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Bacteriophages, or bacterial viruses, are a type of viruses that infect bacteria. Such a virus infects the cell of bacteria and carries out reproduction inside it. They were discovered by F. W. Twort. They can differ in their shapes and genetic material. Viruses contain genetic material in the form of DNA or RNA, but not both. Here, we will have a closer look at the topic, its structure and life cycle along with some important questions. While bacteriophages are generally double-stranded DNA viruses. Their gene count can range from four to several thousand. They have a protein coat, capsid, which is made of capsomeres. These capsomeres can be of helical or polyhedral geometric form.
Read More: Difference between Bacteria and Virus
Keyterms: Bacteriophages, Bacteria, Virus, DNA, RNA, Protien, Gene, Reproduction, capsid, archaea
What are Bacteriophages?
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As the name suggests, Bacteriophages (bacteria-eater) are the viruses that infect and replicate within the bacteria. They are often called a phage and are found everywhere. In their genome, they contain DNA or RNA which is encapsulated in a protein coat. They also infect archaea.
Bacteriophages
Ernest Hanbury Hankin was the one who observed the Bactericidal activity of bacteriophage in 1896 in the water of river Ganges, which could kill cholera bacteria. William Twort discovered bacteriophage in 1915.
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Classification and Examples of Bacteriophages
| Examples | Family of the phage virus | Nucleic acid content | Morphological characteristics |
|---|---|---|---|
| T4 (Escherichia virus T4) | Myoviridae | Linear dsDNA, 169 kbp long |
|
| T2 (Enterobacteria phage T2 | Myoviridae | Linear dsDNA, 170 kbp |
|
| λ (Coliphage, Escherichia virus lambda) | Siphoviridae | Linear dsDNA, 48502 bp |
|
Characteristics of Bacteriophage
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- Bacteriophages can be of many types, but each type can infect only certain types of bacteria.
- Just like other viruses, they have a core genetic material surrounded by a protein capsid. And usually, the genetic material of bacteriophages is double-stranded DNA.
- Once this virus infects the bacterial cell, it disrupts the mechanism of the bacterial cell and produces viral components inside it.
- The infected bacterial cell eventually undergoes lysis.
- Transduction is the process in which a bacteriophage transfers the genetic material of a bacterial cell (donor cell) to another bacterial cell (receptor cell).
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Structure of Bacteriophage
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A bacteriophage has a polyhedral head, a short collar and a helical tail.
- The head comprises 2000 capsomeres with double-stranded DNA or single-stranded RNA as their genetic material.
- The tail comprises an inner hollow tube formed of the sheath. Its end is connected to a plate that has tail fibres. The virus attaches itself to the bacteria with these tail fibres.
Structure of Bacteriophage
Life Cycle of Bacteriophage
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There are two different life cycles that a bacteriophage exhibits:
- Lytic Cycle
- Lysogenic Cycle
1. Lytic Cycle (Virulent Cycle)
In this cycle, a bacteriophage enters bacteria and kills them. It then releases a progeny virus. The cycle goes through the below-mentioned steps:
- Adsorption: In this process, the tail fibres of a bacteriophage attach themselves to the surface of bacteria. The process is called adsorption.
- Penetration: After adsorption, the tail sheath of the virus contracts. Once the tail fibres and the plate are attached to the bacterial cell, the hollow core is pushed downwards and DNA is injected into it.
- Synthesis: After the genetic material of the bacteriophage is released in the cell, the components of the new virus are produced. Specific enzymes called early proteins to carry out this synthesis.
- Maturation: The head and tail protein of the bacteriophage DNA combines on maturation and then each component of the phage DNA is surrounded by a protein coat.
- Release: Now, the infected bacterial cell undergoes lysis and produces new bacteriophages. Due to this, the cell wall of the bacteria weakens.
Life Cycle of Bacteriophage
2. Lysogenic Cycle (Temperate Cycle)
In the lysogenic cycle, the bacteriophage integrates with the chromosome of the host cell and comes to be known as the prophage. This is another way of reproduction in bacteria in which the prophage is transmitted to progenies. In this phase, the infected bacterial cell continues to reproduce normally.
Lysogenic Cycle (Temperate Cycle)
During this process, a bacterium doesn’t go through lysis immediately. But the prophage eventually goes through lysis artificially or naturally using physical or chemical agents.
Bacteria infected with a prophage that hasn’t undergone lysis are called lysogenic bacteria. In the case of multiplication of lysogenic bacteria, due to excision, the prophage might get lost.
Read More: DNA vs RNA
Things To Remember
- Any virus that infects bacteria is called a bacteriophage or bacterial virus.
- The reproduction of a bacteriophage occurs through either of two cycles: Lytic cycle and Lysogenic cycle.
- The genetic material of a bacteriophage is usually double-stranded DNA.
- A process in which the phage transfers the genetic material of one bacterium to another bacterium is called transduction.
- During the lysogenic cycle, the phage attaches itself to the chromosome of the host cell and is then known as prophage.
