The differences result from the way in which the cells make selective use of their genetic instructions according to the cues they get from their surroundings in the developing embryo. The DNA is not just a shopping list specifying the molecules that every cell must have, and the cell is not an assembly of all the items on the list. Rather, the cell behaves as a multipurpose machine, with sensors to receive environmental signals and with highly developed abilities to call different sets of genes into action according to the sequences of signals to which the cell has been exposed. The genome in each cell is big enough to accommodate the
Figure 1–33 Cell types can vary enormously in size and shape. An animal nerve cell is compared here with a neutrophil, a type of white blood cell. Both are drawn to scale.
information that specifies an entire multicellular organism, but in any individual cell only part of that information is used.
A large number of genes in the eukaryotic genome code for proteins that regulate the activities of other genes. Most of these transcription regulators act by binding, directly or indirectly, to the regulatory DNA adjacent to the genes that are to be controlled, or by interfering with the abilities of other proteins to do so. The expanded genome of eukaryotes therefore not only specifies the hardware of the cell, but also stores the software that controls how that hardware is used (Figure 1–34).
Cells do not just passively receive signals; rather, they actively exchange signals with their neighbors. Thus, in a developing multicellular organism, the same control system governs each cell, but with different consequences depending on the messages exchanged. The outcome, astonishingly, is a precisely patterned array of cells in different states, each displaying a character appropriate to its position in the multicellular structure.
many eukaryotes live as solitary cells
Many species of eukaryotic cells lead a solitary life—some as hunters (the protozoa), some as photosynthesizers (the unicellular algae), some as scavengers (the unicellular fungi, or yeasts). Figure 1–35 conveys something of the astonishing variety of the single-celled eukaryotes. The anatomy of protozoa, especially, is often elaborate and includes such structures as sensory bristles, photoreceptors, sinuously beating cilia, leglike appendages, mouth parts, stinging darts, and musclelike contractile bundles. Although they are single cells, protozoa can be as intricate, as versatile, and as complex in their behavior as many multicellular organisms (see Figure 1–27, Movie 1.4, and Movie 1.5).
In terms of their ancestry and DNA sequences, the unicellular eukaryotes are far more diverse than the multicellular animals, plants, and fungi, which arose as three comparatively late branches of the eukaryotic pedigree (see Figure 1–17). As with prokaryotes, humans have tended to neglect them because they are microscopic. Only now, with the help of genome analysis, are we beginning to understand their positions in the tree of life, and to put into context the glimpses these strange creatures can offer us of our distant evolutionary past.
A yeast serves as a minimal model eukaryote
The molecular and genetic complexity of eukaryotes is daunting. Even more than for prokaryotes, biologists need to concentrate their limited resources on a few selected model organisms to unravel this complexity.
Figure 1–34 Genetic control of the program of multicellular development.
The role of a regulatory gene is demonstrated in the snapdragon Antirrhinum. In this example, a mutation in a single gene coding for a regulatory protein causes leafy shoots to develop in place of flowers: because a regulatory protein has been changed, the cells adopt characters that would be appropriate to a different location in the normal plant. The mutant is on the left, the normal plant on the right. (courtesy of enrico coen and Rosemary carpenter.)