Page 107 - Biology_F5

Biology for Advanced Level Secondary Schools

3.2.4 Retroviruses devastating epidemics in human societies,
These are special groups of viruses as they are highly infectious and relatively
that contain an enzyme named reverse easy to produce.
transcriptase, which is capable of
synthesising a DNA copy from the virus Advantages of viruses
RNA. This is unlike the usual transcription in (a) Viruses are widely applied in
FOR ONLINE READING ONLY
which RNA is synthesised from a fragment preparation of vaccines against
of DNA. Literally, the term retro means animal diseases. Currently, there is a
‘reverse or going back.’ Hence, retroviruses number of vaccines against deadly
were so named due to their ability to human diseases, such as poliomyelitis,
undergo this reverse transcription process measles, and yellow fever.
in producing DNA copies from RNA. These (b) Viruses such as bacteriophages play
copies can be incorporated randomly into a great role in marine ecology and
chromosomes of the host DNA. It is in this carbon cycling.
group where the Human Immunodeficiency
Virus (HIV), Hepatitis B viruses and Herpes (c) Viruses are important in the field of
viruses belong. The retrovirus DNA is molecular and cell biology; they help
known as provirus. Provirus can be injected in manipulation and investigation
in the host DNA and remains dormant for a of cells, hence providing valuable
information about the aspects of cell
long time prior to expression and formation biology.
of its new RNA. At this stage, the host
does not portray any sign of infection. (d) Some viruses are used in biological
studies. Example, the enzymes reverse
This explains why it sometimes takes a transcriptase discovered in retroviruses
long time for people infected with HIV to are widely used in various aspects of
develop AIDS symptoms. However, when genetic engineering.
provirus is incorporated into the host cell (e) They stimulate the synthesis and
DNA, it may activate host genes to produce release of interferon in the body. This
RNA which will be packaged inside the suppresses the viral replication and
retrovirus particles and delivered together stimulates the activity of cytotoxic
with retroviral RNA to the next infected cells.
cell. The retrovirus binds on T- helper (f) They are used in gene therapy through
lymphocytes destroys them and cripples genetic manipulation of somatic cells
the immune system. of individuals, as well as in production
of transgenic plants and animals.
3.2.5 Economic importance of viruses (g) Certain pathogenic viruses can be used
Though viruses have some disadvantages; as as a pest control agent.
they cause diseases to organisms by being
endoparasites and infectious pathogens, Disadvantages of viruses
yet some have significant benefits in (a) Viruses are known to cause various
medical industry, biotechnology, research, diseases in living organisms, for
agriculture and ecology. Furthermore, example influenza, measles, ebola,
viruses have been used as biological AIDS, chickenpox, and herpes in
warfare (biological weapons) and cause humans, potato mosaic and tobacco

Form Five Student’s Book
100