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.
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.