Page 102 - Biology_F5

Comparative studies of natural groups of organisms

or lipoprotein layer. Most animal and microscopic nucleocapsids (they are smaller
bacterial viruses have DNA, but other than a molecule), lack cellular organisation
animal and plant viruses have RNA. (acellular) and lack certainty in evolutionary
(c) Viruses lack cellular structural history despite having fragments of genetic
organisation, hence they are acellular. materials, such as DNA or RNA. Thus,
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(d) Viruses can reproduce in a living cell they do not fit precisely into the established
only, that is, they can reproduce inside biological classification system, as they
the cell of a living host. possess both living and non-living
characteristics.
(e) Most viruses are infectious; they can
cause diseases to their hosts. They show Living characteristics of viruses
a high degree of host cell specificity, as While inside the host cell, viruses possess
they can recognise and infect specific a number of characteristics similar to those
types of host cells. For example, a of living organisms as follows:
virus causing disease in plants will (a) They possess nucleic acid; either DNA
not cause disease in animals and vice- or RNA, which is a genetic material
versa. However, the same viruses can helping the virus to produce identical
cause disease to different species in the copies of itself.
same group of organisms. A virus called (b) They penetrate through a host cell by
H5-N1 that causes a fatal birdflu was the help of enzymes derived from the
recently discovered to cause infection protein coat on the cell surface of the
and death to humans. host cell.

(f) In the absence of a host, viruses can (c) They are able to multiply or reproduce
crystallise outside a living medium inside the host cell.
and assume features of non-living (d) They are able to undergo mutation in
organisms. response to host internal changes, and
(g) The ability of a virus to reproduce inside they respond rapidly.
the cell and crystallise in the absence of (e) They possess capsid, which is a protein
a living host places them between living coat protecting or enclosing nucleic
and non-living organisms. acids.

(h) Viruses are obligate endoparasites (f) They show specificity to hosts and
because they can live and replicate have the ability to infect the host
inside their host cell only. cell and take control of its metabolic
activity.
3.2.1 Classification of viruses Non-living characteristics of viruses
Generally, classification of viruses has been As non-living organisms, viruses have the
challenging due to their characteristics following characteristics;
while inside or outside the host cell. Several (a) They do not replicate on their own and
other attributes of viruses intensify the lack cellular structures hence unable
difficulties experienced by taxonomists to carry out any life processes when
in classifying them because they are very outside the host cell.

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