Page 103 - Biology_F5

Biology for Advanced Level Secondary Schools

(b) They do not have any enzyme system rather than RNA. The DNA may be either
such as respiratory enzyme, hence double or single stranded. These viruses
they lack energy releasing processes. infect animals and plants. Examples include
(c) While outside the host cell, viruses herpes virus, smallpox virus, chickenpox
are inert, but retain their ability to virus; which infect animals, and tomato
replicate, if they invade a host cell. yellow leaf curl virus; which infect plants.
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Viruses are therefore crystalline in the Classification of viruses according to the
absence of a host cell. When they exist host range includes plant viruses which
as individual particles outside the host infect plants only. Examples include; Tomato
cell, they are called virions. leaf curl virus that infect tomatoes and
(d) They are filterable; therefore, they can Tobacco mosaic virus which infect plants
pass through a bacterial proof filter like tobacco and other plants including
paper. tomato, potato, orchid and blackcurrant,
(e) They are resistant to very high and cauliflower mosaic virus which infect
temperatures due to lack of enzymes. cauliflower plant. In this classification
Normally, at high temperatures, also there are animal viruses which infect
enzymes are denatured and thus render animals only, examples include influenza
them useless. virus and Herpes simplex virus which infect
Viruses can be classified based on a human. Also, there are bacteria viruses that
number of factors including; phenotypic infect bacteria, for example; bacteriophage,
characteristics, such as type of nucleic acid, such as lambda (l) phage (a lysogenic virus,
host organisms, type of disease they cause, which is less virulent whereby the host and
mode of replication, and morphology. In the a phage can exist together for many years),
classification according to type of nucleic enterobacteria T2, and T4 phages (lytic
acid, there are RNA and DNA viruses. viruses, which infect the bacteria known
RNA virus is a virus that has RNA as its as Escherichia coli). Likewise, there are
genetic material, normally single-stranded insect viruses, which affect insects only, and
RNA, but some viruses may form double these include; Baculovirus, Sacbrood virus,
helix intra-stranded complimentary base Entomopox virus and Granulosis virus.
pair of RNA. Viruses infect both plants Based on the type of disease they cause,
and animals, examples are Tobacco Mosaic viruses can be classified as; measles virus;
Virus (TMV) which infects plants, influenza cause measles, polio virus; cause polio, and
virus, hepatitis virus, and polio virus, which rabies virus; cause rabies.
infect humans. A type of RNA virus known
as Retrovirus has a peculiar character of In the classification of viruses according to
inserting a copy of its RNA into DNA of the morphology there are different categories
host cell by reverse transcription process and based on the symmetry of capsid, which
changing the genetic material of that cell, an include helical and icosahedral viruses.
example is Human Immunodeficiency Virus The helical morphology consists of helical
(HIV). On the other hand, DNA virus is a array of capsid proteins wrapped around a
type of virus whose genetic material is DNA helical filament of genetic material, example

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