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RegulatoryDNA allows gene expression to becontrolled by regulatory proteins, whichare in turn the products of other genes.This diagram shows how a cell’s geneexpression is adjusted according to asignal from the cell’s environment. Theinitial effect of the signal is to activate aregulatory protein already present in thecell; the signal may, for example, triggerthe attachment of a phosphate group tothe regulatory protein, altering itschemical properties....that binds to otherregulatory regions...protein-codingregionregulatoryregion...to produce yet more proteins, includingsome additional gene-regulatory proteinsbinding, directly or indirectly, to the regulatory DNA adjacent to the genes thatare to be controlled (Figure 1–39), or by interfering with the abilities of other proteins to do so.
The expanded genome of eucaryotes therefore not only specifiesthe hardware of the cell, but also stores the software that controls how that hardware is used (Figure 1–40).Cells do not just passively receive signals; rather, they actively exchange signals with their neighbors. Thus, in a developing multicellular organism, thesame control system governs each cell, but with different consequencesdepending on the messages exchanged. The outcome, astonishingly, is a precisely patterned array of cells in different states, each displaying a characterappropriate to its position in the multicellular structure.Many Eucaryotes Live as Solitary Cells: the ProtistsMany species of eucaryotic cells lead a solitary life—some as hunters (the protozoa), some as photosynthesizers (the unicellular algae), some as scavengers (theunicellular fungi, or yeasts).
Figure 1–41 conveys something of the variety offorms of these single-celled eucaryotes, or protists. The anatomy of protozoa,Figure 1–40 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 develop inplace of flowers: because a regulatoryprotein has been changed, the cellsadopt characters that would beappropriate to a different location in thenormal plant. The mutant is on the left,the normal plant on the right.
(Courtesyof Enrico Coen and Rosemary Carpenter.)GENETIC INFORMATION IN EUCARYOTES33I.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 asmany multicellular organisms (see Figure 1–32). <ATGG> <TCGC>In terms of their ancestry and DNA sequences, protists are far more diversethan the multicellular animals, plants, and fungi, which arose as three comparatively late branches of the eucaryotic pedigree (see Figure 1–21).
As with procaryotes, humans have tended to neglect the protists because they are microscopic.Only now, with the help of genome analysis, are we beginning to understandtheir positions in the tree of life, and to put into context the glimpses thesestrange creatures offer us of our distant evolutionary past.A Yeast Serves as a Minimal Model EucaryoteThe molecular and genetic complexity of eucaryotes is daunting.
Even morethan for procaryotes, biologists need to concentrate their limited resources on afew selected model organisms to fathom this complexity.To analyze the internal workings of the eucaryotic cell, without the additional problems of multicellular development, it makes sense to use a speciesthat is unicellular and as simple as possible. The popular choice for this role ofminimal model eucaryote has been the yeast Saccharomyces cerevisiae (Figure1–42)—the same 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 andthus, according to modern views, at least as closely related to animals as it is toplants.
It is robust and easy to grow in a simple nutrient medium. Like otherfungi, it has a tough cell wall, is relatively immobile, and possesses mitochondriabut not chloroplasts. When nutrients are plentiful, it grows and divides almost asFigure 1–41 An assortment of protists: asmall sample of an extremely diverseclass of organisms. The drawings aredone to different scales, but in each casethe scale bar represents 10 mm. Theorganisms in (A), (B), (E), (F), and (I) areciliates; (C) is a euglenoid; (D) is anamoeba; (G) is a dinoflagellate; (H) is aheliozoan. (From M.A. Sleigh, Biology ofProtozoa. Cambridge, UK: CambridgeUniversity Press, 1973.)34Chapter 1: Cells and Genomesnucleuscell wallFigure 1–42 The yeast Saccharomycescerevisiae.
(A) A scanning electronmicrograph of a cluster of the cells. Thisspecies is also known as budding yeast; itproliferates by forming a protrusion orbud that enlarges and then separatesfrom the rest of the original cell. Manycells with buds are visible in thismicrograph. (B) A transmission electronmicrograph of a cross section of a yeastcell, showing its nucleus, mitochondrion,and thick cell wall.
