Bacteriophage - Characteristics, Life Cycle and Functions

Bacteriophage: Bacteriophages are viruses that infect and reproduce within bacteria. They are found in environments like soil and water. Bacteriophages attach to bacterial cells, inject their genetic material, and hijack the host's machinery to make more viruses. This can lead to the bacterial cell bursting open. Bacteriophages vary in genetic makeup and shape, with some having tail-like structures. They follow two primary life cycles: lytic, causing cell destruction, and lysogenic, integrating into the bacterial chromosome. Bacteriophages are studied in molecular biology and offer potential in agriculture and medicine as biocontrol agents and alternatives to antibiotics. Read the NEET Biology Notes below to learn more details about the bacteriophage.

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What is Bacteriophage?

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Bacteriophage Classification

• DNA Phages : These bacteriophages contain DNA as their genetic material, which can be either single-stranded or double-stranded.
• RNA Phages : These bacteriophages have RNA as their genetic material, which can also exist in single-stranded or double-stranded forms.

• Capsid : The protein coat surrounding the genetic material of the phage varies in shape, including icosahedral (spherical), filamentous (rod-shaped), or pleomorphic (irregular).
• Tail : Some bacteriophages possess a tail structure for attaching to and injecting their genetic material into host cells. Tails may be long, short, contractile (capable of shortening), or non-contractile.

• Caudovirales : This primary order encompasses approximately 96% of known phages and includes tailed and tailless icosahedral capsids.
• Myoviridae : characterized by long, contractile tails.
• Siphoviridae : identified by long, non-contractile tails.
• Podoviridae : Known for short, non-contractile tails.
• Other Orders : Smaller orders such as Leviviricetes (comprising small RNA phages), Microviridae (involving small DNA phages), and Inoviruses (which are filamentous RNA phages) exhibit diverse morphologies and genetic compositions.

Bacteriophage Characteristics

1. Virus Specificity : Bacteriophages are viruses that specifically infect bacteria. They do not attack other types of cells.
2. Genetic Material : Bacteriophages have DNA or RNA inside them that is enclosed by a capsid protein coat.
3. Host Specificity : Each bacteriophage targets particular types of bacteria. They are specialized hunters.
4. Structural Components : Bacteriophages typically have a head (capsid) where their genetic material is stored and a tail used to attach to and inject their DNA into bacterial cells.
5. Modes of Replication : Bacteriophages replicate in two ways. In the lytic cycle, they multiply inside the host cell until it bursts. In the lysogenic cycle, they merge their DNA with the bacterial chromosome and copy together.
6. Evolutionary Adaptation : Bacteriophages have evolved in ways to bypass bacterial defences and resist bacterial resistance that ensure their survival and spread.
7. Applications : Bacteriophages are used in various ways, such as tools in research, agents to control harmful bacteria in farming and potential treatments for bacterial infections.

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Life cycles of Bacteriophages

Lytic Cycle

• Attachment : The bacteriophage first latches onto the surface of a bacterial cell using specific receptors.
• Injection : Once attached, the phage injects its genetic material (DNA or RNA) into the bacterial cell.
• Replication : The phage DNA takes over the bacterial cell's machinery, redirecting it to produce viral components instead of bacterial ones.
• Assembly : New phage particles are assembled from the replicated genetic material and protein components inside the hijacked bacterial cell.
• Lysis : Assembled phages burst out of the bacterial cell, destroying it. This cell bursting is called lysis, which releases the newly formed phages to infect more bacterial cells nearby.

Lysogenic Cycle

• Integration: Upon injecting its genetic material into the bacterial cell, some phages do not immediately hijack its machinery. Instead, they integrate their DNA into the bacterial chromosome, becoming what is known as a prophage.
• Replication with Host: The prophage replicates with the bacterial DNA as the host cell divides and grows, essentially becoming part of the bacterial genome.
• Dormancy: In this integrated form, the phage lies dormant, not causing any harm to the bacterial host. This dormancy can last for many generations of bacterial cell division.
• Activation: Under certain conditions, such as exposure to UV radiation or certain chemicals, the prophage may become active again. When activated, it exits the bacterial chromosome and initiates the lytic cycle, leading to cell lysis and the release of new phages.

Bacteriophage Functions

• Regulating Bacterial Populations : Bacteriophages control the numbers of bacteria by killing them directly or integrating their genetic material into bacterial DNA. This affects the balance of microbial communities.
• Ecological Influence : Bacteriophages impact the composition and activities of microbial communities in nature, influencing processes like nutrient recycling and the decomposition of organic matter.
• Research Tools : Bacteriophages are valuable aids for scientists studying bacteria. They help explore bacterial genetics, gene activity, and how proteins work.
• Controlling Harmful Bacteria : Bacteriophages are used in farming and food production to fight harmful bacteria. They offer an eco-friendly option compared to chemical pesticides and antibiotics.
• Alternative to Antibiotics : Bacteriophages are being studied as substitutes for antibiotics in treating bacterial infections in people and animals. They can target specific bacteria without harming beneficial ones.
• Biotech Uses : Bacteriophages find various applications in biotechnology. They are used for modifying proteins, creating vaccines, and detecting bacterial contamination in food and water.