Population size refers to the total number of individuals in a given population.
Population density refers to the total number of individuals that are living in a specific area.
Explanation:
Population size means how much the number of organisms are present in an population of different species while population density means how many organisms are living in a particular area.
Gene regulation as mentioned already, is the process of turning genes on and off. In multicellular organisms, the different cells have the same DNA in them but through gene regulation, different sets of genes are expressed. The genes that are expressed determine the function and appearance of the cell.