Biology Today - May 2018

• Phylogenetic relationships, particularly of lower organisms, are not fully reflected. For example, certain green algae and some
  photosynthetic bacteria get hydrogen from sources other than water but these are assigned to different kingdoms.


• A distinction between unicellular and multicellular organisms is not possible in case of algae. Due to this, unicellular green algae
  such as Chlamydomonas, Volvox, etc., have not been included in the Kingdom Protista.


• Viruses have not been included in this system of classification.


• Archaebacteria differ from other bacteria in structure, composition and physiology.

Three Domains of Life (Six Kingdom Classification)

• The three-domain system was introduced by Carl Woese (1990) that divides cellular life forms into archaea, bacteria and
  eukarya domains.


• It emphasises the separation of prokaryotes into two groups, originally called Eubacteria (now Bacteria) and Archaebacteria (now
  Archaea).


• Thus, the three-domain system divides the monera into two ‘domains’, leaving the remaining eukaryotic kingdoms in the third domain.


• It is actually a six kingdom classification.

Kingdom Archaebacteria Kingdom Eubacteria

Kingdom Protista Kingdom Fungi Kingdom Plantae Kingdom Animalia

Six Kingdom Classification

Domain Archaea

(Contains a single kingdom)

Domain Eukarya

(Contains four kingdoms)

Domain Bacteria

(Contains a single kingdom)

KINGDOM MONERA

• It is a kingdom of prokaryotes, therefore, also known as prokaryota.


• Two major groups of monera include: Archaebacteria and Eubacteria. Eubacteria is further of two types : bacteria and
  cyanobacteria (blue green algae).


• The true nucleus with nuclear membrane is absent in bacterial cell. The nuclear material consisting of naked DNA molecule is
  called as nucleoid.


• A layer of slime over the cell wall is present in bacterial cell. It is usually composed of polysaccharides.


• Bacterial cell wall consists of acetyl glucosamine, acetyl muramic acid and a peptide chain of four or five amino acids. All these
  chemicals together form a polymer called peptidoglycan.


• A thin, elastic and selectively permeable plasma membrane is situated just internal to the cell wall in each bacterial cell.


• In many bacteria the plasma membrane gives rise to infoldings called mesosomes which participate in the separation of
  replicated nucleoid and formation of septa during cell division.


• The ribosomes in bacteria are of 70S type.


• Many bacteria (e.g., E.coli) have accessory rings of DNA called plasmids in addition to bacterial chromosomes. The plasmid
  DNA replicates independently and maintain independent integrity.


• The plasmids which temporarily get associated with nucleoid DNA are known as episomes.


• Bacteria reproduce asexually by binary fission.


• Sexual reproduction or genetic recombination in bacteria takes place by three methods: conjugation, transformation and
  transduction.


• Based on their shape bacteria are grouped under four categories : the spherical coccus, the rod shaped bacillus, the comma
  shaped vibrio and the spiral spirillum.


• Bacteria show both autotrophic and heterotrophic nutrition.


• Autotrophic nutrition is of two types : chemosynthesis and photosynthesis. The bacteria performing these modes of nutrition are
  called chemoautotrophs and photoautotrophs respectively.