Bacteriophage: Overview, Structure, Definition, Structure, Examples and Life
A bacteriophage, also known as a phage, is a virus which infects and replicates within bacteria. The term “phagein” means “to devour”. They are composed of proteins that have DNA or RNA, and have simple or elaborate structures. Their genomes may encode as few as four genes, and as many as a hundred genes. Phages replicate within the bacteria, following the insertion of their genome into its cytoplasm.
This Story also Contains
- What is a Bacteriophage?
- Structure of a Bacteriophage
- Bacteriophage Life Cycle
- Types of Bacteriophage
- Applications and Importance of Bacteriophages
- Recommended Video On Hershey and Chase Experiment
- MCQs on Virus, Virioids, and Prions
Bacyeriophages are one of the most common and diverse entities in the biosphere. They are ubiquitous viruses, found where bacteria exist. Viruses are the most abundant biological entity in the oceans and the second largest component of total biomass after prokaryotes. There are up to 70% of marine bacteria may be infected by bacteriophages. Bacteriophages are important topics of the chapter Molecular Basis of Inheritance. It is an important chapter in the Biology subject.
What is a Bacteriophage?
Definition: A bacteriophage, often known as a phage, is a kind of virus that replicates itself in bacteria by infecting bacterial cells with its genetic material.
Function: Bacteriophages are used in a variety of scientific applications, regulate bacterial populations, and enhance genetic diversity.
Relevance in Molecular Biology: Bacteriophages have applications in molecular biology to help with genetic engineering and recombinant DNA technologies by cloning genes and acting as vectors for DNA manipulation.
Structure of a Bacteriophage
The bacteriophage structure is highly specialised to facilitate bacterial infection. Key components include:
Head(capsid): The phage's genetic material, which may consist of either DNA or RNA, is enclosed by the protein-based head (capsid). The genetic instructions for replication of DNA are carried by the head, which also provides protection.
Tail: Genetic material is transferred into the bacterial host by the tail. It consists of a central core encased in a sheath that contracts to introduce DNA into the bacterium.
Base Plate and Tail Fibres: The phage may attach to particular receptors on the bacterial surface by using the base plate, which is located at the base of the tail and connects to the tail fibres. Which bacteria a phage can infect is determined by its specificity.
Bacteriophage Life Cycle
The lytic cycle and the lysogenic cycle are the two main cycles by which bacteriophages multiply. Both cycles influence the spread of viruses and the destiny of bacteria:
The Lytic Cycle
Attachment: Using its tail fibres, the phage binds to a particular receptor on the surface of the bacterial cell.
DNA Injection and Penetration: When the tail contracts, viral DNA is injected into the bacterial cytoplasm.
Replication and Assembly: The phage DNA replicates by generating viral proteins and putting together new phage particles using the resources of the bacterial cell.
Lysis and Release: Newly created phages are released to infect additional bacteria once the bacterial cell is lysed, or destroyed. The bacterial cell dies as a result of this cycle.
Lysogenic Cycle
Integration: Viral DNA integrates into the bacterial genome, becoming a prophage.
Dormancy: The viral genome is replicated passively along with the bacterial DNA as the cell divides. This state can persist until environmental triggers initiate the lytic cycle.
Induction: Under certain conditions (e.g., stress), the prophage DNA excises from the bacterial genome, entering the lytic cycle, and eventually causing bacterial lysis.
Understanding these cycles is important for students, as it demonstrates how viruses can alter bacterial genetic material and impact bacterial survival.
Types of Bacteriophage
There are various types of bacteriophages based on their shape and the presence of a head and tail. The table below shows the classification of bacteriophage:
Type of Bacteriophage | Description | Example |
Icosahedral Phages | Spherical shape with icosahedral symmetry; often have tail fibres or spikes for attachment. | T4 bacteriophage |
Filamentous Phages | Long, thread-like structure; exits the host without causing cell lysis. | M13 bacteriophage |
Head-Tail Phages | Combination of an icosahedral head and a cylindrical tail; most common type. | Lambda bacteriophage |
Applications and Importance of Bacteriophages
Bacteriophages are important for both practical applications and biological research. The various kinds of applications and importance of bacteriophages are given below:
Applications in Medicine: Phage therapy is being researched as a possible antibiotic substitute, particularly for infections that are resistant to antibiotics. Pathogenic bacteria are specifically targeted by phages, which do not damage helpful bacteria.
Biotechnology and Genetic Engineering: Recombinant DNA technology is made possible by the use of bacteriophages as gene transfer vectors. They play a key role in vaccine development, gene expression research, and cloning.
Ecology and Environmental Impact: By controlling bacterial populations in environments like soil and seawater, bacteriophages affect microbial diversity and nutrient cycling.
Recommended Video On Hershey and Chase Experiment
MCQs on Virus, Virioids, and Prions
Q1. A virus envelope is known as:
Option 1: Capsis
Option 2: Virion
Option 3: Nucleoprotein
Option 4: Core
Correct answer: 1)Capsid.
Explanation:
Composition of virus -
Envelope - Outer loose covering with peplomer proteins.
Capsid - Protein covering the genetic material made of capsomeres.
Nucleoid - Either DNA or RNA.
The virus envelope is known as the Capsid.
Hence, the correct answer is option 1)Capsid.
Q2. Viruses are non-cellular organisms, but replicate themselves once they infect the host cell. To which of the following kingdoms do viruses belong?
Option 1: Monera
Option 2: Protista
Option 3: Fungi
Option 4: None of the above
Correct answer: 4) None of the above.
Explanation:
Viruses could not be classified into any of the five kingdoms of Whittaker (Monera, Protista, Fungi, Plantae, and Animalia) due to their non-cellular nature. Unlike living organisms, viruses lack the basic cellular structure and the machinery needed for metabolism, reproduction, and growth. They are essentially composed of genetic material (DNA or RNA) enclosed in a protein coat, and they can only replicate within the living cells of a host organism. Because they do not meet the criteria for life as defined by the kingdoms in Whittaker's system, viruses are considered acellular and are classified separately from living organisms in a distinct group.
Hence, the correct answer is option 4) None of the above.
Q3. Some viruses contain glycoprotein in their envelope. These glycoproteins are helpful in
Option 1: Phagocytosis
Option 2: Binding the viruses to the host
Option 3: Protecting viruses from harsh conditions
Option 4: Fusion of virus with the plasma membrane
Correct answer: 2) Binding the viruses to the host
Explanation:
The glycoproteins in the envelope possess spikes. These help the binding of viruses with the host surface. These spikes specifically bind to receptors on the host cell membrane, allowing the virus to attach and facilitate infection. In some viruses, they also help in membrane fusion or endocytosis, enabling viral entry.
Hence, the correct answer is option 2) Binding the viruses to the host
Also Read:
Frequently Asked Questions (FAQs)
Bacteriophages replicate through the lytic cycle, which destroys the host cell, and the lysogenic cycle, where the phage DNA integrates into the host's genome.
Bacteriophages attach to bacterial surfaces using tail fibres, inject their genetic material, and take over the cell's machinery to produce new phages.
Bacteriophages consist of a protein coat (capsid) containing genetic material, usually DNA or RNA, and a tail structure used for attaching to bacterial cells.
Bacteriophages are used in phage therapy to treat antibiotic-resistant bacterial infections, providing an alternative to traditional antibiotics.
