B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition), страница 10

The translation of genetic informationfrom the 4-letter alphabet of polynucleotides into the 20-letter alphabet of proteins is a complex process. The rules of this translation seem in some respectsneat and rational but in other respects strangely arbitrary, given that they are(with minor exceptions) identical in all living things. These arbitrary features, itis thought, reflect frozen accidents in the early history of life. They stem from thechance properties of the earliest organisms that were passed on by heredity andhave become so deeply embedded in the constitution of all living cells that theycannot be changed without disastrous effects.THE UNIVERSAL FEATURES OF CELLS ON EARTH(A)FOOD INWASTE OUT7(B)amino acidsnucleotidesbuildingblocksenergycatalyticfunctioncell's collectionof catalystsproteinssequenceinformationpolynucleotidesCELL'S COLLECTION OF CATALYSTSCOLLABORATE TO REPRODUCE THEENTIRE COLLECTION BEFOREA CELL DIVIDESIt turns out that the information in the sequence of a messenger RNA moleculeis read out in groups of three nucleotides at a time: each triplet of nucleotides, orcodon, specifies (codes for)a singleamino acid in a corresponding protein.

SinceMBoC6m1.08/1.08the number of distinct triplets that can be formed from four nucleotides is 43,there are 64 possible codons, all of which occur in nature. However, there are only20 naturally occurring amino acids. That means there are necessarily many casesin which several codons correspond to the same amino acid. This genetic code isread out by a special class of small RNA molecules, the transfer RNAs (tRNAs).Each type of tRNA becomes attached at one end to a specific amino acid, anddisplays at its other end a specific sequence of three nucleotides—an anticodon—that enables it to recognize, through base-pairing, a particular codon or subset ofcodons in mRNA.

The intricate chemistry that enables these tRNAs to translatea specific sequence of A, C, G, and U nucleotides in an mRNA molecule into aspecific sequence of amino acids in a protein molecule occurs on the ribosome, alarge multimolecular machine composed of both protein and ribosomal RNA. Allof these processes are described in detail in Chapter 6.Each Protein Is Encoded by a Specific GeneDNA molecules as a rule are very large, containing the specifications for thousands of proteins. Special sequences in the DNA serve as punctuation, definingwhere the information for each protein begins and ends. And individual segmentsof the long DNA sequence are transcribed into separate mRNA molecules, codingfor different proteins.

Each such DNA segment represents one gene. A complication is that RNA molecules transcribed from the same DNA segment can often beprocessed in more than one way, so as to give rise to a set of alternative versionsof a protein, especially in more complex cells such as those of plants and animals.In addition, some DNA segments—a smaller number—are transcribed into RNAmolecules that are not translated but have catalytic, regulatory, or structural functions; such DNA segments also count as genes. A gene therefore is defined as thesegment of DNA sequence corresponding to a single protein or set of alternativeprotein variants or to a single catalytic, regulatory, or structural RNA molecule.In all cells, the expression of individual genes is regulated: instead of manufacturing its full repertoire of possible proteins at full tilt all the time, the celladjusts the rate of transcription and translation of different genes independently,according to need.

Stretches of regulatory DNA are interspersed among the segments that code for protein, and these noncoding regions bind to special proteinmolecules that control the local rate of transcription. The quantity and organization of the regulatory DNA vary widely from one class of organisms to another,but the basic strategy is universal. In this way, the genome of the cell—that is, thetotality of its genetic information as embodied in its complete DNA sequence—dictates not only the nature of the cell’s proteins, but also when and where theyare to be made.Figure 1–8 Life as an autocatalyticprocess.

(A) The cell as a self-replicatingcollection of catalysts. (B) Polynucleotides(the nucleic acids DNA and RNA, which arenucleotide polymers) provide the sequenceinformation, while proteins (amino acidpolymers) provide most of the catalyticfunctions that serve—through a complexset of chemical reactions—to bring aboutthe synthesis of more polynucleotides andproteins of the same types.8Chapter 1: Cells and GenomesLife Requires Free EnergyA living cell is a dynamic chemical system, operating far from chemical equilibrium.

For a cell to grow or to make a new cell in its own image, it must take infree energy from the environment, as well as raw materials, to drive the necessarysynthetic reactions. This consumption of free energy is fundamental to life. Whenit stops, a cell decays toward chemical equilibrium and soon dies.Genetic information is also fundamental to life, and free energy is requiredfor the propagation of this information. For example, to specify one bit of information—that is, one yes/no choice between two equally probable alternatives—costs a defined amount of free energy that can be calculated.

The quantitativerelationship involves some deep reasoning and depends on a precise definition ofthe term “free energy,” as explained in Chapter 2. The basic idea, however, is notdifficult to understand intuitively.Picture the molecules in a cell as a swarm of objects endowed with thermalenergy, moving around violently at random, buffeted by collisions with oneanother.

To specify genetic information—in the form of a DNA sequence, forexample—molecules from this wild crowd must be captured, arranged in a specific order defined by some preexisting template, and linked together in a fixedrelationship. The bonds that hold the molecules in their proper places on thetemplate and join them together must be strong enough to resist the disorderingeffect of thermal motion.

The process is driven forward by consumption of freeenergy, which is needed to ensure that the correct bonds are made, and maderobustly. In the simplest case, the molecules can be compared with spring-loadedtraps, ready to snap into a more stable, lower-energy attached state when theymeet their proper partners; as they snap together into the bonded arrangement,their available stored energy—their free energy—like the energy of the springin the trap, is released and dissipated as heat.

In a cell, the chemical processesunderlying information transfer are more complex, but the same basic principleapplies: free energy has to be spent on the creation of order.To replicate its genetic information faithfully, and indeed to make all its complex molecules according to the correct specifications, the cell therefore requiresfree energy, which has to be imported somehow from the surroundings. As weshall see in Chapter 2, the free energy required by animal cells is derived fromchemical bonds in food molecules that the animals eat, while plants get their freeenergy from sunlight.All Cells Function as Biochemical Factories Dealing with the SameBasic Molecular Building BlocksBecause all cells make DNA, RNA, and protein, all cells have to contain andmanipulate a similar collection of small molecules, including simple sugars,nucleotides, and amino acids, as well as other substances that are universallyrequired.

All cells, for example, require the phosphorylated nucleotide ATP (adenosine triphosphate), not only as a building block for the synthesis of DNA andRNA, but also as a carrier of the free energy that is needed to drive a huge numberof chemical reactions in the cell.Although all cells function as biochemical factories of a broadly similar type,many of the details of their small-molecule transactions differ. Some organisms,such as plants, require only the simplest of nutrients and harness the energy ofsunlight to make all their own small organic molecules.

Other organisms, such asanimals, feed on living things and must obtain many of their organic moleculesready-made. We return to this point later.All Cells Are Enclosed in a Plasma Membrane Across WhichNutrients and Waste Materials Must PassAnother universal feature is that each cell is enclosed by a membrane—theplasma membrane. This container acts as a selective barrier that enables the cellto concentrate nutrients gathered from its environment and retain the products itTHE UNIVERSAL FEATURES OF CELLS ON EARTHFigure 1–9 Formation of a membrane by amphiphilic phospholipidmolecules. Phospholipids have a hydrophilic (water-loving, phosphate) headgroup and a hydrophobic (water-avoiding, hydrocarbon) tail.

At an interfacebetween oil and water, they arrange themselves as a single sheet with theirhead groups facing the water and their tail groups facing the oil. But whenimmersed in water, they aggregate to form bilayers enclosing aqueouscompartments, as indicated.synthesizes for its own use, while excreting its waste products.