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A bacteriophage or just phage is a virus that infects bacteria. The T4 phage is an example of a bacteriophage. The head and tail of the bacteriophage are constructed from proteins. The head contains the viral DNA and the tail is a hollow tube used to inject a host cell during infection, though not all phages have a tail. The head can be icosahedral (20 sided) or filamentous, and the tail can be coated with a contratile sheath or in the case of the T phage have a base plate attached[1].

Three basic bacteriophage types can be distinguished from their capsid structures. The first of these have icosahedral capsids, in which individual protein subunits (known as protomers) are arranged into a 20-faced geometric structure that surrounds the nucleic acid. Examples of icosahedral bacteriophages are MS2, which infects Escherichia coli, and PM2 which infects pseudomonas aeruginosa. The second type have filamentous capsids, in which the protomers are arranged in a helix, producing a rod shaped structure. The E.coli bacteriophage M13 is an example. The head-and-tail bacteriophages combine the features of the other two types. Their capsid is made up of an icosahedral head, containing the nucleic acid, and a filamentous tail, which facilitates entry of the nucleic acid into the host cell. They may also have have other structures, such as the 'legs' possessed by the E. coli bacteriophage T4. [2]

There are 4 functions bacteriophages must be able to achieve in order to survive. Firstly, they must protect the viral nucleic acid that is injected into bacteria from their surroundings, as otherwise the molecule may be changed. Secondly, they have to be able to transfer that nucleic acid to a bacterium.  They also have to be able to use infected bacteria as machinery to create a phage-producing system that will result in a large number of progenies. Lastly, those newly made progeny phages need to be able to leave the infected bacteria.[3]

The most common nucleic acid within phages is double-stranded linear DNA, though double-stranded circular DNA, single-stranded linear and circular DNA as well as single- an d double stranded RNA may also be found. The nucleic acid is held seperated from the extracellular environment by a capsid or coat, which is a enclosing protein shell, serving as protection of the nucleic acid from harmful substances.

Life cycles of phages can be distinguished into 2 types, lytic and lysogenic.

Phages in lytic life cycles turn infected bacteria into 'phage factories', which results in the production of many progeny.If a cell is only able of lytic growth it is said to be virulent.

Phages in lysogenic life cycles (observed only in dsDNA phages) don't produce any progeny phages. Instead, the phage DNA is usually added into the chromosome of the infected bacterium. Cells able of these life cylces are called temperate.[4]


The Lytic Life Cycle of a Typical Phage[5]

1) Phage adsorbs to specific receptors on bacterial surface 

The bacterium has other uses for these receptors apart from phage absorption. For example, the receptor phage T6 uses is normally involved in the transport of nucleosides into the cell.

2) Phage injects DNA into cell

Some tailed phage types use injection sequences for this, by contraction of the phage's tail sheath. This results in a core protein tube being driven through the bacterium's cell wall.

3) Infected Bacterium converted to Phage-Producing Cell

The bacterium may lose its ability of DNA translation and/or transcription. This DNA/RNA synthesis stop in the host can be achieved through many different ways that depend on the phage species. 

4) Production of phage nucleic acid and proteins

The infected cell's biosynthetic pathways are redirected by the phage and now copy phage nucleic acid and synthesise phage proteins. The phage achieves this by transcription and translation of phage genes. Phage gene transcription is usually initiated by bacterial RNA polymerase. After the first couple of rounds of transcription, the polymerase is usually modified to recognize phage specific promoters or a phage-specific polymerase is made. The transcription is controlles and  the phage proteins are produced when they are needed. This means different classes of mRNA are synthesised, each at a certain time after the infection. The mRNA can be divided into the two broadest categories, early and late mRNA.

5) Assembling Phage Particles (Morphogenesis)

The two types of proteins needed for this are structural proteins that are present in the phage particle, possibly in modified form, and catalytic proteins which take part in the assemly process, but in the end aren't part of the phage partice. Maturation proteins are a sub-class of catalytic proteins and convert the phage DNA in the cell into a form that can be packaged in the phage particle. 

6) Release of newly synthesised phage particles

So-called lysozymes or endolysins (certain phage proteins) are made towards the end of the infection cycle. They cause Lysis: the cell wall is disrupted, causing the phage particles to be released into the surroundings.


  2. Brown, Terry. (2012). Chapter 13: Inheritance of Genes During Virus Infection Cycles. In: Clayton, J. and Owen, E. Introduction to Genetics: A Molecular Approach. New York, USA: Garland Science, Taylor and Francis Group LLC. p255.
  3. Malacinski, George M. (2003). Chapter 14: Transposons, Plasmids and Bacteriophage. In Essentials of Molecular Biology (Fourth Edition). Sudbury: Jones and Bartlett Publishers. p.324
  4. Malacinski, George M. (2003). Chapter 14: Transposons, Plasmids and Bacteriophage. In Essentials of Molecular Biology (Fourth Edition). Sudbury: Jones and Bartlett Publishers. p.326
  5. Malacinski, George M. (2003). Chapter 14: Transposons, Plasmids and Bacteriophage. In Essentials of Molecular Biology (Fourth Edition). Sudbury: Jones and Bartlett Publishers. p.327
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