Lysogenic Cycle

A virus is an infectious submicroscopic organism that only reproduces inside living cells. All living things, including plants, animals, and microbes like bacteria and archaea, are susceptible to virus infection. Since Dmitri Ivanovsky’s 1892 publication revealing a non-bacterial pathogen infecting tobacco plants and Martinus Beijerinck’s 1898 discovery of the tobacco mosaic virus, more than 9,000 virus species—out of the millions of different types of viruses in the environment—have been documented in detail. Viruses are the most common sort of living organism and can be found in practically all ecosystems on Earth. Virology is the study of viruses; it is a branch of microbiology.

Bacteriophage

A virus known as a bacteriophage or phage only infects and replicates inside the bodies of bacteria.

• Frederick W. Twort in the United Kingdom and Félix d’Hérelle in France each separately made the discovery of bacteriophages.
• The terms “bacteria” and “phagein,” which both mean to consume, are the roots of the word “bacteriophage.” Félix d’Hérelle is credited with coining the phrase.
• These are acknowledged as one of the most prevalent biological agents on earth and can be found all over the planet in various situations. These biological particles are the most prevalent in water and make up the second-largest portion of biomass on land after prokaryotes.
• Bacteriophages that infect bacteria can also infect members of the archaeal domain.
• Bacteriophages vary in shape, size, and genomic organization depending on the type of bacteria they infect, but the basic organization is the same.
• All bacteriophages consist of a nucleic acid genome enclosed in a coat of phage-encoded capsid proteins.
• Different phage head structures can vary, and phage sizes range from 24 to 200 nm in length.
• The shape, size, and structure of different bacteriophages vary according to the bacteriophage species.
• Research on bacteriophages has increased over the years as the range of applications has expanded.
• The ability of phages to infect and, in some cases, kill infectious bacteria demonstrates their potential to complement or replace antibiotics. The bacteriophage infection mechanism is much the same, first attaching to the host cell and inserting its genome into the host cell to shut down the host’s cellular machinery.

Structure of Bacteriophage

Phage types vary according to the species or group of bacteria they infect, but all phages share some common characteristics and properties. Some of these features or characteristics of bacteriophages are:

1. Like all viruses, bacteria are also very specific to their host cells. Bacteria infect only one species of bacteria or even several specific strains of bacteria within a species. 
2. The basic structure of all phages is the same. They consist of a core of nuclear matter surrounded by a capsid protein.
3. Bacteriophages exist in three basic structural forms; tetrahedral head with the tail, tetrahedral head without tail, and threadlike. The genetic or nuclear material of a macrophage can be either DNA or RNA, both of which can be double-stranded or single-stranded. 
4. Bacteria are obligate intracellular parasites that exist latently outside the host cell and require the host cell machinery to carry out their metabolic activities. 
5. Like bacteria, phages are also classified into different ranks and families based on their morphology and genetic material. Some commonly studied families include Inoviridae, Tectiviridae, Microviridae, and Rudiviridae.

Life Cycles of Bacteriophage

The virus enters the host cell to reproduce, during which time the virus leads to various forms of infection of the host cell. The overall process of viruses entering, replicating, and exiting host cells includes the life cycle of the virus. Bacteriophages, like all viruses, follow a similar trajectory, where viruses enter the bacterial host cell to replicate. There are two types of life cycles that differ in the mechanism of DNA replication, where one type of viral DNA is incorporated into the host DNA but in another type of cycle, the DNA replicates the host DNA separately. These life cycles can occur independently or alternately in different phages.

• Lytic Cycle

1. The lytic cycle is one of the two life cycles of bacteriophages, in which viral DNA remains as a floating molecule and replicates separately from bacterial DNA. 
2. The lytic cycle usually occurs in virulent phages, as phages cause disruption of infected cell membranes during the release of virus particles. 
3. The lytic cycle is a highly virulent infection that leads to cell destruction.

Lysogenic Cycle

• Lysogenicity is one of two bacteriophage life cycles defined by the integration of the bacteriophage genome into the host genome. 
• During the lysogenic life cycle, host bacteria normally survive and continue to grow after bacteriophage replication. 
• A bacteriophage’s genetic material that is incorporated into the bacterial DNA during the lysogenic life cycle is called a prophage and can be transferred to daughter cells during bacterial cell division. 
• The lysogenic cycle is a moderately nonpathogenic infection because bacteriophages do not kill host cells.