(A, courtesy of IraHerskowitz and Eric Schabatach.)mitochondrion(B)(A)10 mm2 mmrapidly as a bacterium. It can reproduce either vegetatively (that is, by simplecell division), or sexually: two yeast cells that are haploid (possessing a singlecopy of the genome) can fuse to create a cell that is diploid (containing a doublegenome); and the diploid cell can undergo meiosis (a reduction division) to produce cells that are once again haploid (Figure 1–43). In contrast with higherplants and animals, the yeast can divide indefinitely in either the haploid or thediploid state, and the process leading from the one state to the other can beinduced at will by changing the growth conditions.In addition to these features, the yeast has a further property that makes it aconvenient organism for genetic studies: its genome, by eucaryotic standards, isexceptionally small.
Nevertheless, it suffices for all the basic tasks that everyeucaryotic cell must perform. As we shall see later in this book, studies on yeasts(using both S. cerevisiae and other species) have provided a key to many crucialprocesses, including the eucaryotic cell-division cycle—the critical chain ofevents by which the nucleus and all the other components of a cell are duplicated and parceled out to create two daughter cells from one.
The control system that governs this process has been so well conserved over the course of evolution 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 ahuman, the yeast is cured of its defect and becomes able to divide normally.The Expression Levels of All The Genes of An Organism Can BeMonitored SimultaneouslyThe complete genome sequence of S.
cerevisiae, determined in 1997, consists ofapproximately 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 6300 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 techniquesFigure 1–43 The reproductive cycles of the yeast S. cerevisiae. Dependingon environmental conditions and on details of the genotype, cells of thisspecies can exist in either a diploid (2n) state, with a double chromosomeset, or a haploid (n) state, with a single chromosome set.
The diploid formcan either proliferate by ordinary cell-division cycles or undergo meiosis toproduce haploid cells. The haploid form can either proliferate by ordinarycell-division cycles or undergo sexual fusion with another haploid cell tobecome 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)2nnnmating (usuallyimmediately afterspores hatch)nnspores hatchnnnproliferationof haploidcellsnBUDDING YEAST LIFE CYCLEGENETIC INFORMATION IN EUCARYOTES35ACE2FKH1FKH2MBP1MC MNDD 1RM E 1SK 1ST N7S B1SWWI4S I5A WIAS 1 6H1YJL206CUGA3THI2STP21STP 4SIP 1SFPL1SF G3RT 1GRT GT1R U T3PG1DI MS1H E4IM OT3M D1PH 101RIM K2SO 12STE 1SUMABF1DOT6FHL1HIR1HIR2RAP1REB1RGMCAD 1CIN 1CRZ 5CU 1G P9H TS1HA AA1H L9MA SF1C1CHC A4BA BF1SAAR ZF1 1AR O80ARGG818ADR 0ZMS11ZAP1YFL044CYAP7YAP6YAP5N1MS SN2 4M NMS DR1P CS1R X1RF 1RLMX1RO 1RPH 1SKOSMP1YAP1YAP3PP HON HO 4MS RG1 2SM 11M IG1METET4MAL 31MAL1333LEU3IXR1INO4INO2HAP5HAP4HAP3HAP2GLN3GCR21GCRN4GC T3GA T1GA L4GA ZF1F L821DA AL8DDNA/RNA/protein biosynthesiscell cycleenvironmental responsedevelopmental processesmetabolismto be described in Chapter 8, it is now possible, for example, to monitor, simultaneously, the amount of mRNA transcript that is produced from every gene inthe yeast genome under any chosen conditions, and to see how this whole pattern of gene activity changes when conditions change.
The analysis can berepeated with mRNA prepared from mutants lacking a chosen gene—any genethat we care to test. In principle, this approach provides a way to reveal theentire system of control relationships that govern gene expression—not only inyeast cells, but in any organism whose genome sequence is known.Figure 1–44 The network of interactionsbetween gene regulatory proteins andthe genes that code for them in a yeastcell.
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