B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 18
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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 itemson the list. Rather, the cell behaves as a multipurpose machine, with sensors toreceive environmental signals and with highly developed abilities to call differentsets of genes into action according to the sequences of signals to which the cellhas been exposed. The genome in each cell is big enough to accommodate theneuronneutrophil25 µmFigure 1–33 Cell types can varyenormously in size and shape. Ananimal nerve cell is compared here with aneutrophil, a type of white blood cell. Bothare drawn to scale.MBoC6 n1.500/1.3330Chapter 1: Cells and GenomesFigure 1–34 Genetic control of theprogram of multicellular development.The role of a regulatory gene isdemonstrated in the snapdragonAntirrhinum.
In this example, a mutationin a single gene coding for a regulatoryprotein causes leafy shoots to developin place of flowers: because a regulatoryprotein has been changed, the cells adoptcharacters that would be appropriate to adifferent location in the normal plant. Themutant is on the left, the normal plant onthe right. (Courtesy of Enrico Coen andRosemary Carpenter.)information that specifies an entire multicellular organism, but in any individualcell only part of that information is used.A large number of genes inthe eukaryoticgenome code for proteins that regMBoC6m1.40/1.34ulate the activities of other genes. Most of these transcription regulators act bybinding, directly or indirectly, to the regulatory DNA adjacent to the genes that areto be controlled, or by interfering with the abilities of other proteins to do so.
Theexpanded genome of eukaryotes therefore not only specifies the hardware of thecell, but also stores the software that controls how that hardware is used (Figure1–34).Cells do not just passively receive signals; rather, they actively exchange signals with their neighbors. Thus, in a developing multicellular organism, the samecontrol system governs each cell, but with different consequences depending onthe messages exchanged. The outcome, astonishingly, is a precisely patternedarray of cells in different states, each displaying a character appropriate to its position in the multicellular structure.Many Eukaryotes Live as Solitary CellsMany 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, andmusclelike contractile bundles. Although they are single cells, protozoa can beas intricate, as versatile, and as complex in their behavior as many multicellularorganisms (see Figure 1–27, Movie 1.4, and Movie 1.5).In terms of their ancestry and DNA sequences, the unicellular eukaryotes arefar more diverse than the multicellular animals, plants, and fungi, which arose asthree comparatively late branches of the eukaryotic pedigree (see Figure 1–17).
Aswith 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 thesestrange creatures can offer us of our distant evolutionary past.A Yeast Serves as a Minimal Model EukaryoteThe molecular and genetic complexity of eukaryotes is daunting.
Even more thanfor prokaryotes, biologists need to concentrate their limited resources on a fewselected model organisms to unravel this complexity.GENETIC INFORMATION IN EUKARYOTES31(D)(A)(C)(B)(E)(F)To analyze the internal workings of the eukaryotic cell without the additionalproblems of multicellular development, it makes sense to use a species that isunicellular and as simple as possible.
The popular choice for this role of minimalMBoC6 cerevisiaem1.41/1.35(Figure 1–36)—themodel eukaryote has been the yeast Saccharomycessame species that is used by brewers of beer and bakers of bread.S. cerevisiae is a small, single-celled member of the kingdom of fungi and thus,according to modern views, is at least as closely related to animals as it is to plants.It is robust and easy to grow in a simple nutrient medium. Like other fungi, it has atough cell wall, is relatively immobile, and possesses mitochondria but not chloroplasts. When nutrients are plentiful, it grows and divides almost as rapidly as abacterium. It can reproduce either vegetatively (that is, by simple cell division), orsexually: two yeast cells that are haploid (possessing a single copy of the genome)can fuse to create a cell that is diploid (containing a double genome); and the diploid cell can undergo meiosis (a reduction division) to produce cells that are onceagain haploid (Figure 1–37).
In contrast with higher plants and animals, the yeastcan divide indefinitely in either the haploid or the diploid state, and the processleading from one state to the other can be induced at will by changing the growthconditions.In addition to these features, the yeast has a further property that makes it aconvenient organism for genetic studies: its genome, by eukaryotic standards,is exceptionally small. Nevertheless, it suffices for all the basic tasks that everyeukaryotic cell must perform.
Mutants are available for essentially every gene,nucleus10 µm(B)Figure 1–35 An assortment of protozoa:a small sample of an extremely diverseclass of organisms. The drawings aredone to different scales, but in each casethe scale bar represents 10 μm. Theorganisms in (A), (C), and (G) are ciliates;(B) is a heliozoan; (D) is an amoeba;(E) is a dinoflagellate; and (F) is a euglenoid.(From M.A. Sleigh, Biology of Protozoa.Cambridge, UK: Cambridge UniversityPress, 1973.)cell wallmitochondrion(A)(G)2 µmFigure 1–36 The yeast Saccharomycescerevisiae.
