In earlier five topics, preliminary and basic information on principles of genetics was presented in its historical perspective. Further ideas and principles of genetics will be presented in further sections in the background built in these five topics. However, before this additional information is presented, one should understand about the physical entities present in the cell, which axg responsible for the transmission and expression of hereditary characters. This aspect of genetics will be discussed in this and the next three sections. However, the discussion on these entities at the molecular level involving chemistry of the molecules responsible for heredity.