Essential Cell Biology 5th edition
32 CHAPTER 1 Cells: The Fundamental Units of Life(A)1 cmroots of crops. Smaller and simpler than Drosophila, this creature developswith clockwork precision from a fertilized egg cell into an adult thathas exactly 959 body cells (plus a variable number of egg and spermcells)—an unusual degree of regularity for an animal. We now have aminutely detailed description of the sequence of events by which thisoccurs—as the cells divide, move, and become specialized according tostrict and predictable rules. And a wealth of mutants are available fortesting how the worm’s genes direct this developmental ballet. Some 70%of human genes have some counterpart in the worm, and C. elegans, likeDrosophila, has proved to be a valuable model for many of the developmentalprocesses that occur in our own bodies. Studies of nematodedevelopment, for example, have led to a detailed molecular understandingof apoptosis, a form of programmed cell death by which animalsdispose of surplus cells, a topic discussed in Chapter 18. This process isalso of great importance in the development of cancer, as we discuss inChapter 20.(B)ECB5 e1.37/1.381 mmFigure 1–38 Zebrafish are popular modelsfor studies of vertebrate development.(A) These small, hardy, tropical fish—a staplein many home aquaria—are easy and cheapto breed and maintain. (B) They are alsoideal for developmental studies, as theirtransparent embryos develop outside themother, making it easy to observe cellsmoving and changing their characters inthe living organism as it develops. In thisimage of a two-day-old embryo, taken witha confocal microscope, a green fluorescentprotein marks the developing lymphaticvessels and a red fluorescent protein marksdeveloping blood vessels; regions wherethe two fluorescent markers coincide appearyellow. (A, courtesy of Steve Baskauf;B, from H.M. Jung et al., Development144:2070–2081, 2017.)Another animal that is providing molecular insights into developmentalprocesses, particularly in vertebrates, is the zebrafish (Figure 1–38A).Because this creature is transparent for the first two weeks of its life, itprovides an ideal system in which to observe how cells behave duringdevelopment in a living animal (Figure 1–38B).Mammals are among the most complex of animals, and the mouse haslong been used as the model organism in which to study mammaliangenetics, development, immunology, and cell biology. Thanks to modernmolecular biological techniques, it is possible to breed mice withdeliberately engineered mutations in any specific gene, or with artificiallyconstructed genes introduced into them (as we discuss in Chapter 10).In this way, one can test what a given gene is required for and how itfunctions. Almost every human gene has a counterpart in the mouse,with a similar DNA sequence and function. Thus, this animal has provenan excellent model for studying genes that are important in both humanhealth and disease.Biologists Also Directly Study Humans and Their CellsHumans are not mice—or fish or flies or worms or yeast—and so manyscientists also study human beings themselves. Like bacteria or yeast,our individual cells can be harvested and grown in culture, where investigatorscan study their biology and more closely examine the genes thatgovern their functions. Given the appropriate surroundings, many humancell types—indeed, many cell types of animals or plants—will survive,proliferate, and even express specialized properties in a culture dish.Experiments using such cultured cells are sometimes said to be carriedout in vitro (literally, “in glass”) to contrast them with experiments onintact organisms, which are said to be carried out in vivo (literally, “in theliving”).Although not true for all cell types, many cells—including those harvestedfrom humans—continue to display the differentiated properties appropriateto their origin when they are grown in culture: fibroblasts, a major celltype in connective tissue, continue to secrete proteins that form the extracellularmatrix; embryonic heart muscle cells contract spontaneously inthe culture dish; nerve cells extend axons and make functional connectionswith other nerve cells; and epithelial cells join together to formcontinuous sheets, as they do inside the body (Figure 1–39 and Movie1.7). Because cultured cells are maintained in a controlled environment,they are accessible to study in ways that are often not possible in vivo. Forexample, cultured cells can be exposed to hormones or growth factors,
