Page 105 - Biology_F5

Biology for Advanced Level Secondary Schools

(a) Lysogenic cycle divides. However, the phage is not virulent
When a phage contacts a bacterium, its tail at this stage but dormant, and bacterial cells
fibers attach to receptor sites on the bacteria may exist together with the dormant phage
and it sheds its protein coat outside the DNA for many generations. This dormant
host cell. The viral DNA is incorporated in or latent stage of the phage is also termed as
the bacterial chromosomes (simple circles prophage and the host’s cell as a lysogenic
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of DNA) and is replicated along with it. cell. The viral DNA released may enter a
Therefore, replicated copies of viral DNA virulent pathway called lytic cycle, if it is
will be produced each time the bacterial cell activated (Figure 3.2).

Bacterial
Phage
cell DNA

Phage DNA

Bacterial cell The DNA of phage is injected into
the bacterial cell

The DNA of phage becomes incorporated
into bacterial DNA
The bacterial cells divide to produce two
cells whose DNA still incorporates the
DNA of the phage

Each cell continues to divide many times to give large number of cells which may, under special
circumstances, enter a lytic phase whereby phage replication takes place. Then, cell bursts (lysis)
and the phage particles escape.

Figure 3.2: The life cycle of a lysogenic phage

(b) Lytic cycle cell machinery, replicates more repeatedly,
When a phage particle approaches the host and codes for its new coat proteins. New
cell, it attaches to the host wall and injects viral proteins and new virus particles are
its genetic material. Other lysogenic viruses assembled. As a large number of new
may be activated to enter the lytic cycle; viral particles is produced, the phage also
this occurs when their genetic materials are produces lysozymes containing digestive
already inside the host cell. They undergo enzymes which digest the host cell. This
replication and produce more phages, makes the host cell to burst and release
disrupting the host’s DNA as a result, the phages. Due to the digestion of the host
phage DNA takes control of the bacterial cell caused by the phage, this phase is

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