What is the Difference Between T4 and T7 Bacteriophage

The main difference between T4 and T7 bacteriophage is that the T4 bacteriophage has a short tail, whereas the T7 bacteriophage has a long tail.

Bacteriophages are a type of virus that infects bacteria. They replicate only in bacterial cells. Bacteriophages also infect single-celled prokaryotes, the archaea. There are many different types of bacteriophages depending on the genetic material, morphology, and shape. T4 and T7 bacteriophages are two types of bacteriophages that differ structurally.

Key Areas Covered 

1. What is a Bacteriophage
     – Definition, Features
2. What is a T4 Bacteriophage
     – Definition, Structure, Replication, Features
3. What is a T7 Bacteriophage
     – Definition, Structure, Replication, Features
4. Difference Between T4 and T7 Bacteriophage
     – Comparison of Key Differences  

Key Terms

Bacteriophage, T4 Bacteriophage, T7 Bacteriophage

Difference Between T4 and T7 Bacteriophage - Comparison Summary

What is a Bacteriophage

Bacteriophages are very simple organisms. Their structure includes a core genetic material (nucleic acid) and a cover of protein capsid. The nucleic acid may be either a DNA or an RNA (single-stranded or double-stranded). Moreover, there are three types of bacteriophages: icosahedral head with a tail, icosahedral head without a tail, and filamentous form. These bacteriophages are non-motile and perform Brownian motion to reach their target. They are host specific, meaning they affect a few strains of a species or one single species. Furthermore, there are two replication strategies in bacteriophages: lytic (virulent) or lysogenic (temperate). In the lytic cycle of replication, bacteriophage takes over the machinery of the cell, destroys the cell, and then produces new phages. However, in the lysogenic cycle, the phage nucleic acid incorporates into the bacterial chromosome and replicates without destroying the cell.

What is a T4 Bacteriophage

T4 bacteriophage is a bacteriophage that infects Escherichia coli bacteria. T4 bacteriophages undergo a lytic replication cycle, not a lysogenic replication cycle. In fact, it is one of the 7 Escherichia coli phages that affect the bacteria. The T4 phages have a head, tail, and long tail fibers. The head performs the function of protecting the genome, while the tail performs the function of inserting the genome into the host.

T4 vs T7 Bacteriophage

This bacteriophage has a double-strand DNA. The tail is 925Ao long and covered by a sheath that ends in a hexagonal base plate. Moreover, the six long tails of bacteriophage that are attached to the base plate aid in recognition of the host cells. Additionally, the base plate has 134 subunits of 15 different proteins.

What is a T7 Bacteriophage

T7 bacteriophage has DNA as the nuclear material. The structure of the phage includes an icosahedral head, a long tail, and a base plate. The tail of the DNA is 28.5 nm long. The virus also has a complex structure with an icosahedral head of 55 nm. T7 has a smaller genome size than T4. Moreover, T7 infects only E.coli bacteria. For DNA replication, T7 uses its own polymerases and replicase enzymes. Additionally, the replication is seen within the cell.

Compare T4 and T7 Bacteriophage - What's the difference?

This bacteriophage has a lytic life cycle. It is considered the ideal bacteriophage for experimentation, as a model system for molecular biology studies, and also as a biocontrol agent against bacterial pathogens.

Difference Between T4 and T7 Bacteriophage


T4 is a species of bacteriophage that infect bacteria, while T7 is an obligate lytic bacteriophage that infects Escherichia coli.


T4 bacteriophages have a small tail, whereas T7 bacteriophages have a long tail.

Host Specificity

T4 bacteriophages infect a wide range of bacteria, whereas T7 infects only E.coli.

Genome Size

While T4 bacteriophages have a larger genome, T7 bacteriophages have a smaller genome.

Mode of Replication

T4 uses enzymes of the host cell for the replication process, but T7 uses replicase and polymerase enzymes of their own for replication.

Replication Cycle

Moreover, T4 undergoes a lysogenic cycle, whereas T7 does not undergo a lysogenic cycle.


T4 is used in biotechnology for the production of recombinant proteins, whereas T7 is used as a model system for molecular biology studies and experimentation.


In conclusion, bacteriophages are viruses that infect bacteria. There are different types of bacteriophages, and T4 and T7 are two types of them. The main difference between T4 and T7 is that T4 has a shorter tail, whereas T7 has a longer tail.


1. “Bacteriophage.” Scitable by Nature Education.
2. “Overview of Bacteriophage.” Science Direct.

Image Courtesy:

1. “T4likevirus virion” By ViralZone, SIB Swiss Institute of Bioinformatics: Tevenvirinae – (CC BY 4.0) via Commons Wikimedia
2. “Structure of T7 phage” By Karasunoko – Own work (CC0) via Commons Wikimedia

About the Author: Hasini A

Hasini is a graduate of Applied Science with a strong background in forestry, environmental science, chemistry, and management science. She is an amateur photographer with a keen interest in exploring the wonders of nature and science.

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