Часть 1 (B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (5th edition)), страница 7

Since there are 64(= 4 ¥ 4 ¥ 4) possible codons, all of which occur in nature, but only 20 amino acids,there are necessarily many cases in which several codons correspond to the sameamino acid. The code is read out by a special class of small RNA molecules, thetransfer RNAs (tRNAs).

Each type of tRNA becomes attached at one end to a specific amino acid, and displays at its other end a specific sequence of threenucleotides—an anticodon—that enables it to recognize, through base-pairing, aparticular codon or subset of codons in mRNA (Figure 1–9).For synthesis of protein, a succession of tRNA molecules charged with theirappropriate amino acids have to be brought together with an mRNA molecule andmatched up by base-pairing through their anticodons with each of its successivecodons. The amino acids then have to be linked together to extend the growingprotein chain, and the tRNAs, relieved of their burdens, have to be released.

Thiswhole complex of processes is carried out by a giant multimolecular machine,the ribosome, formed of two main chains of RNA, called ribosomal RNAsTHE UNIVERSAL FEATURES OF CELLS ON EARTH7amino acidsFigure 1–8 Life as an autocatalyticprocess. Polynucleotides (nucleotidepolymers) and proteins (amino acidpolymers) provide the sequenceinformation and the catalytic functionsthat serve—through a complex set ofchemical reactions—to bring about thesynthesis of more polynucleotides andproteins of the same types.nucleotidescatalyticfunctionsequenceinformationproteinspolynucleotides(rRNAs), and more than 50 different proteins. This evolutionarily ancient molecular juggernaut latches onto the end of an mRNA molecule and then trundlesalong it, capturing loaded tRNA molecules and stitching together the aminoacids they carry to form a new protein chain (Figure 1–10).The Fragment of Genetic Information Corresponding to OneProtein Is One GeneDNA molecules as a rule are very large, containing the specifications for thousands of proteins.

Individual segments of the entire DNA sequence are transcribed into separate mRNA molecules, with each segment coding for a differentprotein. Each such DNA segment represents one gene. A complication is thatRNA molecules transcribed from the same DNA segment can often be processedin more than one way, so as to give rise to a set of alternative versions of a protein, especially in more complex cells such as those of plants and animals. Agene therefore is defined, more generally, as the segment of DNA sequence corresponding to a single protein or set of alternative protein variants (or to a single catalytic or structural RNA molecule for those genes that produce RNA butnot protein).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 cell adjuststhe rate of transcription and translation of different genes independently, according to need.

Stretches of regulatory DNA are interspersed among the segmentsFigure 1–9 Transfer RNA. (A) A tRNAmolecule specific for the amino acidtryptophan. One end of the tRNAmolecule has tryptophan attached to it,while the other end displays the tripletnucleotide sequence CCA (its anticodon),which recognizes the tryptophan codonin messenger RNA molecules. (B) Thethree-dimensional structure of thetryptophan tRNA molecule. Note that thecodon and the anticodon in (A) are inantiparallel orientations, like the twostrands in a DNA double helix (see Figure1–2), so that the sequence of theanticodon in the tRNA is read from rightto left, while that of the codon in themRNA is read from left to right.amino acid(tryptophan)specifictRNAmoleculetRNAbinds toits codonin mRNAACCanticodonACCUGGbase-pairinganticodoncodon in mRNA(A)NET RESULT: AMINO ACID ISSELECTED BY ITS CODON(B)8Chapter 1: Cells and GenomesFigure 1–10 A ribosome at work.

(A) The diagram shows how a ribosomemoves along an mRNA molecule, capturing tRNA molecules that match thecodons in the mRNA and using them to join amino acids into a proteinchain. The mRNA specifies the sequence of amino acids. (B) The threedimensional structure of a bacterial ribosome (pale green and blue), movingalong an mRNA molecule (orange beads), with three tRNA molecules(yellow, green, and pink) at different stages in their process of capture andrelease.

The ribosome is a giant assembly of more than 50 individualprotein and RNA molecules. (B, courtesy of Joachim Frank, Yanhong Li andRajendra Agarwal.)growing polypeptide chainincoming tRNAloaded withamino acidSTEP 121234PA34STEP 2that code for protein, and these noncoding regions bind to special proteinmolecules that control the local rate of transcription (Figure 1–11).

