The eukaryotic cells of animals, plants, fungi, and microscopic creatures called protists have many similarities in structure and function. They have the structures common to all cells: a plasma membrane, cytoplasm, and ribosomes. All eukaryotic organisms contain cells that have a nucleus, organelles, and many internal membranes.

Recombinant DNA technology can be controversial. People, including scientists, worry about the ethical, legal, and environmental consequences of altering the DNA code of organisms: Genetically modified organisms (GMOs) that contain genes from a different organism are currently used in agriculture, but some people are concerned about the following potential impacts on wild organisms and on small farms: Genetically modified plants may interbreed with wild species, transferring genes for pesticide resistance to weeds.

DNA is so small that you can barely see it with an electron microscope — and yet, people have figured out how to read it, copy it, cut it into pieces, sort it, and put it back together in new combinations. When DNA from two different sources is combined together, the patchwork DNA molecule is called recombinant DNA.

Scientists first discovered the basic principles of gene regulation by studying how gene expression works in bacteria. Bacteria regulate their gene expression in order to respond to an ever-changing environment. For example, the availability of food and water changes constantly, and bacteria must be able to take advantage of their current situation in order to survive.

Metabolism is a vast interlocking web of chemical reactions. If you put your finger on one of the dots (in the following figure) that represents a particular chemical, you can trace a path from that dot, along lines, to other dots, and so on. The path your finger travels represents a subset of the many chemical reactions that are occurring in the cell.