The Life Sciences Recent Progress and Application to Human Affairs The World of Biological Research Requirements for the Future (1970) / Chapter Skim
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The Life and Times of a Cell
The Life and Times of a Cell
Pages 52-70
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From page 52... ...
When such pieces are simply mixed, the complete virus forms without any guiding agent, enzyme, or energy source much as simpler molecules assemble to form orderly crystals.
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From page 53... ...
Still more remarkably, such a structure is capable of accepting additional materials from the environment and proceeding through a complex set of operations that results in the formation of two daughter cells identical in \\~ ~/35 Tail fiber ' 37 j38 ~/36
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From page 54... ...
As the supply of this compound, available from the primordial soup, disappeared, an advantage accrued to those cells that had "learned" to synthesize ATP for themselves by utilizing the energy potentially available in the chemical structure of glucose. Such processes have persisted in all living cells to the present time.
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From page 55... ...
The process may be more familiar in the manufacture of beer, in which yeast conducts an essen tially similar process: glucose + 2ADP + 2Pi ~ 2 ethanol + 2CO, + 2ATP. PHOTOSYNTHESIS As long as the supply of glucose in the medium sufficed, this would have permitted generation of ATP to meet the requirement of the cell for an energy source.
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From page 56... ...
THE LIFE SCIENCES ejected an electron that is transferred to some acceptor and then to an iron-protein called ferredoxin, the chlorophyll being left as a free radical. In the most primitive instance, this electron is transferred in turn from one carrier to another and ultimately returns to the original chlorophyll; in the course of this electron passage along the consecutive carriers, one or two molecules of ATP can be synthesized.
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From page 57... ...
FRONTIERS OF BIOLOGY 57 LIG HT E'o volts +0.008 -0 32 -043 +034 Cytochrome c / Cytochrome b:A ~ P Quinones ADP Flavoprotein Succinic Acid FIGURE 15 Path of electron flow in bacterial photosynthesis.
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From page 58... ...
58 THE LIFE SCIENCES Eo +1.0 +0.8 +0.6 +0.4 +0.2 -0.0 -0.2 -0.4 -0.6 -- - TEN -- - Ferredoxin System 11 Chl + -- -OH /O2L~ l O2 1 -- P700 -- - Plastocyanin ~~ Cytochrome f Light Chl° Acceptor System I _-~ Light - 1- Wo Plastocyanin Cytochrome f -- -Cytochromeb6 | | Cytochromeb6 | _ Rubimedin -- - Quinones Plastoquinone A , Plastoquinone C I TEN 1: / HI Flavoprotein I \ | Ferredoxin | FIGURE 16 Path of electron flow in photosynthesis by higher plants. (From priM ciples of Biocllemis~ry, 4th ea., A
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From page 59... ...
The appearance of animal cells necessarily had to wait until plant-photosynthetic activities were well advanced and had enriched the atmosphere with oxygen while generating a supply of organic compounds. Animal cells developed their own miniature power plants, termed "mitochondria," in which electrons taken from ingested foodstuffs similarly pass, hand to hand as it were, over a series of intermediary carriers and are delivered to oxygen with the formation of water.
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From page 60... ...
METABOLIC CONTROLS A pathway commences with a reasonably readily available material, e.g., glucose, made by the photosynthetic apparatus m glucose >a >basic defend product n Each step thereafter is made possible by the catalytic activity of a specific protein called an enzyme, which serves no other function in the life of the cell. Early in the pathway, there may be branches in which intermediates can be utilized for entry into other pathways, but one step, in our example the reaction basic, is called the "committed step" in the sense that, thereafter, the intermediates serve no purpose in the life of the cell but as stages in the ultimate formation of the desired end product.
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From page 61... ...
The smaller subunits tightly bind CTP but have no catalytic properties. When both types of units are recombined, catalytic property is retained, but CTP binding to the smaller units so alters the three-dimensional structure of the enzyme complex that its A ~Y'C0 .30 B FIGURE 17 Schematic representation of allosteric inhibition.
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From page 62... ...
If, however, a small amount of lactose enters the cell, it preferentially binds to the repressor protein made by the activity of the regulatory gene, removing it from its attachment to the operator gene, thus freeing the rest of this locus for its expression, and the cell rapidly gains ability to metabolize milk sugar. That this is the case has been demonstrated by the actual isolation of the repressor protein, by the finding that it does indeed
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From page 63... ...
In the top figure, repressor protein, made by action of regulatory gene, binds to operator gene, and the other genes of the operon are not functional. Newly arrived substrate, below, binds to repressor protein, which departs from regulator gene, and the structural genes of the operon go to work, as indicated by the growing protein chains on the ribosomes.
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From page 64... ...
Although a few components e.g., water and lipid-soluble gases can move freely, most materials viz., amino acids, sugars, and charged ions cross the membrane only in consequence of a process termed "active transport." As noted earlier, galactose permease takes its place in the bacterial cell membrane when the cell has been derepressed by the presence of milk sugar. Available information indicates that on the surface of this protein there is a site that tightly binds the sugar galactose.
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From page 65... ...
Cys ~ Asn ~ Pro~Tyr ~Lys Server 100 lieu l Thr Asp Cys Arg Glu Thr 22 23 24 25 26 27 28 29 30 31 49 V1U~t5 ~ Vol ~Phe ~Thr ~ Asn ~ Val ~ Pro ~Lys ~ Cys ~Arg ~ Asp ~ Lys ~Ihr~Leu ~ Asn ~ Erg ~ ~ 48 47 46 45 44 43 42 4 1 4C 39 38 37 36 35 34 33 FIGURE 19 The amino acid sequence of bovine pancreatic ribonuclease.
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From page 66... ...
The pairs of balls represent sulfur atoms in disulfide bridges, e.g., between 40 and 95 at the upper right. The single balls represent sulfur atoms of methionine residues.
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From page 67... ...
When a chain of amino acids was synthesized with the sequence that had been ascertained by analytical procedures, it spontaneously assumed the three-dimensional conformation that results in normal catalytic activity of this enzyme, the ultimate triumph capping five decades of research. At the same time, this new capability offers a bold vista for the future, the prospect of synthetic proteins that will serve a variety of purposes.
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From page 68... ...
. : Nucleolus: Nuclear membrane M itochond rion EndoPlasmic reticulum Lysosome FIGURE 21 Modern diagram of a typical animal cell, based on what is seen in electron micrographs.
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From page 69... ...
The life cycle of most cells can be viewed as a sequence of events shown as follows: S- BIG, G1 ~ D Stage S is the period of DNA synthesis, during which the total DNA complement of the cells is doubled. Stage D is the process of mitosis or gene segregation, in which the total DNA is separated into two identical sets of genetic material, after which cell division occurs.
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From page 70... ...
It is organized in chromosomes, structures which, at greatest magnifications in some cells, somewhat resemble a bottle brush. Whereas a bacterial chromosome consists of a single circular DNA molecule, it is unknown whether mammalian chromosomal DNA is circular, whether each chromosome is a single molecule, or whether each of the protruding filaments from the main axis is a separate discrete molecule of DNA.
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