5 Структура нуклеиновых кислот (1160074)
Текст из файла
Лекция 5.Структура нуклеиновых кислотC H A P T E RNucleotides and Nucleic AcidsThe final classes of biomolecules to be considered, the nucleotides andmolecules derived from them, represent a clear case in which last is notleast. Nucleotides themselves participate in a plethora of crucial supporting roles in cell metabolism, and polymers of nucleotides, the nucleic acids, provide the script for everything that occurs in a cell.Nucleotides are energy-rich compounds that drive metabolic processes (primarily biosyntheses) in all cells.
They also serve as chemicalsignals, key links in cellular systems that respond to hormones andother extracellular stimuli, and are structural components of a numberof enzyme cofactors and metabolic intermediates.The nucleic acids, deoxyribonucleic acid (DNA) and ribonucleicacid (RNA), are the molecular repositories for genetic information. Thestructure of every protein, and ultimately of every cell constituent, is aproduct of information programmed into the nucleotide sequence of acell's nucleic acids.This chapter provides an overview of the nucleotides and nucleicacids found in most cells.
The metabolism of nucleotides is discussed inChapter 21, and a more detailed examination of the function of nucleicacids is the focus of Part IV of this text.Some BasicsThe amino acid sequence of every protein and the nucleotide sequenceof every RNA molecule in a cell are specified by that cell's DNA. Thenecessary protein or RNA sequence information is found in corresponding nucleotide sequences in the DNA. A segment of DNA thatcontains the information required for the synthesis of a functional biological product (protein or RNA) is referred to as a gene. A cell typically has many thousands of genes, and DNA molecules, not surprisingly, tend to be very large. The storage of biological information is theonly known function of DNA.Several classes of RNAs are found in cells, each with a distinctfunction.
Ribosomal RNAs (rRNA) are structural components of ribosomes, the large complexes that carry out the synthesis of proteins.Messenger RNAs (mRNA) are nucleic acids that carry the information from one or a few genes to the ribosome, where the correspondingproteins can be synthesized. Transfer RNAs (tRNA) are adapter molecules that faithfully translate the information in mRNA into a specificsequence of amino acids. In addition to these major classes there are a324Chapter 12 Nucleotides and Nucleic Acids325wide variety of special-function RNAs, described in depth in Part IV.We introduce here the chemical structures of nucleotides and nucleicacids.Purine or •pyrimidine jbasePhosphate "O—P—O—CH.Nucleotides Have Characteristic Bases and PentosesNucleotides have three characteristic components: (1) a nitrogenousbase, (2) a pentose, and (3) a phosphate (Fig.
12-la). The nitrogenousbases are derivatives of two parent compounds, pyrimidine and purine (Fig. 12-lb). The bases and pentoses found in the common nucleotides are heterocyclic compounds. The carbon and nitrogen atoms inthe parent structures are conventionally numbered to facilitate naming and identification of the many derivative compounds. The convention for the pentose ring follows rules outlined in Chapter 11, but in thepentoses of nucleotides the carbon numbers are given a prime (') designation (Fig.
12-la) to distinguish them from the numbered atoms ofthe nitrogenous bases.The base is joined covalently (at N-l of pyrimidines and N-9 ofpurines) in an iV-glycosidic linkage to the 1' carbon of the pentose, andthe phosphate is esterified to the 5' carbon. The iV-glycosidic bond isformed by removal of the elements of water (a hydroxyl group from thepentose and hydrogen from the base), as in O-glycosidic bond formation (see Fig. 11-11). Without th,e phosphate group, the molecule iscalled a nucleoside.DNA and RNA both contain two major purine bases, adenine (A)and guanine (G).
DNA and RNA also contain two major pyrimidines;in both types of nucleic acid one of these is cytosine (C). The singleimportant difference between the bases of DNA and those of RNA isthe nature of the second major pyrimidine: thymine (T) in DNA anduracil (U) in RNA. Only rarely does thymine occur in RNA or uracil inDNA. The structures of the five major bases are shown in Figure 12-2,and the nomenclature of their corresponding nucleotides and nucleosides is summarized in Table 12-1.PentoseOH(a)HPyrimidinePurineFigure 12-1 (a) The general structure of nucleotides, showing the numbering convention for thepentose.
The structure shown is that of a ribonucleotide. In deoxyribonucleotides the —OH group onthe 2' carbon (in red) is replaced with —H. (b) Theparent compounds of the pyrimidine and purinebases of nucleotides and nucleic acids, showing thenumbering conventions for the ring structures.ONH 2PurinesAdenineGuanineAdenosineDeoxyadenosineAdenylateDeoxyadenylateGuanosineDeoxyguanosineGuanylateDeoxyguanylateHCytosineCytidineDeoxycytidineCytidylateDeoxycytidylateThymineThymidine ordeoxythymidineThymidylate ordeoxythymidylateUracilUridineUridylateRNADNADNARNA* Nucleoside and nucleotide are generic terms that include both ribo- and deoxyribo- forms. Notethat here ribonucleosides and ribonucleotides are designated simply as nucleosides and nucleotides (e.g., riboadenosine as adenosine) and deoxyribonucleosides and deoxyribonucleotides asdeoxynucleosides and deoxynucleotides (e.g., deoxyriboadenosine as deoxyadenosine).
