Bacteriophage T4 Structure & Lytic Cycle: Biomechanical Syringe & Viral DNA Injection

“Bacteriophage T4 operates as a spring-loaded bionanomechanical syringe: upon docking its tail fibers onto Escherichia coli surface receptors, its helical sheath contracts and injects viral DNA under ~50 atm pressure while leaving the protein coat outside.”

Grade 9–11 · Virology & Hershey–Chase

Mechanism

Bacteriophage T4 is a tailed double-stranded DNA virus (*Myoviridae*) that infects *Escherichia coli* and serves as an archetype of bionanomechanical engineering. Its virion comprises a prolate icosahedral protein capsid (115× 85 nm), a collar, a rigid inner tail tube surrounded by a contractile helical sheath, and a hexagonal baseplate bearing six short pins and six long receptor-binding tail fibers.

Equations and conservation laws

Packed inside the capsid by a powerful ATP-driven portal motor is a 169 kbp (56 μm) dsDNA genome whose bending stiffness and electrostatic repulsion generate an internal pressure of P₀ ≈ 50 atm. When tail fibers lock onto host lipopolysaccharide (LPS) receptors, the baseplate undergoes a hexagon-to-star conformational switch, contracting the sheath by 55% to drive the lysozyme-tipped core through the bacterial envelope and inject the DNA.

Measurements and applications

In the classic 1952 Hershey–Chase experiment, phage DNA was labeled with radioactive phosphorus (³²P) and capsid proteins with radioactive sulfur (³⁵S). Only ³²P-labeled DNA entered the host bacteria to direct progeny virion synthesis, proving that DNA is the hereditary material.

Theoretical derivation and analysis

Bacteriophage T4 is a complex virus infecting *Escherichia coli*, comprising an elongated icosahedral protein head (capsid) packed with double-stranded DNA (approx 169 kbp) pressurized to P ≈ 40..60 atm, coupled to a contractile tail apparatus. When the six long tail fibers bind lipopolysaccharide (LPS) receptors on the host surface, the hexagonal baseplate switches conformation, the outer sheath contracts to less than half its length, and the rigid inner tube (tipped with lysozyme domains) punctures the cell envelope, injecting viral DNA into the cytoplasm. In the Hershey–Chase experiment (1952), radioactive ³²P (DNA) pelleted inside the bacteria and appeared in progeny phages, whereas ³⁵S (protein capsid) remained in the supernatant after agitation, establishing DNA as the genetic material.

Methodological calculation guide

In the Hershey–Chase experiment, only ³²P-labeled viral DNA entered the host bacteria while ³⁵S-labeled protein coats remained outside.

Experimental parameters and SI units

Infection stage (1: Adsorption, 2: Puncture, 3: DNA Injection, 4: Assembly): 1 … 4 stage, standard value 3 stage
Initial internal DNA packaging pressure in phage capsid P₀: 20 … 65 atm, standard value 50 atm
Bacterial surface LPS receptor density: 20 … 100 %, standard value 80 %
Lytic burst size (new virions released per host cell): 50 … 250 phages, standard value 150 phages

Everyday example

Bacteriophage T4 looks and acts like a lunar lander equipped with a high-pressure syringe! Landing on six tail fibers, it contracts its sheath and injects its DNA genome pressurized to 50 atm into the bacterium while leaving its empty protein shell outside.

³²P (DNA) xrightarrowinjection Host Cytoplasm, ³⁵S (Protein Coat) longrightarrow Extracellular Ghost

Question

In the landmark 1952 Hershey–Chase blender experiment, bacteriophages were differentially labeled with radioactive phosphorus ³²P (incorporated exclusively into DNA) and radioactive sulfur ³⁵S (incorporated exclusively into capsid proteins). Which isotope entered the infected bacterial cells, and what fundamental principle did this prove?

Selected answer Option B

Formula

Energy balance E = 0.0 J

Fraction of Viral DNA Injected into Bacterial Cytoplasm (%)

0.0 J

Internal Capsid DNA Packaging Pressure P_capsid (atm)

0.0 J

Mechanical Tail Sheath Contraction (%)

0.0 J

Fig. B-18. 3D model of Bacteriophage T4 on a bacterial cell wall: icosahedral head packed with dsDNA, contractile tail sheath, baseplate with 6 long tail fibers, and inner tail tube puncturing the peptidoglycan envelope.