Model Organisms33(A) (B) (C)50 µm 50 µm 50 µmFigure 1–39 Cells in culture often display properties that reflect their origin. These phase-contrast micrographsshow a variety of cell types in culture. (A) Fibroblasts from human skin. (B) Human neurons make connections withone another in culture. (C) Epithelial cells from human cervix form a cell sheet in culture. (Micrographs courtesy ofScienCell Research Laboratories, Inc.)and the effects that these signal molecules have on the shape or behaviorof the cells can be easily explored. Remarkably, certain human embryocells can be coaxed into differentiating into multiple cell types, whichcan self-assemble into organlike structures that closely resemble a normalorgan such as an eye or brain. Such organoids can be used to studyECB5 n1.101/1.39developmental processes—and how they are derailed in certain humangenetic diseases (discussed in Chapter 20).In addition to studying our cells in culture, humans are also examineddirectly in clinics. Much of the research on human biology has been drivenby medical interests, and the medical database on the human species isenormous. Although naturally occurring, disease-causing mutations inany given human gene are rare, the consequences are well documented.This is because humans are unique among animals in that they reportand record their own genetic defects: in no other species are billions ofindividuals so intensively examined, described, and investigated.Nevertheless, the extent of our ignorance is still daunting. The mammalianbody is enormously complex, being formed from thousands of billionsof cells, and one might despair of ever understanding how the DNA in afertilized mouse egg cell directs the generation of a mouse rather thana fish, or how the DNA in a human egg cell directs the development ofa human rather than a mouse. Yet the revelations of molecular biologyhave made the task seem eminently approachable. As much as anything,this new optimism has come from the realization that the genes of onetype of animal have close counterparts in most other types of animals,apparently serving similar functions (Figure 1–40). We all have a commonevolutionary origin, and under the surface it seems that we sharethe same molecular mechanisms. Flies, worms, fish, mice, and humansthus provide a key to understanding how animals in general are madeand how their cells work.Comparing Genome Sequences Reveals Life’s CommonHeritageAt a molecular level, evolutionary change has been remarkably slow.We can see in present-day organisms many features that have beenpreserved through more than 3 billion years of life on Earth—about onefifthof the age of the universe. This evolutionary conservatism provides
- Page 17 and 18: PrefacexvCONTENTSPreface vAbout the
- Page 19 and 20: ContentsxviiThe Equilibrium Constan
- Page 21 and 22: ContentsxixCHAPTER 6DNA Replication
- Page 23 and 24: ContentsxxiPOST-TRANSCRIPTIONAL CON
- Page 25 and 26: ContentsxxiiiCHAPTER 11Membrane Str
- Page 27 and 28: ContentsxxvCHAPTER 14Energy Generat
- Page 29 and 30: ContentsxxviiCHAPTER 16Cell Signali
- Page 31 and 32: ContentsxxixCHAPTER 18The Cell-Divi
- Page 33 and 34: ContentsxxxiGENETICS AS AN EXPERIME
- Page 35 and 36: CHAPTER ONE1Cells: The FundamentalU
- Page 37 and 38: Unity and Diversity of Cells3mechan
- Page 39 and 40: Unity and Diversity of Cells5nucleo
- Page 41 and 42: Cells Under the Microscope7The Inve
- Page 43 and 44: Cells Under the Microscope9cytoplas
- Page 45 and 46: The Prokaryotic Cell110.2 mm(200 µ
- Page 47 and 48: The Prokaryotic Cell13SUPER-RESOLUT
- Page 49 and 50: The Prokaryotic Cell15(A)HSV10 µmF
- Page 51 and 52: The Eukaryotic Cell17nucleusnuclear
- Page 53 and 54: The Eukaryotic Cell19chloroplastsch
- Page 55 and 56: The Eukaryotic Cell21lysosomenuclea
- Page 57 and 58: The Eukaryotic Cell23duplicatedchro
- Page 59 and 60: PANEL 1-2 CELL ARCHITECTURE 25ANIMA
- Page 61 and 62: Model Organisms27(C)(D)(A) (B) (E)
- Page 63 and 64: Model Organisms29Figure 1-34 Drosop
- Page 65: Model Organisms31INTRODUCE FRAGMENT
- Page 69 and 70: Model Organisms35TABLE 1-2 SOME MOD
- Page 71 and 72: Questions37• Free-living, single-
- Page 73 and 74: CHAPTER TWO2Chemical Components of
- Page 75 and 76: Chemical Bonds41number of protons.
