Page 101 - Biology_F5

Biology for Advanced Level Secondary Schools

saprophytes, such as mushrooms, yeast, and all eukaryotic organisms, thus bringing
Rhizopus was still contradicting, because together the kingdoms Protoctista, Fungi,
they were erratically classified under Plantae, and Animalia.
kingdom Plantae or Protista. This prompted
Robert Whittaker to propose kingdom 3.2 Viruses
Fungi in 1969 under which all multicellular
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saprophytes were placed, making five Task 3.1
kingdoms. However, molecular studies by Use internet sources and the library to
Carl Woese in the 1970s revealed cellular search for information on viruses and
structural differences among prokaryotes make a summary.
based on the membrane structure, ribosomal The first viruses were isolated in 1852
RNA, lipid compounds, among others. This as infectious extract from tobacco plants
necessitated the splitting of the kingdom suffering from tobacco mosaic. In 1898,
Monera (which is also called Prokaryota) Martinus Beijerink named the isolated
into Archaebacteria and Eubacteria. In extract virus a Latin word, meaning
1982, about a decade later, Margulis and 'toxic' or 'infectious'. Viruses are therefore
Schwartz proposed a classification system infectious particles, thought to have
which adopted the five kingdoms, namely genomes detached out of eukaryotic or
the Prokaryotae and four Eukaryote prokaryotic cells. Thus, a virus can be
kingdoms. However, that was not the end, defined as a fragment of nucleic acid
because recent molecular work has further (single or double stranded DNA or RNA),
revealed phylogenetic relatedness among surrounded by a protein coat and capable
protists, which lead to yet another kingdom, of replicating once it is inside a living cell.
Chromista. Generally, viruses are too minute; they are
smaller than bacteria and cannot be seen
In 2015, Cavalier Smith and others introduced by a light microscope, but they can be seen
biological taxonomy of eight kingdoms under the electron microscope. The latter
namely; Eubacteria, Archaebacteria has simplified an understanding on the
(Archae), Archezoa, Protozoa, Chromista, characteristics of viruses, including their
Fungi, Plantae, and Animalia. Biological shapes and ways in which they interact
taxonomy of 8 kingdoms has introduced a with their hosts.
taxonomic rank called domain above the Characteristics of viruses
kingdom. Three domains of life, namely (a) They are the smallest organisms with
domain Archaea, Bacteria and Eukarya are size ranging from 17 nm to 300 nm. On
thus recognised. Domain Archaea includes average, they are 50 times smaller than
Archaea bacteria which are the most bacteria, which range in size between
primitive bacteria with cell wall containing 0.1 µm and 10 µm.
pseudomurein but lacking a peptidoglycan (b) They are simple in structure with a
cell wall. Domain Bacteria consists of all fragment of nucleic acid, which is either
true bacteria, while Eukarya encompasses DNA or RNA, enclosed in a protein

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