life11e_ch16

Viruses undertake two kinds of reproductive cycles

After a viral genome enters a cell, typically the invader takes over the cells’ molecular genetic machinery. But in some cases, there is an alternate series of events, in which the viral genome becomes integrated into the host genome.

LYTIC CYCLE The Hershey–Chase experiment (see Figure 13.4) involved a typical lytic viral reproductive cycle, so named because soon after infection, the host cell bursts (lyses), releasing progeny viruses. In this cycle, the viral genetic material takes over the host’s synthetic machinery for its own reproduction immediately after infection. In the case of some bacteriophages, the process is extremely rapid—within 15 minutes, new phage particles appear in the bacterial cell. Ten minutes later, the “game is over,” and these particles are released from the lysed cell. What happened?

At the molecular level, the reproductive cycle of a typical lytic virus has two stages: early and late, as illustrated in Figure 16.11. Follow along in the text and Figure 16.11 and you’ll see examples of both positive and negative regulation, which stimulate and inhibit, respectively, gene expression:

The viral genome contains a promoter that binds host RNA polymerase. In the early stage (1–2 min after phage DNA entry), viral genes that lie adjacent to this promoter are transcribed (positive regulation).
These early genes often encode proteins that shut down host transcription (negative regulation) and stimulate viral genome replication and transcription of viral late genes (positive regulation). Three minutes after DNA entry, viral nuclease enzymes digest the host’s chromosome, providing nucleotides for the synthesis of viral genomes.
In the late stage, viral late genes are transcribed (positive regulation); they encode the proteins that make up the capsid (the outer shell of the virus) and other protein components of the virus and enzymes that lyse the host cell to release the new virions. This begins 9 min after DNA entry and 6 min before the first new phage particles appear.

Figure 16.11 The Lytic Cycle: A Strategy for Viral Reproduction (A) In a host cell infected with a virus, the viral genome uses its early genes to shut down host transcription while it replicates itself. Once the viral genome is replicated, its late genes produce capsid proteins that package the genome and other proteins that lyse the host cell. (B) Bacteriophages have attached to this E. coli cell, and the reproductive cycle is underway, producing new phage particles. The cell is viewed in transverse section.

The entire process—from binding and infection to release of new phage—takes about 30 min. During this period, the sequence of transcriptional events is carefully controlled to produce complete, infective virions.

LYSOGENIC CYCLE Like all nucleic acid genomes, those of viruses can mutate and evolve by natural selection. Some viruses have evolved an advantageous process called lysogeny that postpones the lytic cycle. In lysogeny, the viral DNA becomes integrated into the host DNA and becomes a prophage (Figure 16.12). As the host cell divides, the viral DNA gets replicated along with that of the host. The prophage can remain inactive within the bacterial genome for thousands of generations, producing many copies of the original viral DNA.

Figure 16.12 The Lytic and Lysogenic Cycles of Bacteriophages In the lytic cycle, infection of a bacterium by viral DNA leads directly to multiplication of the virus and lysis of the host cell. In the lysogenic cycle, an inactive prophage is integrated into the host DNA where it is replicated during the bacterial life cycle.

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However, if the host cell is not growing well, the virus “cuts its losses.” It switches to a lytic cycle, in which the prophage excises itself from the host chromosome and reproduces. In other words, the virus is able to enhance its chances of multiplication and survival by inserting its DNA into the host chromosome, where it sits as a silent passenger until conditions are right for lysis.