Other noncoding DNA is also present, some of it serving, for example, as punctuation,defining where the information for an individual protein begins and ends. Thequantity and organization of the regulatory and other noncoding DNA varywidely from one class of organisms to another, but the basic strategy is universal. In this way, the genome of the cell—that is, the total of its genetic information as embodied in its complete DNA sequence—dictates not only the nature ofthe cell’s proteins, but also when and where they are to be made.two subunitsof ribosome321mRNA42P3A4STEP 32314Life Requires Free Energy2A 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 necessary synthetic reactions. This consumption of free energy is fundamental to life.When it stops, a cell decays towards chemical equilibrium and soon dies.Genetic information is also fundamental to life.

Is there any connection? Theanswer is yes: free energy is required for the propagation of information. Forexample, to specify one bit of information—that is, one yes/no choice betweentwo equally probable alternatives—costs a defined amount of free energy thatcan be calculated. The quantitative relationship involves some deep reasoningand depends on a precise definition of the term “free energy,” discussed inChapter 2. The basic idea, however, is not difficult 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 disordering effect of thermal motion.

The process is driven forward by consumption offree energy, which is needed to ensure that the correct bonds are made, andmade robustly. In the simplest case, the molecules can be compared withspring-loaded traps, ready to snap into a more stable, lower-energy attachedstate when they meet their proper partners; as they snap together into thebonded arrangement, their available stored energy—their free energy—like theenergy of the spring in the trap, is released and dissipated as heat. In a cell, thechemical processes underlying information transfer are more complex, but thesame basic principle applies: free energy has to be spent on the creation of order.To replicate its genetic information faithfully, and indeed to make all itscomplex molecules according to the correct specifications, the cell thereforerequires free energy, which has to be imported somehow from the surroundings.3STEP 42314344new tRNAbringingnext aminoacid545STEP 12313(A)All Cells Function as Biochemical Factories Dealing with the SameBasic Molecular Building BlocksBecause all cells make DNA, RNA, and protein, and these macromolecules arecomposed of the same set of subunits in every case, all cells have to contain and4(B)THE UNIVERSAL FEATURES OF CELLS ON EARTH9manipulate a similar collection of small molecules, including simple sugars,nucleotides, and amino acids, as well as other substances that are universallyrequired for their synthesis.

All cells, for example, require the phosphorylatednucleotide ATP (adenosine triphosphate) as a building block for the synthesis ofDNA and RNA; and all cells also make and consume this molecule as a carrier offree energy and phosphate groups to drive many other chemical reactions.Although all cells function as biochemical factories of a broadly similar type,many of the details of their small-molecule transactions differ, and it is not aseasy as it is for the informational macromolecules to point out the features thatare strictly universal. Some organisms, such as plants, require only the simplestof nutrients and harness the energy of sunlight to make from these almost alltheir own small organic molecules; other organisms, such as animals, feed onliving things and obtain many of their organic molecules ready-made.

We returnto this point below.All Cells Are Enclosed in a Plasma Membrane Across WhichNutrients and Waste Materials Must PassThere is, however, at least one other feature of cells that is universal: each one isenclosed by a membrane—the plasma membrane. This container acts as aselective barrier that enables the cell to concentrate nutrients gathered from itsenvironment and retain the products it synthesizes for its own use, while excreting its waste products.

Without a plasma membrane, the cell could not maintainits integrity as a coordinated chemical system.The molecules forming this membrane have the simple physico-chemicalproperty of being amphiphilic—that is, consisting of one part that is hydrophobic (water-insoluble) and another part that is hydrophilic (water-soluble). Suchmolecules placed in water aggregate spontaneously, arranging their hydrophobic portions to be as much in contact with one another as possible to hide themfrom the water, while keeping their hydrophilic portions exposed. Amphiphilicmolecules of appropriate shape, such as the phospholipid molecules that comprise most of the plasma membrane, spontaneously aggregate in water to form abilayer that creates small closed vesicles (Figure 1–12).

The phenomenon can bedemonstrated in a test tube by simply mixing phospholipids and water together;under appropriate conditions, small vesicles form whose aqueous contents areisolated from the external medium.Although the chemical details vary, the hydrophobic tails of the predominant membrane molecules in all cells are hydrocarbon polymers(–CH2–CH2–CH2–), and their spontaneous assembly into a bilayered vesicle isbut one of many examples of an important general principle: cells producemolecules whose chemical properties cause them to self-assemble into thestructures that a cell needs.The cell boundary cannot be totally impermeable.