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Bacteriophage – Definition, Structure, Life Cycle, Importance

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  • Last Updated : 30 Jun, 2022

Bacteriophage infections attack and reproduce inside microscopic organisms, as the name suggests. A phage is a well-known name for them. They can be tracked down out of control. Their genome comprises DNA or RNA that is encased in a protein sheath. Bacteriophages would be the most well-known and various elements in the biosphere. Bacteriophages are omnipresent infections, found any place microbes exist. It is assessed there are more than 100000000000000 bacteriophages in the world, more than each and every organic entity on Earth.




Bacteriophages would have a capsid, which is a protein coat that encases the DNA. A polyhedral head is available. It very well may be wrapped or not and arrive in that frame of mind of morphologies, including bar formed, filamentous, isometric, etc. Numerous capsomeres make up the capsid. Changing species have various sizes and shapes.
The genome is composed of straight or round ss or ds DNA or RNA. Proteins going from 5 to 110 are encoded in the genome. The genome of the MS2 bacteriophage codes for four proteins. A bacteriophage’s greatest genome is 735 kbp in size. The tails may be long or short, contractile or non-contractile, and contractile or non-contractile. The infection would connect to the cell mass of the microscopic organisms with the assistance of the tail strands.

Bacteriophage life cycles

Bacteriophages can contaminate the host bacterium in one of two ways.

Life Cycle


Lytic Cycle (Virulent disease)

They make the bacterial cell totally lyse, and this would call as lytic life cycle. Examples are t2 and t4 phages. After the viral genome is reproduced, the bacterial cell is altogether annihilated. This is known as a destructive contamination, and it is generally utilized in phage treatment.

The means in the lytic cycle are as per the following:

  • Adsorption is the course of a bacteriophage’s tail strings, mooring it to the bacterial cell divider.
  • Infiltration involves infusing phage DNA into microscopic organisms.
  • Replication and combination are disturbed, and the viral genome assumes command over the bacterial apparatus.
  • The parts of the phage are assembled to frame new infection particles.
  • Bacterial cells are lysed, and new infection particles are delivered into the climate to contaminate more cells.


  1. Attachment– An infection joins itself to the outside have cell.
  2. Infusion- The infection’s hereditary material (either RNA or DNA) is infused into the host cell through a recently made opening in the layer of cell.
  3. Integration- The infection’s hereditary material provides the cell with another arrangement of directions: construct a greater amount of the infection.
  4. Replication- The ribosomes inside the host cell assemble the parts of the infection.
  5. Gathering – The host cell collects the parts into new duplicates of the infection.
  6. Lysis- The cell blasts open, bringing about its demise and furthermore in the arrival of a greater amount of the infection, which can now proceed to taint different cells.

Lysogenic Cycle (Temperate disease)

  • Calm phages are bacteriophages that go through lysogeny. The viral DNA is coordinated into the bacterial genome and duplicates close by it. A prophage is a viral genome that has been consolidated.
  • It is very harmless and will stay set up until the lytic cycle is started. It could happen suddenly or because of outward factors like radiation openness. The prophage then becomes dynamic, and a lytic cycle starts, bringing about cell divider lysis.
  • The phage DNA is integrated into bacterial DNA and replicated close by the bacterial genome after infiltration.
  • The viral genome is passed on to the descendants as the genome of microscopic organisms would be integrated into the bacterial genome and microbes would carry out normal role as a rule.
  • Lysogenic cells are bacterial cells that contain a prophage.


  1.  A bacteriophage infection taints a microorganism by infusing its DNA into the bacterial cytoplasm, or fluid space within the cell divider.
  2.  The viral DNA is perused and duplicated by the very bacterial proteins that imitate bacterial DNA.
  3. The viral DNA can keep utilizing the bacterial apparatus to imitate, or it can change to the lytic cycle. In the event that the viral DNA stays in the lysogenic cycle, one duplicate, or hardly any duplicates, of the DNA exist in numerous microorganisms. In the lysogenic cycle, the DNA possibly gets reproduced when the microorganisms are recreating their own DNA.
  4.  Eventually, the viral DNA will change to the lytic cycle, in which the bacterial components are utilized to deliver bunches of DNA and loads of capsids, or protein covers, for the DNA.
  5. These capsids get delivered into the climate, contaminate another microbes, and the lysogenic cycle might begin once more. Assuming that the microbes are frail or biting the dust, the infection might enter straight into the lytic cycle, to try not to color with the microorganisms.

Bacteriophage’s Importance

  • Bacteriophages serve an assortment of capacities. They’re usually utilized in clinical and research settings.
  • Phage treatment Due to their comparable technique for activity, they are used as anti-toxins against microscopic organisms.
  • They’re utilized to kill microorganisms in meat and dairy items in the food business.
  • Bacteriophages are utilized to make analyze.
  • In exploration and examinations, they act as a model.
  • In hereditary recombination, they are used as a cloning vector.
  • Now phage therapy would be used in place of antibiotics.

Conceptual Questions 

Question 1: Cycles in the bacteriophage?


Lytic and the lysogenic cycle would happen in the bacteriophage.

Question 2: Greatest genome in the bacteriophage?


Genome in bacteriophage would be 735kbp in the size.

Question 3: Vector utilized In the hereditary designing?


Bacteriophage is the normally utilized vector.

Question 4: Strains of lysogenic would be known as?


They are called as a virulent and the mild phages.

Question 5: Genome of the bacteriophage made of?


It would be made of DNA or the RNA

Question 6: Bacteriophage was discovered by?


It was found by T.O Diener.

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