• Options C and D are correct: Given appropriate surroundings, most plant and animal cells can live, multiply, and even express differentiated properties in a tissue-culture dish. The cells can be watched continuously under the microscope or analyzed biochemically, and the effects of adding or removing specific molecules, such as hormones or growth factors, can be explored. In addition, by mixing two cell types, the interactions between one cell type and another can be studied. Experiments performed on cultured cells are sometimes said to be carried out in vitro (literally, “in glass”) to contrast them with experiments using intact organisms, which are said to be carried out in vivo (literally, “in the living organism”). • Options B is correct: DNA is an organic chemical molecule made from atoms of carbon, hydrogen, nitrogen, oxygen, and phosphorous. Like many other organic molecules that are made inside living cells, DNA can also by synthesized in test tubes using the tools of organic chemistry. In most descriptions of DNA synthesis technology, we hear that DNA