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Previous Years Questions
- Black (stem) rust of wheat is caused by… (NEET 2010)
- A free living nitrogen-fixing cyanobacterium which can also form symbiotic association with the water fern Azolla is...(NEET 2004)
- Which fungal disease spreads by seed and flowers...(NEET 2002)
- The thalloid body of a slime mould (Myxomycetes) is known as...(NEET 2006)
- There exists a close association between the alga and the fungus within a lichen. The fungus...(NEET 2005)
- Puccinia forms uredia and … (NEET 1998)
- A hyphae which is multinucleated and asptate is known as … (WBJEE 1999)
- Animals which possess cleidoic eggs exhibit….[KCET 2011]
- Which among these is not a post fertilization event ?...[KCET 2016]
- Type of asexual reproduction found in Hydra is..[KEAM]
- Which of the following is having longitudinal binary fission ?….[KEAM]
- In grafting, the stock and scion should be joined….
- The mode of asexual reproduction in Euglena is….[CUCET 2010]
- Archaebacteria differ from eubacteria in...(NEET 2014)
- Azotobacter and Bacillus polymyxa are the examples of...(NEET 1996)
- Apomixis in plant means development of a plant… [BHU UET 2008]
- Which one of the following plants reproduces vegetatively by epiphyllous buds?
Sample Questions
Ques: What is a bacteriophage? (1 mark)
Ans: A bacteriophage is a virus that infects bacteria and reproduces inside the bacterial cell. It is also referred to as a bacterial virus.
Ques: Generally, the genetic material of a bacteriophage is: (1 mark)
(a) ss RNA
(b) ss DNA
(c) ds RNA
(d) ds DNA
Ans: Viruses consist of DNA or RNA as their genetic material, but not both. Bacteriophages usually have double-stranded DNA (ds DNA) as their genetic material.
Ques: What is the process by which a bacteriophage transfers the genetic material of one bacterium to another called? (2 marks)
(a) Transduction
(b) Conjugation
(c) Transformation
(d) Viral replication
Ans: The process by which a bacteriophage transfers the genetic material of a bacterial cell (donor cell) to another bacterial cell (receptor cell) is called transduction.
Ques: A bacteriophage is a virus that infects ____. (1 mark)
(a) insects
(b) plants
(c) bacteria
(d) animals
Ans: A bacteriophage or bacterial virus is a virus that infects bacteria.
Ques: Explain the replication process of a bacteriophage. (3 marks)
Ans: A bacteriophage reproduces inside a bacterial cell that it has infected. After it has injected its genetic component into the bacteria, reproduction occurs due to one of the two cycles: Lytic cycle and Lysogenic cycle. In the lytic cycle, the infected bacterial cell undergoes lysis and produces progeny viruses. While in the lysogenic cycle, the phage integrates with the host cell chromosome and is called a prophage. In this, the bacterial cell reproduces normally. Eventually, the prophage undergoes lysis artificially or naturally through chemical or physical agents.
Ques: What is the lifecycle of bacteriophages? (2 marks)
Ans: There are two different life cycles that a bacteriophage exhibits:
- Lytic Cycle: In this cycle, a bacteriophage enters bacteria and kills them.
- Lysogenic Cycle: In the lysogenic cycle, the bacteriophage integrates with the chromosome of the host cell and comes to be known as the prophage.
Ques: Write the structure of the bacteriophage. (2 marks)
Ans: A bacteriophage has a polyhedral head, a short collar and a helical tail.
- The head comprises 2000 capsomeres with double-stranded DNA or single-stranded RNA as their genetic material.
- The tail comprises an inner hollow tube formed of the sheath. Its end is connected to a plate that has tail fibres. The virus attaches itself to the bacteria with these tail fibres.
Ques: What are the steps related to the lytic or virulent cycle of the bacteriophage? (4 marks)
Ans: The cycle goes through the below-mentioned steps:
- Adsorption: In this process, the tail fibres of a bacteriophage attach themselves to the surface of bacteria. The process is called adsorption.
- Penetration: After adsorption, the tail sheath of the virus contracts. Once the tail fibres and the plate are attached to the bacterial cell, the hollow core is pushed downwards and DNA is injected into it.
- Synthesis: After the genetic material of the bacteriophage is released in the cell, the components of the new virus are produced. Specific enzymes called early proteins to carry out this synthesis.
- Maturation: The head and tail protein of the bacteriophage DNA combines on maturation and then each component of the phage DNA is surrounded by a protein coat.
- Release: Now, the infected bacterial cell undergoes lysis and produces new bacteriophages. Due to this, the cell wall of the bacteria weakens.
Ques: What is the lysogenic lifecycle of the bacteriophage? (4 marks)
Ans: In the lysogenic cycle, the bacteriophage integrates with the chromosome of the host cell and comes to be known as the prophage. This is another way of reproduction in bacteria in which the prophage is transmitted to progenies. In this phase, the infected bacterial cell continues to reproduce normally. During this process, a bacterium doesn’t go through lysis immediately. But the prophage eventually goes through lysis artificially or naturally using physical or chemical agents. Bacteria infected with a prophage that hasn’t undergone lysis are called lysogenic bacteria. In the case of multiplication of lysogenic bacteria, due to excision, the prophage might get lost.
Ques: Write the importance of bacteriophage. (5 marks)
Ans: Bacteriophages are utilized for various purposes. They are widely used in medicine and research.
- They are used as antibiotics against bacteria due to the same mode of action in phage therapy.
- In order to kill bacteria in meat or cheese products, they are used in the food industry.
- Bacteriophages are also used for diagnostic purposes.
- In research and studies, they act as a model.
- They are used as a cloning vector in genetic recombination techniques.
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