Bacteriophage |multiplication of bacteriophage |Lytic, and Lysogenic Cycle

Bacteriophages are viruses that are obligatory parasites on bacteria (Viruses that parasitize bacteria are called bacteriophages). These were first described by Heeler (1917). It contains DNA as genetic material. These are tadpole-shaped viruses.

Bacteriophages that infect E.coli are known as coliphage or T4 phages.

The bacteriophage is divisible into five regions. They are head, tail, collar, endplate, and tail fibril.

The head is hexagonal, consisting of a core containing double-stranded DNA and capsid made up of proteins.

The tail consists of an empty core surrounded by a protein sheath. The head and tail jointed by a collar.

A hexagonal plate is present at the tip of the tail. It Produces six tail like fibers which are useful for attachment with the bacterial cell during infection.

Lytic cycle

Multiplication of Bacteriophages  or life cycle of bacteriophage

Bacteria phages can multiply by two alternative mechanisms the lytic cycle or the glycogenic cycle. The lyric cycle end with the lysis and death of the host cell, whereas the cell remains alive in the lysogenic cycle.

The Lytic Cycle 

The virions of T-even bacteria phages are large, complex, and non-enveloped, with a characteristic head and tail.
The length of DNA contained in these bacteria phages is only about six percent of that contained in E. coli. The multiplication cycle of these phages is similar to all viruses and occurs in five distinct stages.
(i) Attachment
(ii) Penetration
(iii) Biosynthesis
(iv) Maturation
(v) Release

(i) Attachment   

During this process, the virus attaches to a complementary receptor site on the bacterial cell. This attachment may be a chemical interaction during which weak bonds are formed between the attachment and receptor sites.

T-even bacteria phages use fibers at the end of the tail as attachment sites.

The complementary receptor sites are on the bacterial cell wall.

(ii) Penetration 

After attachment, the T-even bacteriophage injects it, s DNA into the bacterium. After this, the bacteriophage’s tail releases an enzyme (phage lysozyme ), which breaks down some of the bacterial cell membranes.

During the method of penetration, the tail sheath of the phage contracts and tail care is driving through the cell membrane. When the tip of the core reaches the cell membrane, the DNA from the bacteriophage’s head passes through the tail core, through the cell membrane, and enters the bacterial cell.

The capsids remain outside the cell. Therefore, the phage particle functions as a hypodermic to inject DNA into the bacterial cell. 

(iii) Biosynthesis 

Once the bacteriophage DNA has reached to the cytoplasm of the host cell, the biosynthesis of viral nucleic acid and protein occurs. 

The virus stops host protein synthesis– induce degradation of the host DNA, viral proteins interfere with transcription, or the repression of translation.

Initially, the phage uses the host cell’s nucleotide and several of its enzymes to synthesize many copies of phage DNA. After that, the biosynthesis of viral protein begins. 

mRNA transcribed from phage DNA for the synthesis of phage enzymes and capsid proteins. The host cell’s ribosome, enzyme, and amino acids are used for translation. 

Genetic control regulates when different regions of phage DNA are transcribed into mRNA during the multiplication cycle. For several minutes of the following infection, complete phages can’t be found in the host cell. Only separate components of DNA and protein can be detected. 

The period during viral multiplication when complete, Infective virion is not yet present, is called the eclipse period. 


In this bacteriophage, DNA and capsid are assembling into complete virions. The viral components essentially make into a viral particle spontaneously and eliminating the need for many nonstructural genes and gene products. 

The phase head and tail are separately assembled from protein subunits, and the head is filled with phage DNA and attached to the tail.


The final step of viral multiplication is the release of the virion from the host cell. The term “lysis’’ is generally used for this stage in the multiplication of T-even phages because, in this case, the plasma membrane breaks open (lyses).

Lysozyme is encoded by a phage gene, is synthesized within the cell. This enzyme causes the bacterial cell membrane to break down, and thus the newly produced bacteria phages are releasing from the host cell.

The released bacteriophages infect other susceptible cells within the vicinity, and therefore the viral multiplication cycle is repeating within these cells.


The Lysogenic cycle (Bacteriophage Lambda)

In contrast to T-even bacteria phage, some viruses don’t cause lysis and death of the host cell when they multiply. These lysogenic phages are also called temperate phages.

A temperate phase is called as bacteriophage lambda. Bacteriophage lambda is similar to T4, but its tail has only one short tail fiber.

The lysogenic phages also proceed through a lytic cycle, but they are also capable of inculpating their DNA into the host cell s DNA to begin a lysogenic cycle.  

Lytic-and-lysogenic-cycle (3)
Lytic-and-lysogenic-cycle (3)

In lysogenic, the phage remains latent or inactive, and the participating bacterial host cells are known as lysogenic cells.

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