Question

Describe the difference between the lytic cycle and lysogeny when bacteriophage infection occurs.

Short answer

Answer: The key differences between the lytic and lysogenic cycles during bacteriophage infection in bacteria are the outcome for the bacterial cell, viral replication, and environmental factors. In the lytic cycle, the bacterial cell is lysed and killed, while in the lysogenic cycle, the bacterial cell remains alive and the phage genome is integrated into the bacterial genome. Active viral replication occurs in the lytic cycle, while the viral genes are dormant in the lysogenic cycle. The lysogenic cycle can be triggered to revert back to the lytic cycle under specific stress conditions.

Step-by-step solution

1. Understand Bacteriophages and Their Infections

Bacteriophages are viruses that infect bacteria. They have two main types of life cycles when infecting a bacterial cell: the lytic cycle and the lysogenic cycle. In both cycles, the bacteriophage injects its genetic material into the bacterial cell, but the subsequent steps are different, which leads to diverse outcomes for the infected bacteria.

2. The Lytic Cycle

In the lytic cycle, the bacteriophage uses the bacterial cell's machinery to replicate its genetic material and produce its viral components. Afterward, the newly assembled phages are released by breaking open (lysing) the bacterial cell, leading to cell death. The newly released bacteriophages can then infect other bacterial cells, and the cycle repeats.

3. The Lysogenic Cycle

In the lysogenic cycle, the bacteriophage's genetic material gets integrated into the bacterial genome, forming a prophage. The bacterial cell is not harmed, and it can continue to multiply and replicate the integrated viral genetic material along with its own. The viral genes are dormant during this stage, and the bacteriophage is not being actively replicated. However, under certain stress conditions (e.g., UV radiation, chemical exposure), the prophage can be induced to exit the lysogenic cycle, reenter the lytic cycle, and start producing new bacteriophages, ultimately leading to bacterial cell lysis.

4. Key Differences Between Lytic and Lysogenic Cycles

Here are the main differences between the lytic and lysogenic cycles: - Outcome for the bacterial cell: In the lytic cycle, the bacterial cell is lysed and killed, while in the lysogenic cycle, the bacterial cell remains alive, and the phage genome is integrated into the bacterial genome. - Viral replication: Active viral replication occurs in the lytic cycle, while the viral genes are dormant in the lysogenic cycle. - Environmental factors: The lysogenic cycle can be triggered to revert back to the lytic cycle under specific stress conditions. By comparing these two cycles, we can understand that the lytic cycle is characterized by active bacteriophage replication, leading to bacterial cell death, while the lysogenic cycle allows the bacteriophage's genetic material to integrate into the bacterial genome and lie dormant until specific conditions trigger it to reenter the lytic cycle.

Key concepts

Lytic Cycle

In the world of tiny invaders known as bacteriophages, the lytic cycle is like an all-out attack on bacterial cells. When a bacteriophage infects a bacterial host during this cycle, it hijacks the cell's machinery to make more of itself. This process unfolds in several steps. First, the phage attaches to the bacterial cell membrane and injects its DNA inside.

Once inside, the phage DNA takes the reins, directing the cell's ribosomes and enzymes to switch from their regular job—keeping the bacterial cell alive—to the new task of replicating phage components. As the cell fills with new viral particles, energy and resources are depleted until the cell can no longer sustain itself. The endgame is the production of enzymes that break down the bacterial cell wall, causing the cell to burst and release the new phages to repeat the cycle. This violent and efficient cycle leads to the quick spread of the virus, as each burst cell unleashes numerous phages that can infect surrounding bacteria.

Lysogenic Cycle

Unlike the destructive path of the lytic cycle, the lysogenic cycle is akin to a covert operation. Here, the invading bacteriophage opts for a stealthy approach—it incorporates its genetic material into the host's chromosome, a process known as lysogeny. The hybrid DNA, now called a prophage, is replicated along with the host's genetic material when the bacterial cell divides.

This integration means that the phage's presence is unassuming, waiting like a sleeper agent within the host's genome. The incorporated viral DNA can remain dormant for many generations. However, it's not a completely benign arrangement. The prophage can sometimes influence the host bacterium's properties via gene expression. Importantly, the prophage can be triggered—often by environmental stressors like UV light or certain chemicals—to activate and initiate the lytic cycle, leading to a sudden outbreak.

Prophage Integration

Prophage integration is a pivotal step in the lysogenic cycle, distinguishing it from the all-or-nothing lytic cycle. The process begins when the phage injects its DNA into a bacterial cell. Instead of commandeering the cell's reproductive machinery, this DNA is spliced into the bacterial chromosome through an enzyme-mediated process called site-specific recombination.

The phage genetic material silently resides within the host's genome and is copied alongside it during cell division. This integration helps the phage to persist in a dormant state through multiple bacterial generations. The bacterium, unknowingly, becomes a carrier of the virus, enabling it to spread throughout a bacterial population without immediately destroying it. The viral DNA can be excised from the bacterial chromosome to re-enter the lytic cycle under certain conditions, leading to a tactical switch from dormancy to active infection.

Viral Replication

Viral replication is the process by which viruses reproduce and propagate. This process can vary widely among different viruses, but for bacteriophages, it often involves either the lytic or lysogenic cycle. During the lytic cycle, replication is aggressive and immediate, as the phage DNA commandeers the host cell to synthesize new viral particles, which ultimately leads to the cell's destruction and the release of the progeny.

Conversely, in the lysogenic cycle, replication is surreptitious—phage DNA integrates into the host chromosome and passively replicates along with it during cell division. This persistence allows a virus to distribute its genetic material widely and to weather unfavorable conditions until it's strategically advantageous to initiate the synthesis of new viruses. Regardless of the method, viral replication is fundamental to the survival and spread of viruses and plays a critical role in their evolutionary success.