2. Systematics of Living Organisms- part 02 - Three domains of life, Chemotaxonomy and DNA barcoding

 

2. Systematics of Living Organisms- part 02 - Three domains of life, Chemotaxonomy and DNA barcoding


Three domains of life :
  • It is believed that the life originated on earth in its very simple form. 
  • Constant struggle of the early living beings gave rise to more and more perfect forms of life. 
  • This struggle and progress is evolution which led to formation of diverse life forms. 
  • Carl Woese in 1990 proposed three domains of life to classify life forms. They are -
  1. Archaea
  2. Bacteria and 
  3. Eukarya. 
  • Domain is an unit larger than Kingdom in the system of classification.
  • Bacteria and Archaea both have prokaryotic cellswhere as Eukarya have eukaryotic cell. 
  • All the three domains have very unique ribosomal RNA (rRNA). 
  • Archaea are known for their survival in very extreme conditions like high tempreature, salinity, acidic conditions, etc. 
  • Bacteria, though are prokaryotes differ from Archaea in structure of cell wall.

Chemotaxonomy :
  • It is method of biological classification based on similarities and differences in structure of certain compounds present among the organisms being classified. 
  • In short, it is the classification based on chemical constituents of organisms. 
  • e.g. Archaea cell wall is without peptidoglycan and that of Prokarya is with peptidoglycan. 
  • Among Eukarya, fungi havechitinous cell wall while plants have cellulosic cell wall.

Numerical taxonomy :
  • The system is based on quantification of characters and develops an algorithm for classification. 
  • The basic aim of this taxonomy was to create a taxonomy using numeric algorithms like cluster analysis rather than using subjective evaluation of their propertise.
  • This system was first proposed by Sokel and Sneath in 1963.

Cladogram :

  • It is a typical branching pattern.  
  • It represents a hypothetical relationship denoting a comparison of organisms and their common ancestors.

Phylogeny :
  • It is evolutionary relationship of organism. 
  • It is an important tool in classification as it takes into account not merely the morphological status but also the relationship of one group of organism with other groups of life. 
  • The system helps to understand the evolution and also focuses on the similarities of their metabolic functioning. 
  • Woese’s three domain concept as well as Whittakar’s five kingdom system are very good examples of phylogenetic relationship.

DNA barcoding :

  • DNA barcoding, is a new method for the identification of any species based on its DNA sequence from a tiny tissue sample of the organism under study.
  • It helps to study newly identified species as well as understanding ecological and evolutionary relationships between living beings. 
  • The process of DNA barcoding includes two basic steps: 
  1. Collecting DNA barcode data of known species and 
  2. Matching thebarcode sequence of the unknown sampleagainst the barcode library for identification.
Application of DNA Barcoding :
  1. protection of endangered species
  2. preservation of natural resources
  3. pest control in agriculture
  4. identifying disease vectors
  5. authentication of natural health products and 
  6. identification of medicinal plants.

Taxonomic Categories :

  • Classification is not a single step process but involves hierarchy of steps in which each step represents a rank or category. 
  • Since the category is a part of overall taxonomic arrangement it is called taxonomic category and all categories together constitute the taxonomic hierarchy. 
  1. Kingdom
  2. Division
  3. Class
  4. Order
  5. Family
  6. Genus
  7. Species are the categories in hirarchial sequence. 
  • These are compulsory categories. Besides, there are some facultative categories like - 
  1. Sub-order
  2. Sub-family, etc. to be used as per need.

Taxonomic Hierarchy :
Taxon : 

  • A taxon is the taxonomic group of any rank in the system of classification (H.J. Lam 1948) 
  • e.g. in plant kingdom each one of the following such as Angiosperms,Dicotyledonae, Polypetalae, Malvaceae represents a taxonomic group i.e. a taxon.

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