The lysogenic life cycle process  occurs in the following steps:

Attachment and Penetration 

1. The first step of the lysogenic life cycle is the same as the first step of the lytic life cycle. 
2. Bacteriophage ligands attach to receptors on the surface of the bacterial cell wall. 
3. Binding is highly specific as it is determined by the interaction between the ligand and the receptor present on the surface of the bacterial cell wall. 
4. After attachment, the viral genome is injected into the cytoplasm of the host cell. 
5. The infectious viral DNA or prophage then integrates into the host chromosome, converting the infectious prophage into a non-infectious prophage.

Replication

1. The viral DNA then uses the host machinery to replicate and continues to replicate with the host chromosome during cell division.
2. In some cases, prophages can be expelled from the host chromosome and viral DNA can enter the lytic cycle.
3. In contrast to the lytic cycle, the bacterial cell mechanism is not carried over to the viral particle and viral protein biosynthesis does not occur.
4. Prophages, on the other hand, can be transferred to daughter cells during bacterial cell division.
5. The replication process can be either a physical stressor like ultraviolet radiation, undernutrition, or a chemical that can lead to a transition from the lysogenic to lytic cycle.
6. It continues until it exists. After conversion to the lytic cycle,  viral DNA undergoes transcription to produce viral proteins. The proteins and viral genome are then assembled to form a complete virus particle, which is released from the host cell by lysis.

Difference between Lytic Cycle and Lysogenic Cycle

Lytic Cycle	Lysogenic Cycle
The lytic cycle is a type of bacteriophage life cycle in which viral DNA remains as a floating molecule and replicates separately from bacterial DNA.	Lysogenicity is another type of bacteriophage life cycle defined by the integration of the bacteriophage genome into the host genome.
The lytic cycle is also known as the infectious or virulence cycle.	The lysogenic cycle is also known as the temperate cycle or the avirulent cycle.
The productiveness of viral DNA and viral proteins is high.	Viral DNA and viral protein productivity are low.
The lytic cycle is immediate and completed in a short time.	The lysogenic cycle takes longer.
Since this cycle is an infectious cycle, symptoms of viral infection are seen.	This cycle is a non-infectious cycle that causes no symptoms.
The lytic cycle does not allow the genetic recombination of host chromosomes.	The lysogenic cycle enables the genetic recombination of host chromosomes.
A lytic cycle cannot transition to a lysogenic cycle.	A lysogenic cycle can transition to a lytic cycle.
The host’s cellular mechanisms are completely hijacked by the viral DNA.	The host’s cellular mechanisms are unaffected.
The lytic cycle ends with the lysis of host cells.	The lysogenic cycle does not result in host cell lysis.

FAQs on Lysogenic Cycle

Question 1: Which viruses follow the lysogenic cycle?

Answer:

Bacteriophages are viruses that infect and multiply in bacteria. Temperate phages (such as lambda phage) can replicate using both lytic and lysogenic cycles.

Question 2: What triggers lysogenic to lytic?

Answer:

When prophage-bearing bacteria are exposed to stressors such as UV light,  nutrient deficiencies, and chemicals such as mitomycin C, prophages are naturally extracted from the host genome and can enter the lytic cycle in a process called induction.

Question 3: How do viruses reproduce the lysogenic cycle?

Answer:

These viruses break down or lyse cells and spread to other cells to continue the cycle. Similar to the lytic cycle, in the lysogenic cycle the virus attaches to  host cells and injects its DNA. From there, the viral DNA integrates into the host’s DNA and  cells.

Question 4: What is the advantage of a lysogenic cycle?

Answer:

The lysogenic cycle allows phages to replicate without killing the host.

Question 5: Which stage of the virus occurs first?

Answer:

The first stage is an entry. The entry includes an attachment, where the virus particle encounters a host cell and attaches to the cell surface, entry, where the virus particle reaches the cytoplasm, and exposure, where the virus releases the capsid.

Question 6: How are lysogenic viruses different from lytic viruses?

Answer:

The main difference between lysogenic and lysogenic cycles is that the lysogenic cycle does not destroy host cells whereas the lysogenic cycle destroys host cells. The viral DNA destroys the host cell’s DNA, halting the cell’s function in the lytic cycle. However, in the lysogenic cycle, viral DNA can fuse with host DNA.