Both formsof naming are acceptable, but the shortened names are more commonly used.CHyNHGuaninePurines00NH 2CH 3CHPyrimidinesN\/^H2NAdenineNucleic acidRNADNARNADNAHN1CH1Table 12—1 Nucleotide and nucleic acid nomenclatureNucleotide*9/^NH(b)1Nucleoside*,NA\sCHHCJN\^ \BaseOH !NHThymine(DNA)^CNi1CHiiII^CH0^^ a ^N^HCytosineHN'~CHii0nII^N^HUracil(RNA)PyrimidinesFigure 12-2 The major purine and pyrimidinebases of nucleic acids. Some of the common namesof these bases reflect the circumstances of their discovery. Guanine, for example, was first isolatedfrom guano (bird manure), and thymine was firstisolated from thymus tissue.326CH2OHAldehydeFigure 12-3 The straight-chain (aldehyde) andring (/3-furanose) forms of ribose.
When ribose isfree in solution, the two forms are in equilibrium.RNA contains only the ring form, /3-n-ribofuranose.Deoxyribose undergoes a similar interconversion insolution, but in DNA exists solely as /3-2'-deoxy-Dribofuranose.Two kinds of pentoses are found in nucleic acids. The recurringdeoxyribonucleotide units of DNA contain 2'-deoxy-D-ribose, and theribonucleotide units of RNA contain D-ribose. In nucleotides, bothtypes of pentoses are in their /3-furanose (closed five-member ring)form (Fig.
12-3).Figure 12-4 gives the structures and names of the four major deoxyribonucleotides (deoxyribonucleoside 5'-monophosphates), thestructural units of DNAs, and the four major ribonucleotides (ribonucleoside 5'-monophosphates), the structural units of RNAs. Specificlong sequences of A, T, G, and C nucleotides in DNA encode the geneticinformation. Although nucleotides bearing one of these major basesare most common, both DNA and RNA also contain some minor basesO"O-P-O—CHOONucleotide: Deoxyadenylate,deoxyadenosineDeoxyguanylate,deoxyguanosineDeoxythymidylate,deoxythymidineDeoxycytidylate,deoxycytidine5 '-monophosphate5'-monophosphate5'-monophosphate5 '-monophosphateSymbols: A, dA, dAMPG, dG, dGMPT, dT, dTMPC, dC, dCMPNucleoside: DeoxyadenosineDeoxyguanosineDeoxythymidineDeoxycytidine(a)NBOO"OIO-P-O-CH.OIO—P—O—CH-0—P—O— CH 2 /OOoCytidylate, cytidine5 '-monophosphateGuanylate, guanosine5'-monophosphateUridylate, uridine5' -monophosphateSymbols: A, AMPG, GMPU, UMPC, CMPNucleoside: AdenosineGuanosineUridineCytidineNucleotide: Adenylate, adenosine5'-monophosphate(b)Figure 12-4 (a) The deoxyribonucleotide units ofDNA in free form at pH 7.0.
In DNA they are usually symbolized as A, G, T, and C, and sometimesas dA, dG, dT, and dC. In their free form thesenucleotides are commonly abbreviated dAMP,dGMP, dTMP, and dCMP. (b) The ribonucleotideunits of RNAs. All abbreviations assume that thephosphate group is at the 5' position. The nucleoside portion of each molecule is shaded in red.
Inthis and the following illustrations, the ring carbons are not shown in the purine and pyrimidinebases, as is also the convention for the pentoses.Chapter 12 Nucleotides and Nucleic AcidsNH 2327OCH,N 6-MethyladenineCHoOHHINH9NCH3-iV2-Methylguanine7-Methylguanine5-Hydroxymethylcytosine(a)(b)(Fig. 12-5).
In DNA the most common of these are methylated forms ofthe major bases, but in some viral DNAs certain bases may behydroxymethylated or glucosylated. Such altered or unusual bases inDNA molecules are in many cases specific signals for regulating orprotecting the genetic information. Minor bases of many types are alsofound in RNAs, especially in tRNA.The nomenclature used for the minor bases can be confusing. Asindicated in Figures 12-4 and 12-5, many of the minor bases (such ashypoxanthine) have common names, just as the major bases do. Forsubstituted forms of these bases, when the substitution involves anatom in the purine or pyrimidine rings, the usual convention (usedhere) is simply to indicate the ring position of the substitution by itsnumber (e.g., 5-methylcytosine, 7-methylguanine, and 5-hydroxymethylcytosine in Fig.
Характеристики
Тип файла PDF
PDF-формат наиболее широко используется для просмотра любого типа файлов на любом устройстве. В него можно сохранить документ, таблицы, презентацию, текст, чертежи, вычисления, графики и всё остальное, что можно показать на экране любого устройства. Именно его лучше всего использовать для печати.
Например, если Вам нужно распечатать чертёж из автокада, Вы сохраните чертёж на флешку, но будет ли автокад в пункте печати? А если будет, то нужная версия с нужными библиотеками? Именно для этого и нужен формат PDF - в нём точно будет показано верно вне зависимости от того, в какой программе создали PDF-файл и есть ли нужная программа для его просмотра.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