- Page 77 and 78: Chemical Bonds43+atoms+SHARING OFEL
- Page 79 and 80: Chemical Bonds45Four electrons can
- Page 81 and 82: Chemical Bonds47Because of the favo
- Page 83 and 84: Chemical Bonds49Some Polar Molecule
- Page 85 and 86: Small Molecules in Cells51with othe
- Page 87 and 88: Small Molecules in Cells53optical i
- Page 89 and 90: Small Molecules in Cells55can be re
- Page 91 and 92: Small Molecules in Cells57O _O _ ph
- Page 93 and 94: Macromolecules in Cells59Figure 2-3
- Page 95 and 96: Macromolecules in Cells61ultracentr
- Page 97 and 98: Macromolecules in Cells63BBAAthe su
- Page 99 and 100: Questions65KEY TERMSacid electrosta
- Page 101 and 102: 67C-O COMPOUNDSMany biological comp
- Page 103 and 104: 69WATER AS A SOLVENTMany substances
- Page 105 and 106: 71ELECTROSTATIC ATTRACTIONSElectros
- Page 107 and 108: 73α AND β LINKSThe hydroxyl group
- Page 109 and 110: 75LIPID AGGREGATESFatty acids have
- Page 111 and 112: 77ACIDIC SIDE CHAINSNONPOLAR SIDE C
- Page 113 and 114: 79NOMENCLATUREThe names can be conf
- Page 115 and 116: CHAPTER THREE3Energy, Catalysis, an
Model Organisms
33
(A) (B) (C)
50 µm 50 µm 50 µm
Figure 1–39 Cells in culture often display properties that reflect their origin. These phase-contrast micrographs
show a variety of cell types in culture. (A) Fibroblasts from human skin. (B) Human neurons make connections with
one another in culture. (C) Epithelial cells from human cervix form a cell sheet in culture. (Micrographs courtesy of
ScienCell Research Laboratories, Inc.)
and the effects that these signal molecules have on the shape or behavior
of the cells can be easily explored. Remarkably, certain human embryo
cells can be coaxed into differentiating into multiple cell types, which
can self-assemble into organlike structures that closely resemble a normal
organ such as an eye or brain. Such organoids can be used to study
ECB5 n1.101/1.39
developmental processes—and how they are derailed in certain human
genetic diseases (discussed in Chapter 20).
In addition to studying our cells in culture, humans are also examined
directly in clinics. Much of the research on human biology has been driven
by medical interests, and the medical database on the human species is
enormous. Although naturally occurring, disease-causing mutations in
any given human gene are rare, the consequences are well documented.
This is because humans are unique among animals in that they report
and record their own genetic defects: in no other species are billions of
individuals so intensively examined, described, and investigated.
Nevertheless, the extent of our ignorance is still daunting. The mammalian
body is enormously complex, being formed from thousands of billions
of cells, and one might despair of ever understanding how the DNA in a
fertilized mouse egg cell directs the generation of a mouse rather than
a fish, or how the DNA in a human egg cell directs the development of
a human rather than a mouse. Yet the revelations of molecular biology
have made the task seem eminently approachable. As much as anything,
this new optimism has come from the realization that the genes of one
type of animal have close counterparts in most other types of animals,
apparently serving similar functions (Figure 1–40). We all have a common
evolutionary origin, and under the surface it seems that we share
the same molecular mechanisms. Flies, worms, fish, mice, and humans
thus provide a key to understanding how animals in general are made
and how their cells work.
Comparing Genome Sequences Reveals Life’s Common
Heritage
At a molecular level, evolutionary change has been remarkably slow.
We can see in present-day organisms many features that have been
preserved through more than 3 billion years of life on Earth—about onefifth
of the age of the universe. This evolutionary conservatism provides
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