H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003), страница 9

White blood cells normally look like the cell on the left.Cells undergoing programmed cell death (apoptosis), like the cellon the right, form numerous surface blebs that eventually arereleased. The cell is dying because it lacks certain growth signals.Apoptosis is important to eliminate virus-infected cells, removecells where they are not needed (like the webbing thatdisappears as fingers develop), and to destroy immune systemcells that would react with our own bodies. [Gopal Murti/VisualsUnlimited, Inc.](a)(b)To build an integrated understanding of how the various molecular components that underlie cellular functions work together in a living cell, we must draw on various perspectives.Here, we look at how five disciplines—cell biology, biochemistry, genetics, genomics, and developmental biology—cancontribute to our knowledge of cell structure and function.The experimental approaches of each field probe the cell’sinner workings in different ways, allowing us to ask different types of questions about cells and what they do.

Cell division provides a good example to illustrate the role ofdifferent perspectives in analyzing a complex cellular process.The realm of biology ranges in scale more than a billionfold (Figure 1-20). Beyond that, it’s ecology and earth science(c)(d)PHOTODNA base pairsNanometersSmallmoleculesAssembliesMacromoleculesAtomsGlucoseC_C bond10-10 m0.1 nm10-8 m10 nmMulticellular organismsCellsRibosomeHemoglobin10-9 m1 nmMetersMillimetersMicrometersC. elegansBacteriumMitochondrionRed bloodcell10-7 m100 nm10-5 m10 µm10-6 m1 µm▲ FIGURE 1-20 Biologists are interested in objects rangingin size from small molecules to the tallest trees. A samplingof biological objects aligned on a logarithmic scale. (a) The DNAdouble helix has a diameter of about 2 nm.

(b) Eight-cell-stagehuman embryo three days after fertilization, about 200 mNewborn humanBumblebee10-4 m100 µm10-3 m1 mm10-2 m10 mm10-1 m100 mm100 m1macross. (c) A wolf spider, about 15 mm across. (d) Emperorpenguins are about 1 m tall. [Part (a) Will and Deni McIntyre. Part (b)Yorgas Nikas/Photo Researchers, Inc. Part (c) Gary Gaugler/VisualsUnlimited, Inc. Part (d) Hugh S. Rose/Visuals Unlimited, Inc.]20CHAPTER 1 • Life Begins with Cellsat the “macro” end, chemistry and physics at the “micro” end.The visible plants and animals that surround us are measuredin meters (100–102 m).

By looking closely, we can see a biological world of millimeters (1 mm 103 m) and even tenthsof millimeters (104 m). Setting aside oddities like chickeneggs, most cells are 1–100 micrometers (1 m 106 m) longand thus clearly visible only when magnified.

To see the structures within cells, we must go farther down the size scale to10–100 nanometers (1 nm 109 m).Cell Biology Reveals the Size, Shape,and Location of Cell ComponentsActual observation of cells awaited development of the first,crude microscopes in the early 1600s. A compound microscope, the most useful type of light microscope, has twolenses. The total magnifying power is the product of themagnification by each lens.

As better lenses were invented,the magnifying power and the ability to distinguish closelyspaced objects, the resolution, increased greatly. Moderncompound microscopes magnify the view about a thousandfold, so that a bacterium 1 micrometer (1 m) long looks likeit’s a millimeter long. Objects about 0.2 m apart can be discerned in these instruments.Microscopy is most powerful when particular components of the cell are stained or labeled specifically, enablingthem to be easily seen and located within the cell. A simpleexample is staining with dyes that bind specifically to DNAto visualize the chromosomes. Specific proteins can be detected by harnessing the binding specificity of antibodies,the proteins whose normal task is to help defend animalsagainst infection and foreign substances.

In general, eachtype of antibody binds to one protein or large polysaccharide and no other (Chapter 3). Purified antibodies can bechemically linked to a fluorescent molecule, which permitstheir detection in a special fluorescence microscope (Chapter 5). If a cell or tissue is treated with a detergent that partially dissolves cell membranes, fluorescent antibodies candrift in and bind to the specific protein they recognize.When the sample is viewed in the microscope, the boundfluorescent antibodies identify the location of the target protein (see Figure 1-15).Better still is pinpointing proteins in living cells with intact membranes.

One way of doing this is to introduce anengineered gene that codes for a hybrid protein: part of thehybrid protein is the cellular protein of interest; the otherpart is a protein that fluoresces when struck by ultravioletlight. A common fluorescent protein used for this purposeis green fluorescent protein (GFP), a natural protein thatmakes some jellyfish colorful and fluorescent. GFP “tagging” could reveal, for instance, that a particular proteinis first made on the endoplasmic reticulum and then ismoved by the cell into the lysosomes. In this case, first theendoplasmic reticulum and later the lysosomes would glowin the dark.▲ FIGURE 1-21 During the later stages of mitosis,microtubules (red) pull the replicated chromosomes (black)toward the ends of a dividing cell.

This plant cell is stainedwith a DNA-binding dye (ethidium) to reveal chromosomes andwith fluorescent-tagged antibodies specific for tubulin to revealmicrotubules. At this stage in mitosis, the two copies of eachreplicated chromosome (called chromatids) have separated andare moving away from each other. [Courtesy of Andrew Bajer.]Chromosomes are visible in the light microscope onlyduring mitosis, when they become highly condensed. The extraordinary behavior of chromosomes during mitosis firstwas discovered using the improved compound microscopesof the late 1800s. About halfway through mitosis, the replicated chromosomes begin to move apart. Microtubules, oneof the three types of cytoskeletal filaments, participate in thismovement of chromosomes during mitosis. Fluorescent tagging of tubulin, the protein subunit that polymerizes to formmicrotubules, reveals structural details of cell division thatotherwise could not be seen and allows observation of chromosome movement (Figure 1-21).Electron microscopes use a focused beam of electrons instead of a beam of light.

In transmission electron microscopy,specimens are cut into very thin sections and placed under ahigh vacuum, precluding examination of living cells. The resolution of transmission electron microscopes, about 0.1 nm,permits fine structural details to be distinguished, and theirpowerful magnification would make a 1-m-long bacterialcell look like a soccer ball. Most of the organelles in eukaryotic cells and the double-layered structure of the plasmamembrane were first observed with electron microscopes(Chapter 5). With new specialized electron microscopy techniques, three-dimensional models of organelles and largeprotein complexes can be constructed from multiple images.But to obtain a more detailed look at the individual macromolecules within cells, we must turn to techniques within thepurview of biochemistry.1221Ioch n exro chm anatog geraphyGech l firo ltram tiat onographyAfch finiro tymatographySafra ltctionationBiochemists extract the contents of cells and separate theconstituents based on differences in their chemical or physical properties, a process called fractionation.

Of particularinterest are proteins, the workhorses of many cellularprocesses. A typical fractionation scheme involves use ofvarious separation techniques in a sequential fashion.These separation techniques commonly are based on differences in the size of molecules or the electrical charge ontheir surface (Chapter 3). To purify a particular protein ofinterest, a purification scheme is designed so that each stepyields a preparation with fewer and fewer contaminatingproteins, until finally only the protein of interest remains(Figure 1-22).The initial purification of a protein of interest from a cellextract often is a tedious, time-consuming task.