Kinds of Factories

There are a number of ways Life organizes its factories or cells. In the simplest, the factory is one big room containing the Instruction Book and all the workers. This kind of living thing is called a prokaryote, the most common example of which is bacteria.

If the Book, the dispatchers, promoters and repressors are in a central office in the factory, the living thing is called a eukaryote. Yeast, amoebae, and many kinds of algae are eukaryotes. The special office in the factory that contains the Instruction book is called the nucleus. Eukaryotes also usually have other rooms in the factory

Not all eukaryotes have just one factory. Those that do are called unicellular. Many kinds of eukaryote consist of many factories built one against each other; these living things are multicellular. People, birds, fish, and trees are all multicellular living things.

Multicellular organisms are distinguished from unicellular organisms not only by the number of cells they have, but by their complexity. Each factory in a multicellular living thing has its own copy of the organism's Instruction Book. All the copies are identical, or nearly so. However, in multicellular living things, the factories are organized into groups that do different things. Groups of factories doing the same thing are called tissues and groups of tissues working together are called organs. Even though the factories in different organs are doing different things, they all have the same copy of the organism's Instruction Book.

Multicellular life forms make different tissues and organs through the intertwined processes of development and differentiation. Development is how an organism changes as it grows; differentiation is how cells become different from each other to produce development. Cells differentiate by producing cascades of activators and repressors that change the kinds of workers they produce. The cascades of activators and repressors often produce new activators and repressors, further changing the kinds of workers the cell is making. This means that even though all the factories in a multicellular organism have a copy of the same Instruction Book, they carry out a different set of Instructions from it, and make a different set of workers. For example, while bone-making cells (osteoblasts) produce workers that gather up and transform the materials needed to make bones, cells in the eye's retina produce workers sensitive to light, allowing us to see. Of course, many of the Instructions being followed are the same for all cells, since each factory in a multicellular organism has to do many of the same things, like gather raw materials, take out the trash, and make new workers.

Just as the Instructions in Life's Book are unlabeled, the steps in the process of differentiation are unlabeled. There are no single Instructions or promoters that make a finger, or even a fingerprint. The way that a cell reads its own copy of the Book to figure out what kind of cell it should be involves an interplay among the Instructions it's currently following, the Instructions cells around it are following, and messages it receives from elsewhere in its organism. Readers interested in learning more about differentiation should see the bibliography.

Stealing from Life's Factories

We didn't mention viruses among the kinds of factories. That's because they aren't factories. A virus is a kind of parasite that needs a factory to make it. A virus consists of a very simple Instruction Book with a few instructions, and a shell made out of workers. It also has a few workers that break into the factory and help take it over.

Many viruses take over a factory by luring away its transcription factors, which they do by using their own strong promoters or activators. Some also make repressors to stop the factory's normal operations. When this kind of virus invades a factory, its workers and the workers made by lured dispatchers race the factory's loyal workers for control over the factory. If the loyal workers win, the virus is destroyed, and its Instruction book and workers are recycled. If the virus's workers win, the factory is recycled into more viruses. Both the common cold and the flu are caused by this kind of virus.

Retro-viruses are considered "retro" because of the way they make workers. Their way is backwards from the way living things work. See Making Workers for details.