The study of biology at the molecular level is known as molecular biology. Other branches of biology and chemistry, particularly genetics and biochemistry, intersect with this field. Understanding the interactions between the many systems of a cell, such as the interrelationships of DNA, RNA, and protein synthesis, and knowing how these interactions are regulated, is the focus of molecular biology. Cell biology encompasses both the vast diversity of single-celled creatures such as bacteria and the numerous specialized cells found in multicellular species such as humans. The basic and fundamental principle of cellular biology is that the cell is the primary unit of life.