Biology_Unit_5 (1110837), страница 3
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This DNA molecule is the prokaryotic chromosome. (Chapter 17 discusses the genetics of prokaryotes.)Individual genes in the DNA molecule encode the information required to make proteins. This information is copied into atype of RNA molecule called messenger RNA (mRNA). Small,roughly spherical particles in the cytoplasm, the ribosomes, usethe information in the mRNA to assemble amino acids into proteins. A prokaryotic ribosome consists of a large and a small subunit, each formed from a combination of ribosomal RNA (rRNA)and protein molecules.
Each prokaryotic ribosome containsthree types of rRNA molecules, which are also copied from theDNA, and more than 50 proteins.In almost all prokaryotes, the plasma membrane is surrounded by a rigid external layer of material, the cell wall, whichranges in thickness from 15 to 100 nm or more (a nanometer isNucleoidCytoplasmCell wallDr. G. Cohen-BazirePlasmamembraneone-billionth of a meter). The cell wall provides rigidity to prokaryotic cells and, with the capsule, protects the cell from physical damage. In many prokaryotic cells, the wall is coated with anexternal layer of polysaccharides called the glycocalyx (a “sugarcoating” from glykys sweet; calyx cup or vessel). When theglycocalyx is diff use and loosely associated with the cells, it is aslime layer; when it is gelatinous and more firmly attached tocells, it is a capsule.
The glycocalyx helps protect prokaryotic cellsfrom physical damage and desiccation, and may enable a cell toattach to a surface, such as other prokaryotic cells (as in forminga colony), eukaryotic cells (as in Streptococcus pneumoniae attaching to lung cells), or nonliving substrate (such as a rock).The plasma membrane itself performs several vital functions in prokaryotes. Besides transporting materials into and outof the cells, it contains most of the molecular systems that metabolize food molecules into the chemical energy of ATP.
In photosynthetic prokaryotes, the molecules that absorb light energyand convert it to the chemical energy of ATP are also associatedwith the plasma membrane or with internal, saclike membranesderived from the plasma membrane.Many prokaryotic species contain few if any internal membranes; in such cells, most cellular functions occur either on theplasma membrane or in the cytoplasm.
But some prokaryoteshave more extensive internal membrane structures. For example,photosynthetic bacteria and archaeans have complex layers ofintracellular membranes formed by invaginations of the plasmamembrane on which photosynthesis takes place. And membersof the bacterial phylum Planctomycetes have complex internalmembranes that form distinct compartments.As mentioned earlier, prokaryotic cells have fi lamentous cytoskeletal structures with functions similar to those in eukaryotes. Prokaryotic cytoskeletons play important roles in creatingand maintaining the proper shape of cells, in cell division and,for certain prokaryotes, in determining polarity of the cells.0.5 μmRibosomesCytoplasmNucleoidBacterialflagellumPiliPlasmamembraneCellwallCapsuleFIGURE 5.7Prokaryotic cell structure. An electron micrograph (left) and a diagram (right) of the bacterium Escherichia coli.
The pili extending from the cell wall attach bacterialcells to other cells of the same species or to eukaryotic cells as a part of infection. A typical E. coli has four flagella.CHAPTER 5THE CELL: AN OVERVIEW93INSIGHTS FROM THEMolecular RevolutionAn Old Kingdom in a NewDomain: Do archaeans define adistinct domain of life?Many archaeans live in extreme environmentsthat can be tolerated by no other organisms,suggesting that they might belong in a distinctdomain of life. For example, Methanococcusjannaschii was first found in an oceanic hotwater vent at a depth of more than 2,600 m(8,500 feet).
