The eons are defined as the longest portion of the geological time scale, which records all the major distinct events that have taken place in the past. It's unit time is a billion years.
This eon is divided into 3 eras, namely-
(1) Paleozoic Era
(2) Mesozoic Era
(3) Cenozoic era
These eras are distinctly divided on the basis of the first appearance and disappearance of life forms on earth, which has been studied and determined through the study of the fossil assemblages that have been obtained. In addition to this, they are also classified based on the climatic variation that has favored the growth and expansion of different life forms that have occurred on earth in the geological past.
Phanerozoic Eon is divided into three eras on the basis of prevailing climatic conditions on earth and dominant life forms present during the time.
Explanation:
The Phanerozoic Eon is divided into three eras namely the Paleozoic, the Mesozoic, and the Cenozoic era. The basis for classification of the Phanerozoic eon into three eras are the prevailing climatic conditions on earth and dominant life forms present during the time. Changes in climatic conditions of earth altered the species composition and dominant life forms present in each era.
When directed mutagenesis which is also called reverse genetics is used, only a single mutation per genome is introduced. In contrast, the number of mutations per genome introduced by random mutagenesis is usually difficult to control. It is likely that the strain you obtained with a mutation in gene X after random mutagenesis contained an additional mutation or perhaps multiple mutations which ultimately was/were responsible for loss of virulence in your favorite pathogen.
