The study of structural and functional properties of biological systems, pursued within the context of understanding the roles of the various molecules in living cells and the relationship between them. Historically, molecular biology evolved as a biological discipline with roots derived from biophysics, genetics, and biochemistry. Since its inception, a prime focus of the field has been the molecular basis of genetics, and with the demonstration in the mid-1940s that deoxyribonucleic acid (DNA) is the genetic material, emphasis has been on structure, organization, and regulation of genes. Initially, molecular biologists restricted their studies to bacterial and viral systems, largely because of the systems' genetic and biochemical simplicity. However, a series of conceptual and technological developments occurred rapidly during the late 1970s that permitted molecular biologists to approach a broad spectrum of plant and animal cells with experimental techniques. One of the major factors that rendered higher plant and animal cells amenable to molecular analysis has been the development and applications of genetic engineering. The implementation of recombinant DNA technology allowed the isolation and selective modification of specific genes, thereby reducing both their structural and functional complexity and facilitating the study of gene expression in higher cells.