H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003), страница 2

C. Cross and K. L. Mercer, 1993, Cell and Tissue Ultrastructure:A Functional Perspective, W. H. Freeman and Company.]4CHAPTER 1 • Life Begins with CellsAnimalsPlantsFungiCiliatesEuglenaMicrosporidiaEUKARYOTASlime moldsDiplomonads(Giardia lamblia)EUBACTERIAE. coliSulfolobusARCHAEAB. subtilusThermotogaThermococcusMethanobacteriumHalococcusFlavobacteriaGreen sulfurbacteriaBorreliaburgdorferiHalobacteriumMethanococcusjannaschiiso many similarities. In recent years, detailed analysis of theDNA sequences from a variety of prokaryotic organisms hasrevealed two distinct types: the so-called “true” bacteria, or eubacteria, and archaea (also called archaebacteria or archaeans).Working on the assumption that organisms with more similargenes evolved from a common progenitor more recently thanthose with more dissimilar genes, researchers have developedthe evolutionary lineage tree shown in Figure 1-3. According tothis tree, the archaea and the eukaryotes diverged from the truebacteria before they diverged from each other.Many archaeans grow in unusual, often extreme, environments that may resemble ancient conditions when lifefirst appeared on earth.

For instance, halophiles (“salt loving”) require high concentrations of salt to survive, andthermoacidophiles (“heat and acid loving”) grow in hot (80 C)sulfur springs, where a pH of less than 2 is common. Stillother archaeans live in oxygen-free milieus and generatemethane (CH4) by combining water with carbon dioxide.Unicellular Organisms Help and Hurt UsPresumed common progenitorof all extant organismsPresumed common progenitorof archaebacteria and eukaryotes▲ FIGURE 1-3 All organisms from simple bacteria tocomplex mammals probably evolved from a common, singlecelled progenitor. This family tree depicts the evolutionaryrelations among the three major lineages of organisms.

Thestructure of the tree was initially ascertained from morphologicalcriteria: Creatures that look alike were put close together. Morerecently the sequences of DNA and proteins have beenexamined as a more information-rich criterion for assigningrelationships. The greater the similarities in these macromolecularsequences, the more closely related organisms are thought tobe.

The trees based on morphological comparisons and the fossilrecord generally agree well with those those based on moleculardata. Although all organisms in the eubacterial and archaeanlineages are prokaryotes, archaea are more similar to eukaryotesthan to eubacteria (“true” bacteria) in some respects. Forinstance, archaean and eukaryotic genomes encode homologoushistone proteins, which associate with DNA; in contrast, bacterialack histones. Likewise, the RNA and protein components ofarchaean ribosomes are more like those in eukaryotes thanthose in bacteria.generally much larger than bacteria.

A typical human fibroblast, a connective tissue cell, might be about 15 macross with a volume and dry weight some thousands oftimes those of an E. coli bacterial cell. An amoeba, a singlecelled protozoan, can be more than 0.5 mm long. An ostrichegg begins as a single cell that is even larger and easily visible to the naked eye.All cells are thought to have evolved from a common progenitor because the structures and molecules in all cells haveBacteria and archaebacteria, the most abundant single-celledorganisms, are commonly 1–2 m in size.

Despite their smallsize and simple architecture, they are remarkable biochemical factories, converting simple chemicals into complex biological molecules. Bacteria are critical to the earth’s ecology,but some cause major diseases: bubonic plague (Black Death)from Yersinia pestis, strep throat from Streptomyces, tuberculosis from Mycobacterium tuberculosis, anthrax fromBacillus anthracis, cholera from Vibrio cholerae, food poisoning from certain types of E. coli and Salmonella.Humans are walking repositories of bacteria, as are allplants and animals.

We provide food and shelter for a staggering number of “bugs,” with the greatest concentration inour intestines. Bacteria help us digest our food and in turnare able to reproduce. A common gut bacterium, E. coli isalso a favorite experimental organism. In response to signalsfrom bacteria such as E.

coli, the intestinal cells form appropriate shapes to provide a niche where bacteria can live, thusfacilitating proper digestion by the combined efforts of thebacterial and the intestinal cells. Conversely, exposure to intestinal cells changes the properties of the bacteria so thatthey participate more effectively in digestion.

