BIOLOGICAL ORDER 



Bacteriophage 



Life-cycle of a virulent bacteriophage. The bacteriophage 

 particle is more complicated than the poliovirus. It has a so-called 

 "head" and a so-called "tail" (Figure 25). The head is a hexagonal 

 prism, composed of a ball of DNA surrounded by a protein coat. 

 The tail is a long cylindrical structure, which is composed of 

 (a) a contractile outer sheath, (b) an internal tube, and (c) a. tip. 



Figure 25. Schematic Representation of a Bacteriophage Particle. 



The bacteriophage is not motile. But when the tip of its tail 

 comes into contact with the bacterial wall, the bacteriophage becomes 

 attached to the bacterium (Figure 26). An enzyme present at the 

 extremity of the phage tip attacks the bacterial wall. The external 

 sheath of the tail contracts. The internal tube penetrates through 

 the membrane inside the bacterium. The genetic material is injected 

 through the tube into the bacterial cytoplasm. All this happens 

 within a minute. Then the so-called vegetative phase takes place. 

 The synthesis of phage protein is initiated, and 5 minutes after the 



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