Page 71 - General and Inorganic Chemistry for Advanced Secondary Schools Students Book Form Five and Six

Biology for Secondary Schools

Characteristics of viruses (c) They cannot reproduce on their
(a) They do not have a nucleus, own. They must attack a host cell
cytoplasm, or cell organelle. and use the materials in that cell to
(b) They have a simple structure reproduce. This is called obligate
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consisting of a small piece of nucleic parasitism. The ability of a virus
acid (DNA or RNA) surrounded to reproduce inside the cell and
by a protein coat called a capsid. crystallize in the absence of a living
Some viruses have viral envelopes. host places them between living and
These are membranes enclosing the non-living things. Figure 5.2 shows
capsids. The envelopes are made up how some viruses attacking and
of proteins from the host cell. reproducing inside the host cell.

(a) Virus attach itself to a host cell (b) The virus DNA undergoes
and inject DNA into the cell replication inside the host cell and
forms viral components

(c) The viral components are (d) The host bursts to release new
assembled to form new viruses viruses
which are complete with a
protein coat

Figure 5.2: Viral reproduction

(d) When outside a host cell, they show no symptoms of life (do not grow, feed,
excrete, or respire). They exist in a dormant state.

(e) They are host-specific. This means that a certain type of virus attacks only a
specific host. For example, the rabies virus affects only mammals and HIV
attacks only certain types of white blood cells in human beings.

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