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Berg JM, Tymoczko JL, Stryer L. Biochemistry. Fifth edition. New York: W H Freeman; 2002.

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The main point acids DNA and also RNA are well suitable to duty as the carrier of geneticinformation through virtue of their covalent structures. These macromolecules arelinear polymers accumulated from similar units associated end toend (Figure 5.1). Every monomer unit withinthe polymer consists of 3 components: a sugar, a phosphate, and a base. Thesequence that bases unique characterizes a nucleic acid and also represents a kind oflinear information.


5.1.1. RNA and also DNA differ in the street Component and One of the Bases

The sugar in deoxyribonucleic mountain (DNA) isdeoxyribose. The deoxy prefix suggests that the 2′ carbonatom the the street lacks the oxygen atom the is connected to the 2′ carbon atom ofribose (the sugar in ribonucleic acid, orRNA), as displayed in Figure5.2. The sugars in nucleic acids are connected to one one more byphosphodiester bridges. Specifics the 3′-hydroxyl (3′-OH) team of the sugarmoiety the one nucleotide is esterified come a phosphate group, i m sorry is, in turn,joined come the 5′-hydroxyl group of the nearby sugar. The chain of sugarslinked through phosphodiester bridges is described as the backboneof the nucleic mountain (Figure 5.3). Whereasthe backbone is continuous in DNA and also RNA, the bases differ from one monomer come thenext. 2 of the bases are derivatives of purine—adenine (A)and guanine (G)—and 2 of pyrimidine—cytosine (C) and thymine(T, DNA only) or uracil (U, RNA only), as shown in figure 5.4.


Figure 5.2

Ribose and Deoxyribose. Atoms space numbered through primes to distinguish them from atom inbases (see figure 5.4).


Figure 5.3

Backbones of DNA and also RNA. The backbones that these nucleic acids are formed by 3′-to-5′phosphodiester linkages. A sugar unit is highlighted in red and aphosphate team in blue.


Figure 5.4

Purines and also Pyrimidines. Atoms within bases are numbered without primes. Uracil instead ofthymine is provided in RNA.

RNA, choose DNA, is a long unbranched polymer consists of nucleotides joined by3′→5′ phosphodiester bonds (see Figure5.3). The covalent framework of RNA different from the of DNA in tworespects. As stated earlier and also as indicated by the name, the sugar devices in RNAare riboses quite than deoxyriboses. Ribose has a 2′-hydroxyl group notpresent in deoxyribose. As a consequence, in addition to the traditional 3′→5′linkage, a 2′→5′ link is possible for RNA. This later on linkage is essential inthe removal of introns and also the joining of exons for the formation of tires RNA(Section 28.3.4). The otherdifference, as currently mentioned, is that among the four major bases in RNA isuracil (U) rather of thymine (T).

Note that each phosphodiester bridge has a negative charge. This an unfavorable chargerepels nucleophilic varieties such together hydroxide ion; consequently, phosphodiesterlinkages space much less susceptible come hydrolytic assault than are various other esterssuch together carboxylic acid esters. This resistance is critical for maintaining theintegrity of information stored in nucleic acids. The lack of the 2′-hydroxylgroup in DNA additional increases that is resistance come hydrolysis. The greaterstability the DNA probably accounts for its use rather than RNA as the hereditarymaterial in all contemporary cells and also in plenty of viruses.

Structural Insights, nucleic Acids

offers a three-dimensional perspective on nucleotide structure, basepairing, and also other facets of DNA and RNA structure.

