The molecular clock concept

The microbiologist should be able to place any organism in the circumstance of its relationship to other organisms and its evolution from a general ancestor. To be able to do this an evolutionary clock must be identified that reflects small changes in the organism over   time.  The Cellular macromolecules  like  as nucleotides  and  proteins,  have   the potential to  act  as  evolutionary chronometers but  to  be  ideal  they  must meet the following area:

? universally distributed – example for, present in all presumably and known yet to be discovered organisms.

? functionally homologous – example for, the molecule must perform the similar action in all organisms.  The Molecules with different functions could be expected to become too diverse to show any relevant sequence similarity.

?  Possess sequence conservation – example for, an  ideal  chronometer should have  regions of sequence which are highly conserved and therefore expected to change only very slowly over long  periods of time  coupled with  other regions with  moderate variability or hyper- variability to illuminate more present changes.

Many macromolecular chronometers have been proposed, including cytochrome c ATPase, RecA, and 16S or 18S rRNA. Many  of the  protein  chronometers have  failed  to satisfy  the  universality needs particularly when examining the  extremophilic Archaea and Bacteria  which lack  a conventional electron transport chain. To date the most broadly used chronometer is 16S rRNA of Archaea and Bacteria along with its 18S rRNA equivalent in Eukarya.