Such communication and response is a common feature of cells.The normal, peaceful mutualism of humans and bacteriais sometimes violated by one or both parties. When bacteriabegin to grow where they are dangerous to us (e.g., in the bloodstream or in a wound), the cells of our immune system fightback, neutralizing or devouring the intruders. Powerful antibiotic medicines, which selectively poison prokaryotic cells,provide rapid assistance to our relatively slow-developingimmune response. Understanding the molecular biology of bacterial cells leads to an understanding of how bacteria are normally poisoned by antibiotics, how they become resistant toantibiotics, and what processes or structures present in bacterial but not human cells might be usefully targeted by new drugs.1.1 • The Diversity and Commonality of Cells(a)5(b)Sporulation2MerozoitesMerozoites4GametocytesLiver5Human1MosquitoSporozoites6SpermEgg78OocystZygote▲ FIGURE 1-4 Plasmodium organisms, the parasites thatcause malaria, are single-celled protozoans with aremarkable life cycle.

Many Plasmodium species are known,and they can infect a variety of animals, cycling between insectand vertebrate hosts. The four species that cause malaria inhumans undergo several dramatic transformations within theirhuman and mosquito hosts. (a) Diagram of the life cycle.Sporozoites enter a human host when an infected Anophelesmosquito bites a person 1 . They migrate to the liver where theydevelop into merozoites, which are released into the blood 2 .Merozoites differ substantially from sporozoites, so thistransformation is a metamorphosis (Greek, “to transform” or“many shapes”).

Circulating merozoites invade red blood cells(RBCs) and reproduce within them 3 . Proteins produced bysome Plasmodium species move to the surface of infectedRBCs, causing the cells to adhere to the walls of blood vessels.This prevents infected RBCs cells from circulating to the spleenwhere cells of the immune system would destroy the RBCs andthe Plasmodium organisms they harbor. After growing andreproducing in RBCs for a period of time characteristic of eachPlasmodium species, the merozoites suddenly burst forth insynchrony from large numbers of infected cells 4 .

It is thisevent that brings on the fevers and shaking chills that arethe well-known symptoms of malaria. Some of thereleased merozoites infect additional RBCs, creating acycle of production and infection. Eventually, somemerozoites develop into male and female gametocytes5 , another metamorphosis.

These cells, which contain halfthe usual number of chromosomes, cannot survive for longunless they are transferred in blood to an Anophelesmosquito. In the mosquito’s stomach, the gametocytes aretransformed into sperm or eggs (gametes), yet anothermetamorphosis marked by development of long hairlikeflagella on the sperm 6 . Fusion of sperm and eggsgenerates zygotes 7 , which implant into the cells of thestomach wall and grow into oocysts, essentially factoriesfor producing sporozoites. Rupture of an oocyst releasesthousands of sporozoites 8 ; these migrate to the salivaryglands, setting the stage for infection of another humanhost.

(b) Scanning electron micrograph of mature oocystsand emerging sporozoites. Oocysts abut the externalsurface of stomach wall cells and are encased within amembrane that protects them from the host immunesystem. [Part (b) courtesy of R. E. Sinden.]Like bacteria, protozoa are usually beneficial members ofthe food chain. They play key roles in the fertility of soil, controlling bacterial populations and excreting nitrogenous andphosphate compounds, and are key players in waste treatment systems—both natural and man-made. These unicellular eukaryotes are also critical parts of marine ecosystems,consuming large quantities of phytoplankton and harboringphotosynthetic algae, which use sunlight to produce biologically useful energy forms and small fuel molecules.However, some protozoa do give us grief: Entamoebahistolytica causes dysentery; Trichomonas vaginalis, vagini-tis; and Trypanosoma brucei, sleeping sickness.

Each year theworst of the protozoa, Plasmodium falciparum and relatedspecies, is the cause of more than 300 million new cases ofmalaria, a disease that kills 1.5 to 3 million people annually.These protozoans inhabit mammals and mosquitoes alternately, changing their morphology and behavior in responseto signals in each of these environments. They also recognize receptors on the surfaces of the cells they infect.

Thecomplex life cycle of Plasmodium dramatically illustrateshow a single cell can adapt to each new challenge it encounters (Figure 1-4). All of the transformations in cell type thatMEDIA CONNECTIONS3Video: Plasmodium Sporozoite Entering and Exiting a Liver CellRed blood cell6CHAPTER 1 • Life Begins with Cellsoccur during the Plasmodium life cycle are governed by instructions encoded in the genetic material of this parasite andtriggered by environmental inputs.The other group of single-celled eukaryotes, the yeasts,also have their good and bad points, as do their multicellularcousins, the molds.