Question

Which of the following steps is NOT part of the life cycle of a lytic phage?

A. The phage DNA is transcribed, and the resulting mRNA is translated to make capsid proteins.

B. The phage DNA integrates into the bacterial chromosome.

C. Phage DNA is injected into the bacterial cell.

D. All of the steps listed are part of the life cycle of a lytic phage.

E. Many copies of phage DNA are made

Answer 1

Answer: option (B) is the correct answer. The statement “The phage DNA integrates into the bacterial chromosome” is incorrect in relation to the lytic phage.

In lytic phage, the phage DNA does not integrates into the bacterial chromosome. Whereas in temperate Phage (Lysogenic phage), the phage DNA integrates into the bacterial chromosome.

Detail explanation:

Lytic (Virulent phage) and Lysogenic (temperate phage) cycles are the method of viral multiplication. Lytic cycle is a method of viral multiplication wherein the virus infects a host cell and uses its machinery to multiply and then destroys the host cell completely. Overall virulent phages typically lead to the death of the cell through cell lysis. Temperate phages (Lysogenic cycle), on the other hand, can become part of a host chromosome (virus integrates its genetic information with that of the host chromosome) and are replicated with the cell genome until such time as they are induced to make newly assembled viruses, or progeny viruses.

The Lytic Cycle

During the lytic cycle of virulent phage, the bacteriophage takes over the cell, reproduces new phages, and destroys the cell. The bacteriophage lytic cycle are having many stages. See below:

1. Attachment is the first stage in the infection process in which the phage interacts with specific bacterial surface receptors (e.g., lipopolysaccharides and OmpC protein on host surfaces). Phages are very host specific. They have a narrow host range and may infect one species of bacteria or one strain within a species. This unique recognition can be exploited for targeted treatment of bacterial infection by phage therapy.

2. The second stage of infection is entry or penetration. This occurs through contraction of the tail sheath, which acts like a hypodermic needle to inject the viral genome through the cell wall and membrane. The phage head and remaining components remain outside the bacteria.

3. The third stage of infection is biosynthesis of new viral components. After entering the host cell, the virus synthesizes virus-encoded endonucleases to degrade the bacterial chromosome. It then hijacks the host cell machinery to replicate, transcribe, and translate the necessary viral components (capsomeres, sheath, base plates, tail fibers, and viral enzymes) for the assembly of new viruses. Polymerase genes are usually expressed early in the cycle, while capsid and tail proteins are expressed later.

4. During the maturation phase, new virions are created by assembling the viral components. To liberate free phages, the bacterial cell wall is disrupted by phage proteins such as lysin, endolysin, holin or lysozyme.

5. The final stage is release of mature phages. Mature viruses burst out of the host cell in a process called lysis and the progeny viruses are liberated into the environment to infect new cells.

Lytic vs Lysogenic Cycle

	Lytic Cycle	Lysogenic Cycle
1.	The viral genome does not integrate into the host genome.	The viral genome integrates into the host genome and stays there throughout replication until the lytic cycle is triggered.
2.	A lytic phage is capable of using only lytic cycles	A temperate phage is capable of using both the lysogenic and lytic cycles
3.	Host DNA hydrolyzed	Host DNA not hydrolyzed
4.	Absence of prophage stage	Presence of prophage stage
5.	Viral DNA replication occurs independently from the host DNA replication	Viral DNA replication occurs along with the host DNA replication
6.	Occurs within a short period of time	Takes time
7.	Symptoms of viral replication are evident	Symptoms of viral replication not evident
8.	Genetic recombination in the host bacterium not allowed	Genetic recombination in the host bacterium allowed
9.	The cellular mechanism of the host cell completely taken over by the viral genome	The cellular mechanism of the host cell slightly disturbed by the viral genome