The Bacteriophage T4 exists in a dormant state, devoid of any biological activity. Comprised entirely of dna encased within a protein shell, it remains inactive until it comes into contact with an appropriate host cell.
An extended 'leg' of the Bacteriophage T4 has come into contact with an E. Coli. Chemical sensors at the ends of its 'leg' respond to specific chemicals on the surface of the E. Coli cell. This triggers a response in the virus, and it binds itself to the surface of the cell.
The Bacteriophage T4 contracts its long sheath, forcing its tube through the cell envelope and into the cell.
The Bacteriophage T4 injects its DNA into the host. This is the viruses last action. Devoid of DNA, all that remains is an empty protein husk. The newly introduced DNA redirects the host cells biosynthetic machine, and the production of new viruses begins.