Life Cycle of Bacteriophage | Lytic Cycle | Lysogenic Cycle | Induction

A bacteriophage is a virus that replicates inside a bacterial cell by utilizing its machinery. The structure of bacteriophage allows it to land and inject its genetic material into a bacterial cell. 

What is a bacteriophage?

The size of a virus is smaller than bacteria. It has been found that the virus also causes infection in bacteria. The bacteriophage is a virus that infects bacteria. It is also known as a phage or bacterial virus. Bacteriophage term was coined by D'Hérelle which is made up of two words “bacteria” and Greek work “φαγεῖν” (phagein), collectively means “bacteria eater”.


Early research mostly focused on phages that infect Escherichia coli. A series of 7 phages that infect Escherichia coli are known as T-phages (T for type). The T2, T4, and T6 phages are called T-even phages. The T1, T3, and T7 phages are known as dependent virulent, while the T5 phage is called "autonomously virulent". Out of all the T-phages, T4 bacteriophage is well studied.

Structure of T4 bacteriophage

The structure of T4 bacteriophage is different from other virus. The T4 phage belongs to the subfamily Tevenvirinae from the family Myoviridae. Under an electron microscope, the T4 phage resembles a tadpole having a head and tail. The detailed structure of the T4 bacteriophage is discussed below.

structure of t4 bacteriophage
Structure of T4 Bacteriophage

I. Head

The head is made up of capsid head and nucleic acid.

a. Capsid head

The head of the T4 phage is 120 nm long and 86 nm wide having hexagonal and prism shapes. The capsid head stores and protects the genome.

b. Nucleic acid

T4 phage contains double-stranded DNA that contains approximately 170000 bases. The modified bases present in T4 DNA protect the genome from breakdown by bacterial restriction endonucleases.

II. Tail

The structure of the tail is more complicated compared to the head. It contains a tail tube, sheath, collar, whisker, spike, base plate, and tail fiber.

a. Tail tube

It is an inner non-contractile tube made up of protein. It provides the passage for the movement of DNA from head to the bacterial cell.

b. Sheath

The tail tube is enclosed by a contractile envelope known as a sheath which is made up of tail sheath proteins. The contraction of the sheath pushes the tail tube through the outer membrane of bacteria, forming a canal for the delivery of the viral genome.

c. Collar and whisker

Collar and whisker are made up of fibers present near the head-to-tail interference of the page. They control the withdrawals of the long tail fibers and also helps in the attachment of tail fibers to phage particles during assembly.

d. Baseplate

It is made up of multiple protein molecules. It controls sheath contraction, host recognition, DNA ejection, and attachment.

e. Spike

A spike pierces the cell membrane of bacteria and makes an opening for the entrance of the tail tube.

f. Tail fiber

It is like appendages. It is a thin rod-like structure used for the attachment of bacteriophage on the surface of bacteria.

Video lesson on structure of bacteriophage

Initial steps of life cycle of bacteriophage

The first step involved in the replication of bacteriophages is the injection of its genetic material into the bacterial cell. The steps involved in this process are discussed below.

life cycle of bacteriophage

Attachment or absorption

Bacteriophage attaches to the surface of the bacterial cell wall at a specific receptor site by their tail fibers. This process of its attachment is called absorption. This attachment is the courtesy of week chemical bonding between receptor site and bacteriophage.


The tail of the bacteriophage discharges lysozyme which is an antibacterial enzyme to digest the small part of the cell wall. After digestion, the tail sheath contracts, and a rigid tube is propelled out of the sheath to puncture a hole on the bacterial cell membrane. 

Genetic material is then passed through the tail tube and injected into the bacterial cell. The protein coat and tail of the bacteriophage do not enter the cell and remain outside.

Life cycle of bacteriophage

There are three different fates of genetic material after the entry into the bacterial cell. Absorption and penetrations steps are the same for all types of cycles. The life cycle of bacteriophage can be divided into three categories i.e.,

  1. Lytic cycle
  2. Lysogenic cycle
  3. Induction

1. Lytic cycle

The lytic cycle or virulent cycle results in the lysis of the bacterial cell. The steps of lytic cycle are discussed below.

lytic cycle of bacteriophage

a. DNA replication and protein synthesis

After the penetration, viral genetic material hijacks the biosynthetic machinery of the bacterial cell and starts multiplying by synthesizing its components such as DNA, protein, and tail structures. 

b. Assembly of phages

The new genetic material is packed into the head and then the tail is attached to complete the construction of the bacteriophage. It takes 25 minutes for a bacteriophage to make about 200 new bacteriophages after entry into the cell. 

c. Lysis

As the number of daughter bacteriophages increases in the bacterial cell, it starts to weaken and newly formed bacteriophages are released causing lysis of the bacterial cell. Such phage which causes lysis of bacterial cell is known as lytic phage.

2. Lysogenic cycle

In the lysogenic cycle of the temperate cycle, no bursting of the bacterial cell takes place and it keeps on living normally. 

lysogenic cycle of bacteriophage

a. Incorportaion of viral genome

Instead, genetic material becomes incorporated into the genetic material of bacteria. The phage at this stage is called a prophage. The prophage does not take over host machinery and remains in a dormant state as a result no viral protein and genetic material are synthesized. The size of viral genetic material is very small and its incorporation into bacterial genetic material does not cause any harm.

b. Daughter bacterial cells

Bacterial keeps on living and viral genome keeps on passing from parent to daughter bacterial cells.

3. Induction

The conversion of the lysogenic cycle into the lytic cycle is called induction. During this process, viral genetic material detaches itself from the bacterial genome and starts the lytic cycle.

Video lesson on life cycle of bacteriophage

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