Cell Biology
The lytic and lysogenic cycles are the life stages of bacteriophages, viruses that infect bacteria. Bacteriophages, or phages, inject their genetic material (which can be DNA or RNA) into the host cell. From there, the phage will undergo either the lytic or lysogenic cycle.
During the lytic cycle, the phage's genetic material replicates in the host cell cytoplasm, and viral proteins are synthesized using the host cell's ribosomes. A large number of progeny viruses are assembled inside the host cell. Late in the lytic cycle, the phages cause lysis of the host cell, releasing the viral progeny to infect new cells. On the other hand, in the lysogenic cycle, the phage's genetic material, called a prophage, is integrated into the host cell genome and replicated along with the host DNA when the host cell divides. In certain conditions, such as a scarcity of nutrients or exposure to toxins, the prophage can be released from the host cell's genome and revert to the lytic cycle.
Lesson Outline
<ul> <li>Lytic and lysogenic cycles</li> <ul> <li>Both begin with injection of phage genetic material into host cell</li> <li>Genetic material can be DNA or RNA</li> <li>Factors determining lytic or lysogenic cycle</li> <ul> <li>Example: Habitability of the host cell</li> <li>Note: Some bacteriophages can only undergo the lytic cycle</li> </ul> </ul> <li>Lytic Cycle</li> <ul> <li>Replication of phage's genetic material in host cell cytoplasm</li> <li>Production of viral proteins</li> <li>Assembly of new bacteriophages within the host cell</li> <li>Lysis of the host cell and release of viral progeny</li> <li>Progeny mature and infect new host cells</li> </ul> <li>Lysogenic Cycle</li> <ul> <li>Integration of phage genetic material (prophage) into host cell genome</li> <li>Prophage is inactive, producing no proteins or new phages</li> <li>Reproduction of phage through host cell division</li> <li>Potential transition to lytic cycle</li> <ul> <li>Triggered by factors such as nutrient scarcity or toxin exposure</li> <li>Phage enters the lytic cycle, leading to host cell lysis</li> </ul> </ul> </ul>
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FAQs
The lytic cycle is a process in which the bacteriophage infects the host cell, replicates, and destroys the host cell, resulting in the release of the newly created viruses. In contrast, the lysogenic cycle is when the bacteriophage’s genetic material integrates into the host cell's DNA, forming a prophage, and reproduces along with the host cell without destroying it. The lysogenic cycle can eventually lead to a lytic cycle if certain conditions are met.
Bacteriophages invade a host cell by first attaching to specific receptor sites on the bacterial cell surface through their tail fibers. Once attached, the phage tail sheath contracts, injecting the phage's genetic material into the host cell. The process of injecting genetic material is aided by enzymatic degradation of a portion of the host bacterial cell wall, creating a passage for the viral DNA or RNA.
During the lytic cycle, viral proteins play a crucial role in the replication and assembly of new bacteriophages within the host cell. The phage's genetic material redirects the host cell's machinery to produce necessary viral proteins that are involved in the synthesis of new phage particles. Some viral proteins are responsible for degrading the host cell's DNA, while others help in the packaging of the viral genetic material into the newly formed phage heads or aid in the assembly of phage tails and tail fibers.
In the lysogenic cycle, the bacteriophage's genetic material integrates into the host cell's DNA, becoming a prophage. The host cell is not destroyed during this process, and the prophage replicates along with the host cell's DNA when the cell divides. The host cell can continue to function normally, but it now carries the viral genes. Under certain circumstances, such as exposure to stress or other environmental factors, the prophage can become active and initiate the lytic cycle, leading to the eventual destruction of the host cell.
Several factors can trigger a prophage to exit the lysogenic cycle and enter the lytic cycle. These factors include exposure to UV light, DNA damage, chemical agents, or other environmental stressors that cause host cell stress.
