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Comparative studies of natural groups of organisms

TMV, whereas in icosahedral the protein enable viruses to crystallise and assume
subunits are arranged in a form of hollow, a non-living matter. The capsid, together
quasi spherical structure, example; Herpes with the genetic material (DNA or RNA),
simplex virus. constitute a nucleocapsid. Some viruses
may have additional envelope of lipoprotein
3.2.2 General structure of viruses (lipid rich protein) around the capsid. The
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Viruses are composed of fragments of the lipoprotein layer is usually derived from the
genetic material (DNA or RNA), which cell membrane of the host cell. The viruses
forms a nucleic acid core. The DNA with this layer are called enveloped viruses,
contains a few genes and can either be examples are influenza and herpes viruses.
single or double stranded. The nucleic acid Those with no envelope are referred to as
core is enclosed by a protein coat called naked viruses, for instance, bacteriophage or
capsid, as in bacteriophage. The capsid is simply a phage; viruses that attack bacteria
made up of small identical protein subunits (Figure 3.1 a, b and c).
called capsomeres which are structures that
Capsid Envelope protein
Nucleic acid
(DNA) Nucleic acid
Viral envelope
Sheath Reverse
transcriptase
Base RNA Protein
Capsule protein
Matrix protein

(a) (b) (c)

Figure 3.1: Structure of (a) Bacteriophage (b) the Human Immunodeficiency Virus (HIV) and (c)
Tobacco mosaic virus (TMV)
3.2.3 Viral replication show uniqueness at a certain stage of their
Viral replication involves formation of replication due to their ability to convert
viruses during the infection process, and their RNA back into a DNA copy. Some
they replicate only when in living cells; viruses called lytic phages kill host cells
that is, the viruses must first get into immediately after they enter. On the other
the host cell before viral replication can hand, other viruses, such as lambda phages
occur. Replication among viruses differs may remain dormant for a long time after
significantly and depends on the type of inserting their DNA into the host DNA,
genes in them. Moreover, most DNA viruses but they may eventually be activated to
gather in the nucleus while most RNA complete their life cycle. These types of
viruses develop merely in the cytoplasm. viruses are described as lysogenic phage and
the dormant stage is called prophage. The
The life cycle of a bacteriophage viral replication therefore, occurs in two
In principle, viral life cycle is the same in major stages or phases, namely; lysogenic
all bacteriophages. Retroviruses, such as and lytic phases or cycles as summarised
Human Immunodeficiency Viruses (HIV) in Figure 3.2, and Figure 3.3 respectively.

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