Hooke first observed dead cells, various organelles were discovered, the classical genetic theory was established, and the discovery of the double helix DNA structure marked the beginning of molecular biology.

What is life? Life has 7 characteristics: cells are unit, metabolism, homeostasis, growth, reproduction, response to stimuli, evolution. Some people think virus is not life.

There are 13 levels of organization from smallest to largest. In sequence, they are represent as atoms, molecules, bio-macromolecules, organelles, cells, tissues, organs, organ systems, organisms, population, community, ecosystem and biosphere.