Lytic Cycle

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Step 1: Absorption
Attachment of absorption of tail fibres of the phages on to a specific receptor site on the bacterial cell wall.

Step 2: Injection
Injection of viral genome into the host through the hollow tubes of the tail.

Step 3: Protein synthesis
Inside the host, the viral genome directs the synthesis of viral proteins using the machinery of the host. Viral genome generally encodes for some enzymes and coat proteins.

Step 4: Viral genome synthesis
Viral genome replicates inside the host making several copies of it self.

Step 5 & 6: Packaging and release
The viral genome gets packed inside the protein coat.

These intact mature infectious particles are called virions. The crowding of virions inside the host ultimately cause cell lysis and liberation of mature viral particles – about 200 mature phages are liberated.

Mode of Multiplication of Bacteriophages

Two modes of multiplication cycle in Bacteriophages namely lytic cycle and lysogenic cycle.

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Lytic cycle :
Lytic cycle or lytic phages called as Virulent phages. Multiples inside the host bacterium and new viral particles comes out by lysing  or by rupturing the host bacterial cell wall.
Example: T phages, T2,  T4, T6 etc…

Lysogenic cycle :
Lysogenic cycle or lysogenic phages called as temperate phages. It does not undergo multiplication or induce lysis, here the viral DNA get integrated into the bacterial DNA without causing lysis.
Example: Lambda phages.

The life cycle of a bacteriophage

Phage must carry out specific set of reactions in order to make more of themselves. 

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First, the phage must be able to recognise a bacterium that can multiply in by binding to the bacterial cell surface.
Next, the phage must inject it’s genome and the genome must be protected from the bacterial nucleases in the cytoplasm.

The phage genome must be replicated transcribed and translated so that a large number of genomes, capsid proteins, and tail proteins, if present, are produced at the same time. Complete phage particles are then assembled and the phage must get back out of bacterium.

Classification of Bacteriophages

Broadly pages can be classified as either virulent or temperate.

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The process of phage infecting a bacterium and producing progeny is referred to as lytic Infection. A virulent phage subverts the cellular apparatus of its bacteria host for multiplification, and release of progeny virions. Eg. T4.

Temperate phages have alternative replication cycles: lytic infection or a lysogenic pathways.

Lysogenic pathway is a pathway in which phage remains latent in the host. Phages are capable of maintaining their chromosome in a stable, silent state within the bacteria. When the bacterium contains a silent phage chromosome, it is referred to as a Lysogen. The incorporated phage genome is referred to as a Prophage.

Bacteriophages – Overview

Bacteriophages are also known as bacterial Viruses or phages .They are the viruses that Infect bacteria. phages are first discovered independently by Frederick w.Twort in great britain C 1915) and Felix d’ Herelle In France (1917). Like all viruses, phages are Viruses that consist of a core of genetic material surrounded by a protein coat Or capsid.

The phage genomemay be DNA or RNA, single or doublestranded, circular or linear, generally present as a single copy. Morphologically varies from simple, Icosahedral and filamentous phage to more complex tailed phages with 9n Icosahedral head.

They are obligate intracellular parasites that multiply inside bacteria by making use of some or all of the host biosynthetic machinery.

Introduction to virology

Virology is the study of viruses – submicroscopic, parasitic particles of genetic material contained in a protein coat and virus-like agents. It focuses on the following aspects of viruses: their structure, classification and evolution, their ways to infect and exploit host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy. Virology is considered to be a subfield of microbiology or of medicine.

Virus contribute significantly to the global burden of infectious disease.
We experience countless infections throughout their lives, with particularly high frequency in early childhood. While most of these are mild, viruses may cause severe disease in susceptible individuals, such as the mal-nourished, immuno-compromised, the very old and the very young. Recent years have also seen the emergence of new viral diseases such as HIV, SARS and “swine flu”.

Introduction to virology

Virology is the study of viruses – submicroscopic, parasitic particles of genetic material contained in a protein coat and virus-like agents. It focuses on the following aspects of viruses: their structure, classification and evolution, their ways to infect and exploit host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy. Virology is considered to be a subfield of microbiology or of medicine.

Virus contribute significantly to the global burden of infectious disease.
We experience countless infections throughout their lives, with particularly high frequency in early childhood. While most of these are mild, viruses may cause severe disease in susceptible individuals, such as the mal-nourished, immuno-compromised, the very old and the very young. Recent years have also seen the emergence of new viral diseases such as HIV, SARS and “swine flu”.