Basic Structure of a Bacteriophage

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What is a Bacteriophage?

A bacteriophage is a virus that preys on bacteria. It infects bacteria by inserting its genetic material into the host cell. To do this, the bacteriophage attaches itself to the surface of the host with tail fibers. Afterward, the genetic material uses the bacteria’s replication apparatus to duplicate itself. To do this, it utilizes the ribosomes in the bacteria to translate the bacteriophage’s genetic material and generate viral proteins. It also utilizes the host cell’s amino acids, nucleotides, and lipids. The metabolic functions of the bacteriophage are dependent on the host bacteria.


There are two main components of a bacteriophage: nucleic acid and protein. The bacteriophage carries ribonucleic acid (RNA) or deoxyribonucleic acid (DNA). The bases that make up the genetic material of the bacteriophage are slightly different than those that make up the genetic material of the host bacteria. This prevents the host’s replication enzymes from cleaving the genetic material of the bacteriophage.

The proteins of the bacteriophage are responsible for binding to the host bacteria and transferring the genetic material. The proteins also protect the genetic material from enzymes.


A bacteriophage is a very small organism, ranging in length from 24 to 200 nanometers. One of the largest groups of bacteriophages is the T4 bacteriophage. It is 200 nanometers in length and 80 to 100 nanometers wide.

The size of the nucleic acid varies between different bacteriophages. Small DNA or RNA strands may contain enough genetic information to produce three to five products. Larger strands may contain enough genetic information for more than 100 gene products.

Head or Capsid Structure

The genetic material of the bacteriophage is stored in the head or capsid region. It is part of the overall structure of a bacteriophage. The capsid is usually in a geometric shape, and it consists of protein. Most have 20 sides and are called icosahedral. Other bacteriophage capsids are not as compact and are referred to as filamentous.

The capsid is composed of one or two different proteins. The main function of the capsid is to protect the genetic material from the environment. Specifically, the capsid prevents enzymes from breaking down the genetic material.

Tail Structure

From the capsid, a long string of proteins make up the tail structure of a bacteriophage. The tail is a hollow tube that serves as a passageway for the genetic material to pass from the capsid to the host bacteria. The entire tail is covered with a sheath, and some bacteriophages feature a base plate and fibers at the bottom of the tail. The base plate and fibers are used to securely attach the bacteriophage to the bacteria sheath, which contracts during an infection.


1. Mayer, Gene Dr. “Bacteriology - Ch. 7 Bacteriophage.” Microbiology and Immunology On-line Textbook - University of South Carolina School of Medicine.

2. Davidson, Michael W. “Virus Structure.” Molecular Expressions Cell Biology - Florida State University.