A unit consists of a base bonded to a street is referred to as anucleoside. The four nucleosideunits in RNA are called adenosine, guanosine, cytidine, anduridine, conversely, those in DNA room calleddeoxyadenosine, deoxyguanosine, deoxycytidine, andthymidine. In each case, N-9 that a purine or N-1 the apyrimidine is attached to C-1′ of the sugar (Figure 5.5). The base lies above the aircraft of sugar when thestructure is composed in the conventional orientation; the is, the construction ofthe N-glycosidic link is β. Anucleotide is a nucleosidejoined come one or an ext phosphate teams by one ester linkage. The most usual siteof esterification in naturally emerging nucleotides is the hydroxyl groupattached come C-5′ that the sugar. A compound formed by the attachments of aphosphate team to the C-5′ that a nucleoside sugar is dubbed a nucleoside5′-phosphate or a5′-nucleotide. For example, ATP isadenosine 5′-triphosphate. Anothernucleotide is deoxyguanosine 3′-monophosphate (3′-dGMP; figure 5.6). This nucleotide different from ATP in that itcontains guanine quite than adenine, consists of deoxyribose fairly than ribose(indicated through the prefix “d”), contains one quite than 3 phosphates, andhas the phosphate esterified to the hydroxyl team in the 3′ quite than the 5′position. Nucleotides room the monomers that are linked to kind RNA and also DNA. Thefour nucleotide units in DNA are referred to as deoxyadenylate, deoxyguanylate,deoxycytidylate, and also deoxythymidylate, andthymidylate. Note that thymidylate has deoxyribose; byconvention, the prefix deoxy is not added because thymine-containing nucleotidesare only rarely found in RNA.

Figure 5.6

Nucleotides adenosine 5′ -triphosphate (5′-ATP) anddeoxyguanosine 3′-monophosphate (3′-dGMP).

The abbreviated symbol pApCpG or pACG denote a trinucleotide of DNA consistingof the structure blocks deoxyadenylate monophosphate, deoxycytidylatemonophosphate, and deoxyguanylate monophosphate linked by a phosphodiesterbridge, wherein “p” denotes a phosphate group (Figure 5.7). The 5′ end will regularly have a phosphate attached to the5′-OH group. Note that, prefer a polypeptide (see section 3.2), a DNA chain has actually polarity. One finish ofthe chain has actually a cost-free 5′-OH group (or a 5′-OH group attached to a phosphate),whereas the other end has a 3′-OH group, neither of i m sorry is attached to anothernucleotide. By convention, the base sequence is composed in the5′-to-3′ direction. Thus, thesymbol ACG indicates that the unlinked 5′-OH group is ~ above deoxyadenylate, whereasthe unlinked 3′-OH team is on deoxyguanylate. Thus polarity, ACG andGCA correspond to different compounds.

Figure 5.7

Structure of a DNA Chain. The chain has a 5′ end, i beg your pardon is usually attached to a phosphate, anda 3′ end, which is generally a totally free hydroxyl group.

A striking characteristic of naturally emerging DNA molecules is your length. ADNA molecule must consist of many nucleotides to carry the genetic informationnecessary for also the easiest organisms. Because that example, the DNA of a virus suchas polyoma, which can cause cancer in details organisms, is as long as 5100nucleotides in length. We can quantify the info carrying volume ofnucleic acids in the adhering to way. Each position deserve to be one of 4 bases,corresponding to two bits of information (22 = 4). Thus, a chain of5100 nucleotides coincides to 2 × 5100 = 10,200 bits, or 1275 bytes (1 byte =8 bits). The E. Coli genome is a single DNA molecule consistingof 2 chains of 4.6 million nucleotides, matching to 9.2 million bits, or1.15 megabytes, of info (Figure5.8).

Figure 5.8

Electron Micrograph of component of the E. Coligenome.

DNA molecule from higher organisms deserve to be lot larger. The human being genomecomprises about 3 exchange rate nucleotides, divided among 24 unique DNAmolecules (22 autosomes, x and also y sex chromosomes) of different sizes. Among thelargest known DNA molecules is uncovered in the Indian muntjak, one Asiatic deer; itsgenome is almost as huge as the person genome yet is spread on only 3chromosomes (Figure 5.9). The biggest ofthese chromosomes has chains of an ext than 1 exchange rate nucleotides. If such a DNAmolecule could be completely extended, it would certainly stretch much more than 1 foot in length.Some tree contain even larger DNA molecules.

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Figure 5.9

The Indian Muntjak and Its Chromosomes. Cell from a mrs Indian muntjak (right) contain three pairs ofvery big chromosomes (stained orange). The cell shown is a hybridcontaining a pair of human chromosomes (stained green) forcomparison. <(Left) (more...)

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