Figure 1–47 A very simple regulatory circuit—a single gene regulating its own expression by the binding of its protein product to its own regulatory DNA. simple schematic diagrams such as this are found throughout this book. They are often used to summarize what we know, but they leave many questions unanswered. When the protein binds, does it inhibit or stimulate transcription from the gene? how steeply does the transcription rate depend on the protein concentration? how long, on average, does a molecule of the protein remain bound to the dnA? how long does it take to make each molecule of mRnA or protein, and how quickly does each type of molecule get degraded? As explained in chapter 8, mathematical modeling shows that we need quantitative answers to all these and other questions before we can predict the behavior of even this single-gene system. For different parameter values, the system may settle to a unique steady state; or it may behave as a switch, capable of existing in one or another of a set of alternative states; or it may oscillate; or it may show large random fluctuations.
Eukaryotic cells, by definition, keep their DNA in a separate membrane-enclosed compartment, the nucleus. They have, in addition, a cytoskeleton for support and movement, elaborate intracellular compartments for digestion and secretion, the capacity (in many species) to engulf other cells, and a metabolism that depends on the oxidation of organic molecules by mitochondria. These properties suggest that eukaryotes may have originated as predators on other cells. Mitochondria—and, in plants, chloroplasts—contain their own genetic material, and they evidently evolved from bacteria that were taken up into the cytoplasm of ancient cells and survived as symbionts.
Eukaryotic cells typically have 3–30 times as many genes as prokaryotes, and often thousands of times more noncoding DNA. The noncoding DNA allows for great complexity in the regulation of gene expression, as required for the construction of complex multicellular organisms. Many eukaryotes are, however, unicellular—among them the yeast Saccharomyces cerevisiae, which serves as a simple model organism for eukaryotic cell biology, revealing the molecular basis of many fundamental processes that have been strikingly conserved during a billion years of evolution. A small number of other organisms have also been chosen for intensive study: a worm, a fly, a fish, and the mouse serve as “model organisms” for multicellular animals; and a small milkweed serves as a model for plants.
Powerful new technologies such as genome sequencing are producing striking advances in our knowledge of human beings, and they are helping to advance our understanding of human health and disease. But living systems are incredibly complex, and mammalian genomes contain multiple closely related homologs of most genes. This genetic redundancy has allowed diversification and specialization of genes for new purposes, but it also makes biological mechanisms harder to decipher. For this reason, simpler model organisms have played a key part in revealing universal genetic mechanisms of animal development, and research using these systems remains critical for driving scientific and medical advances.
Which statements are true? explain why or why not.
What new approaches might provide a clearer view of the anaerobic archaeon that is thought to have formed the nucleus of the first eukaryotic cell? how did its symbiosis with an aerobic bacterium lead to the mitochondrion? somewhere on earth, are there cells not yet identified that can fill in the details of how eukaryotic cells originated?
dnA sequencing has revealed a rich and previously undiscovered world of microbial cells, the vast majority of which fail to grow in a laboratory. how might these cells be made more accessible for detailed study?
What new model cells or organisms should be developed for scientists to study? Why might a concerted focus on these models speed progress toward understanding a critical aspect of cell function that is poorly understood?
how did the first cell membranes arise?