(A) A scanning electronmicrograph of a cluster of the cells. Thisspecies is also known as budding yeast;it proliferates by forming a protrusion orbud that enlarges and then separatesfrom the rest of the original cell. Many cellswith buds are visible in this micrograph.(B) A transmission electron micrograph ofa cross section of a yeast cell, showingits nucleus, mitochondrion, and thick cellwall. (A, courtesy of Ira Herskowitz and EricSchabatach.)32Chapter 1: Cells and GenomesFigure 1–37 The reproductive cycles of the yeast S. cerevisiae.Depending on environmental conditions and on details of the genotype,cells of this species can exist in either a diploid (2n) state, with a doublechromosome set, or a haploid (n) state, with a single chromosome set.
Thediploid form can either proliferate by ordinary cell-division cycles or undergomeiosis to produce haploid cells. The haploid form can either proliferate byordinary cell-division cycles or undergo sexual fusion with another haploidcell to become diploid.
Meiosis is triggered by starvation and gives rise tospores—haploid cells in a dormant state, resistant to harsh environmentalconditions.2n2nproliferationof diploid cells2nmeiosis and sporulation(triggered by starvation)2nnand studies on yeasts (using both S. cerevisiae and other species) have provideda key to many crucial processes, including the eukaryotic cell-division cycle—thecritical chain of events by which the nucleus and all the other components of a cellare duplicated and parceled out to create two daughter cells from one.
The controlsystem that governs this process has been so well conserved over the course ofevolution that many of its components can function interchangeably in yeast andhuman cells: if a mutant yeast lacking an essential yeast cell-division-cycle geneis supplied with a copy of the homologous cell-division-cycle gene from a human,the yeast is cured of its defect and becomes able to divide normally.The Expression Levels of All the Genes of An OrganismCan Be Monitored SimultaneouslyThe complete genome sequence of S. cerevisiae, determined in 1997, consistsof approximately 13,117,000 nucleotide pairs, including the small contribution(78,520 nucleotide pairs) of the mitochondrial DNA. This total is only about 2.5times as much DNA as there is in E.
coli, and it codes for only 1.5 times as manydistinct proteins (about 6600 in all). The way of life of S. cerevisiae is similar inmany ways to that of a bacterium, and it seems that this yeast has likewise beensubject to selection pressures that have kept its genome compact.Knowledge of the complete genome sequence of any organism—be it a yeastor a human—opens up new perspectives on the workings of the cell: things thatonce seemed impossibly complex now seem within our grasp. Using techniquesdescribed in Chapter 8, it is now possible, for example, to monitor, simultaneously, the amount of mRNA transcript that is produced from every gene in theyeast genome under any chosen conditions, and to see how this whole pattern ofgene activity changes when conditions change.
The analysis can be repeated withmRNA prepared from mutant cells lacking a chosen gene—any gene that we careto test. In principle, this approach provides a way to reveal the entire system ofcontrol relationships that govern gene expression—not only in yeast cells, but inany organism whose genome sequence is known.Arabidopsis Has Been Chosen Out of 300,000 SpeciesAs a Model PlantThe large multicellular organisms that we see around us—the flowers and treesand animals—seem fantastically varied, but they are much closer to one anotherin their evolutionary origins, and more similar in their basic cell biology, thanthe great host of microscopic single-celled organisms.
Thus, while bacteria andarchaea are separated by perhaps 3.5 billion years of evolution, vertebrates andinsects are separated by about 700 million years, fish and mammals by about 450million years, and the different species of flowering plants by only about 150 million years.Because of the close evolutionary relationship between all flowering plants,we can, once again, get insight into the cell and molecular biology of this wholeclass of organisms by focusing on just one or a few species for detailed analysis.Out of the several hundred thousand species of flowering plants on Earth today,molecular biologists have chosen to concentrate their efforts on a small weed,nmating (usuallyimmediately afterspores hatch)nnspores hatchnnnproliferationof haploidcellsnBUDDING YEAST LIFE CYCLEMBoC6 m1.43/1.37GENETIC INFORMATION IN EUKARYOTES33the common Thale cress Arabidopsis thaliana (Figure 1–38), which can be grownindoors in large numbers and produces thousands of offspring per plant after8–10 weeks.
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