Review&Se ssion&by&TAs:&Friday&from&3-3:50pm&in& SSL&228263An Overview of Gene Expression a whole organism. The various cell types of an organism therefore differ not because they contain different genes, but because they express them differently.Different Cell Types Produce Different Sets of ProteinsThe extent of the differences in gene expression between different cell types may be roughly gauged by comparing the protein composition of cells in liver, heart, brain, and so on. In the past, such analysis was per-formed by two-dimensional gel electrophoresis (see Panel 4–5, p. 167). Nowadays, the total protein content of a cell can be rapidly analyzed by unfertilized eggnucleus destroyedby UV lightadult frog(A)(B)(C)skin cells inculture dishnucleus in pipettenucleusinjectedinto eggnormal embryotadpolesection of carrotproliferatingcell massseparatedcells in richliquidmediumsinglecellclone ofdividingcellsyoungembryoyoungplantcarrotcowsepithelial cellsfrom oviductunfertilizedegg cellmeioticspindleMEIOTIC SPINDLE AND ASSOCIATEDCHROMOSOMES REMOVEDDONOR CELL PLACED NEXT TO ENUCLEATED EGGreconstructedzygoteELECTRICPULSE CAUSESDONOR CELLTO FUSE WITHENUCLEATED EGG CELLCELLDIVISIONembryo placed infoster mothercalfECB4 e8.02/8.02embryoUVFigure 8–2 Differentiated cells contain all the genetic instructions necessary to direct the formation of a complete organism. (A) The nucleus of a skin cell from an adult frog transplanted into an egg whose nucleus has been destroyed can give rise to an entire tadpole. The broken arrow indicates that to give the transplanted genome time to adjust to an embryonic environment, a further transfer step is required in which one of the nuclei is taken from the early embryo that begins to develop and is put back into a second enucleated egg. (B) In many types of plants, differentiated cells retain the ability to “de-differentiate,” so that a single cell can proliferate to form a clone of progeny cells that later give rise to an entire plant. (C) A nucleus removed from a differentiated cell from an adult cow can be introduced into an enucleated egg from a different cow to give rise to a calf. Different calves produced from the same differentiated cell donor are all clones of the donor and are therefore genetically identical. (A, modified from J.B. Gurdon, Sci. Am. 219:24–35, 1968, with permission from the Estate of Bunji Tagawa.)A&striking&observation&about&multicellular&organisms:What&does&this&im ply?1)#Instructions# for#making# the# organism#are# still#in#the#differentiated#cell2)#D iffer ent#instructions# are# r ead# in# different#cellsGene#expression# differences#are#at#the#heart#of#this.RNA#se que ncing# experiments# show#that#a#typical#human#cel l#mak es #5000-15,000#protein-coding#genes# from#a#total #of#~21,000Timescales&implied&here:Times cale# 1:#Once#the#organism#is #developed,# the #e xpres si on# difference s#have#to#become#long-lasting/permanent.Times cale# 2:#During#the #organism’s#development,# leading,# for#example ,#to#skin#v.#liver#cell,#the#expression# differences# have#to#occur#over#the#timeframe#of#development.Times cale# 3:#On#an#e ven#shorter#ti mes cale,# cel ls #have#to#mak e #decisions# base d# on#their#environment#all#the#time.What#are#the#mechanisms# behind#this#control,#this#“cyber netics”?265(6) regulating how rapidly specific proteins are destroyed after they have been made; in addition, the activity of individual proteins can be further regulated in a variety of ways. These steps are illustrated in Figure 8–3.Gene expression can be regulated at each of these steps. For most genes, however, the control of transcription (step number 1 in Figure 8–3) is paramount. This makes sense because only transcriptional control can ensure that no unnecessary intermediates are synthesized. So it is the regulation of transcription—and the DNA and protein components that determine which genes a cell transcribes into RNA—that we address first.HOW TRANSCRIPTIONAL SWITCHES WORKUntil 50 years ago, the idea that genes could be switched on and off was revolutionary. This concept was a major advance, and it came originally from studies of how E. coli bacteria adapt to changes in the composition of their growth medium. Many of the same principles apply to eukaryotic cells. However, the enormous complexity of gene regulation in higher organisms, combined with the packaging of their DNA into chromatin, creates special challenges and some novel opportunities for control—as we will see. We begin with a discussion of the transcription regulators, proteins that bind to DNA and control gene transcription.Transcription Regulators Bind to Regulatory DNA SequencesControl of transcription is usually exerted at the step at which the proc-ess is initiated. In Chapter 7, we saw that the promoter region of a gene binds the enzyme RNA polymerase and correctly orients the enzyme to begin its task of making an RNA copy of the gene. The promoters of both bacterial and eukaryotic genes include a transcription initiation site, where RNA synthesis begins, plus a sequence of approximately 50 nucleotide pairs that extends upstream from the initiation site (if one likens the direction of transcription to the flow of a river). This upstream region contains sites that are required for the RNA polymerase to recognize the promoter, although they do not bind to RNA polymerase directly. Instead, these sequences contain recognition sites for proteins that associate with the active polymerase—sigma factor in bacteria (see Figure 7–9) or the general transcription factors in eukaryotes (see Figure 7–12).In addition to the promoter, nearly all genes, whether bacterial or eukary-otic, have regulatory DNA sequences that are used to switch the gene on or off. Some regulatory DNA sequences are as short as 10 nucleotide pairs and act as simple switches that respond to a single signal; such simple regulatory switches predominate in bacteria. Other regulatory DNA sequences, especially those in eukaryotes, are very long (some-times spanning more than 10,000 nucleotide pairs) and act as molecular DNA1transcriptionalcontrol2RNAprocessingcontrolRNAtranscriptmRNA mRNA3mRNAtransportandlocalizationcontrol5translation control6NUCLEUS CYTOSOLmRNA degradationcontrolproteindegradationcontrol7proteinactivitycontroldegraded mRNA protein4ECB4