Molecular characteristics :-This is the most powerful approaches to study taxonomy by analyzing proteins and nucleic acids. Because these are either direct gene products or the genes themselves, comparisons of proteins and nucleic acids yield considerable information about true relatedness.
- 1. Comparing amino acid sequences:- Comparison of amino acid sequences of proteins from different organisms reveals its taxonomic relations. The most direct approach is to determine the amino acid sequence of proteins with the same function. If the sequences of proteins with the same function are similar, the organisms possessing them are probably closely related . The electrophoretic mobility of proteins is useful in studying relationships at the species and subspecies levels. Antibodies can discriminate between very similar proteins, and immunologic techniques are used to compare proteins from different microorganisms.
2. Nucleic acid composition:- By direct comparison of microbial genomes and based on the G+C content of different organisms (Escherichia coli 48-52 %). And genomic fingerprinting (RFLP, AFLP) reveals its relatedness with others.
3. Nucleic acid hybridization:- It uses the property of complementarities in double stranded DNA. More distantly related organism can be identified based on DNA-RNA hybridization
4. Nucleic acid sequencing :- Techniques are now available to sequence both DNA and RNA. 5S and 16S RNA (prokaryotes), 18S (fungi) analysis of microorganisms can reveal their relatedness because of its functional role is same in all ribosomes and slow structural changes with time.
Microbial evolution and Diversity
It has been estimated that our planet is about 4.6 billion years old. Around 3.5 to 3.8 billion years old fossilized remains of prokaryotic cells have been discovered in sedimentary rocks. Thus earlier prokaryotes were anaerobic and arose shortly after the earth cooled. Cyanobacteria and oxygen-producing photosynthesis probably developed 2.5 to 3.0 billion or more years ago.
It appears likely that modern eukaryotic cells arose from prokaryotes about 1.4 billion years ago.
Two hypotheses for the evolution of eukaryotic cells
1. Organelles arose within prokaryotes from the invagination of the plasma membrane
2. Endosymbiotic hypothesis
Fusion of ancient true bacteria and archaea to form a nucleus. They proposed that the eukaryotic line diverged from the Archaea and then the nucleus formed, possibly from the Golgi apparatus
Mitochondria and chloroplasts develop later from a permanent symbiotic relationship with other bacteria, e.g., cyanelle (cyanobacterium) living inside the protist Cyanophora paradoxa
Cyanobacteria have been considered the most likely ancestors of chloroplasts. More recently Prochloron has become the favorite candidate. The existence of this bacterium suggests that chloroplasts arose from a common ancestor of prochlorophytes and cyanobacteria. Mitochondria arose from an endosymbiotic relationship between the free-living primitive eukaryotic and bacteria with aerobic respiration (possibly an ancestor of three modern groups: Agrobacterium, Rhizobium, and Rickettsia).
Divisions of Life
Kingdom systems of classification
- Five-kingdom system (Whittaker, 1960s) - based upon cell type, organization, and the means of nutrient acquisition (Monera, Protista, Fungi, Plantae, Animalia)
- Six-kingdom system - differs from five-kingdom system by dividing prokaryotes into bacteria and archaea (Bacteria, Archaea, Protista, Fungi, Plantae, Animalia)
- Eight-kingdom system (Cavalier-Smith) - further division of the protists using rRNA data and grouping organisms into two empires (Eucaryota and Bacteria) containing a total of eight kingdoms [(Bacteria, Archaea), (Archezoa, Protista, Plantae, Chromista, Fungi, Animalia)]