It can live at temperatures as highas 94°C, which is almost the temperature ofboiling water, and can tolerate pressures as highas 200 times the pressure of air at sea level!Research Question: Are archaeans a distinctdomain of life?Experiment: In 1996, Carol J. Bult, Carl R.Woese, J. Craig Venter, and 37 other scientists at the Institute for Genomic Researchpublished the complete genomic DNA sequence of Methanococcus jannaschii. (Thesequence was obtained using techniquesoutlined in Chapter 18.) Using computer algorithms, the scientists compared the sequence with the already known genome sequences of several bacteria and of brewer’syeast (Saccharomyces cerevisiae), the first eukaryote to be sequenced completely.Results: The researchers found genes coding for 1,738 proteins in the Methanococcusgenome.
Of these, only 38% were related togenes coding for known proteins in eitherbacteria or eukaryotes. The remaining 62%have no known relatives in organisms ofthose two groups.Some features of Methanococcus DNA aretypically prokaryotic. Its single, circular chromosome is in a nucleoid, which is not boundedby a membrane. Its protein-coding genes areorganized into functional groups called operons, each having several genes copied as a unitinto a single mRNA molecule (see discussionin Section 16.1).
By contrast, each proteincoding gene in eukaryotes is copied into aMany bacteria and archaeans can move through liquids andacross wet surfaces. Most commonly they do so using long, threadlike protein fibers called flagella (singular, flagellum, meaningwhip), which extend from the cell surface (see Figure 5.2A). Thebacterial flagellum, which is helically shaped, rotates in a socketin the plasma membrane and cell wall to push the cell through aliquid medium (see Chapter 25). In E.
coli, for instance, rotatingbundles of flagella propel the bacterium. Archaeal flagella function similarly to bacterial flagella, but the two types differ significantly in their structures and mechanisms of action. Both types ofprokaryotic flagella are also fundamentally different from themuch larger and more complex flagella of eukaryotic cells, whichare described in Section 5.3.Some bacteria and archaeans have hairlike shafts of proteincalled pili (singular, pilus) extending from their cell walls.
Themain function of pili is attaching the cell to surfaces or othercells. A special type of pilus, the sex pilus, attaches one bacteriumto another during mating (see Chapter 17).Although prokaryotic cells appear relatively simple, theirsimplicity is deceptive. Most can use a variety of substances asenergy and carbon sources, and they are able to synthesize almost all of their required organic molecules from simple inorganic raw materials.
In many respects, prokaryotes are moreversatile biochemically than eukaryotes. Their small size andmetabolic versatility are reflected in their abundance; prokaryotes vastly outnumber all other types of organisms and live successfully in almost all regions of Earth’s surface.The two domains of prokaryotes, the Bacteria and the Archaea, share many biochemical and molecular features. However,94UNIT ONEMOLECULES AND CELLSseparate mRNA molecule. Some of the proteins encoded in Methanococcus DNA, including enzymes active in energy metabolism,membrane transport, and cell division, aresimilar to those of bacteria.
Other proteinsencoded in the Methanococcus DNA are similar to those of eukaryotes, including enzymesand other proteins that carry out DNA replication and the copying of genes into mRNA.Conclusion: Methanococcus has a majority ofgenes that are unique, some that are typicallybacterial, and some that are typically eukaryotic. This finding supports the proposal, firstadvanced by Woese, that Methanococcus andits archaean relatives are a separate domain oflife, the Archaea, with the Bacteria and the Eukarya as the other domains. Together, Bacteriaand Archaea are the prokaryotes. Woese’sthree-domain system is used in this book.Source: C. J. Bult et al. 1996.
Complete genomesequence of the methanogenic archaeon, Methanococcus jannaschii. Science 273:1058–1073.the archaeans also share some features with eukaryotes and haveother characteristics that are unique to their group. Insights fromthe Molecular Revolution describes the discovery of features thatsupport the classification of the Archaea as a separate domain.STUDY BREAK 5.2<Where in a prokaryotic cell is DNA found? How is that DNAorganized?5.3 Eukaryotic CellsThe domain of the eukaryotes, Eukarya, is divided into four majorgroups: the protists, fungi, animals, and plants.
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