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Comparative studies of natural groups of organisms

           termed as lytic phase from the word lysis  with the bacterial DNA while in a lytic
           meaning ‘digest’. The host cell lysis and  cycle, the viral DNA and the host DNA
           release of phages mark the culmination     replicate separately within the host cell,
           of the lytic phase (Figure 3.3). The newly   resulting in many copies of the virus being
           produced phages are capable of infecting a   produced very quickly. Also, the lysogenic
           new bacteria and the cycle starts over again   cycle does not lyse the host cell straight
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           as lysogenic cycle with or without a break   away, while in the lytic cycle, the host cell
           or dormant stage.                          is lysed or destroyed.


           The lysogenic and lytic cycles can be      Phages that replicate using both lytic and
           distinguished due to the fact that, in
           lysogenic cycles, the spread of the viral   lysogenic cycles are called temperate phages
           DNA occurs through the usual bacterial     while phages that replicate through the lytic
           reproduction as their DNA has incorporated   cycle are called virulent phages.


                                                 Bacterial
              Phage                              cell DNA



             Bacterial cell                           Phage particle approaches a bacterial cell

                                                      Phage attaches to a bacterial cell wall. DNA
                                                      is injected into bacterial cell and protein
                                                      ‘ghost’ is left outside
           Empty protein coat
                                                      The DNA of phage becomes incorporated
                                                      into bacterial DNA



                                                                The bacterial cells divides to
                                                                produce two cells whose DNA still
                                                                incorporates the DNA of the phage.


                                                       Phage DNA inside the bacterial cell multiplies
                                                       using bacterial nucleic acid and other substances
                                                       absorbed by the bacterium



                                                       Phage DNA induces formation of protein
                                                       coat. Eventually, bacterial cell bursts (lysis),
                                                       releasing about 200 exact copies of the
                                                       original phage



                                Figure 3.3: The life-cycle of a lytic (virulent) phage



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