occurs, how many bacterial cells will carry the prophage after 2 hours, starting from one lysogenic cell?
- 2 A single infected bacterium releases 120 phages when it lyses. If 50 bacteria complete the lytic cycle with the same burst size, how many phages are released in total?
- 3 A culture of lysogenic bacteria is exposed to UV light, and soon afterward many cells burst open. Explain which cycle became active and why the UV exposure caused this change.