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Lodish H, Berk A, Zipursky SL, et al. Molecular cabinet Biology. Fourth edition. New York: W. H. Freeman; 2000.

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DNA and RNA have good chemical similarities. In your primarystructures both are linear polymers (multiple chemical units) composed of monomers (single chemical units), referred to as nucleotides. Moving RNAs selection inlength from less than one hundred to many thousands that nucleotides. Cellular DNAmolecules have the right to be as long as several hundred million nucleotides. These large DNAunits in association through proteins have the right to be stained with dyes and visualized in thelight microscopic lense as chromosomes.

Polymerization that Nucleotides develops Nucleic Acids

DNA and RNA each consists of only four various nucleotides. Every nucleotideshave a usual structure: a phosphate group linked by aphosphoester bond to a pentose (a five-carbon sugar molecule)that in turn is linked to an organic base (Figure 4-1a). In RNA, the pentose isribose; in DNA, it is deoxyribose (Figure 4-1b). The just other difference inthe nucleotides the DNA and RNA is that among the 4 organic bases differsbetween the two polymers. The bases adenine, guanine, and also cytosine are discovered inboth DNA and also RNA; thymine is discovered only in DNA, and also uracil is uncovered only in RNA.The bases are frequently abbreviated A, G, C, T, and U, respectively. Because that conveniencethe single letters are likewise used when long sequences of nucleotides space writtenout.


Figure 4-1

All Nucleotides have actually a usual structure. (a) Chemical framework of adenosene 5′-monophosphate (AMP),a nucleotide that is current in RNA. All nucleotides room composed ofa phosphate moiety, containing increase to 3 phosphate groups, linkedto the (more...)

The base materials of main point acids are heterocyclic compounds with the ringscontaining nitrogen and carbon. Adenine and guanine space purines, i m sorry contain a pair offused rings; cytosine, thymine, and uracil room pyrimidines, i m sorry contain a single ring (Figure 4-2). The acidic character ofnucleotides is because of the visibility of phosphate, i beg your pardon dissociates in ~ the pHfound within cells, releasing hydrogen ions and also leaving the phosphate negativelycharged (see figure 2-22). Since thesecharges attract proteins, many nucleic mountain in cells are associated withproteins. In nucleotides, the 1′ carbon atom that the street (ribose ordeoxyribose) is attached to the nitrogen at place 9 of a purine(N9) or at place 1 that a pyrimidine (N1).


Figure 4-2

The chemical frameworks of the primary bases in nucleicacids. In nucleic acids and nucleotides, nitrogen 9 that purines and also nitrogen1 the pyrimidines (red) space bonded to the 1′ carbon ofribose or deoxyribose.

Cells and also extracellular fluids in biology contain little concentrations of nucleosides, combinations of a baseand a sugar without a phosphate. Nucleotides space nucleosides that have actually one, two,or three phosphate groups esterified at the 5′ hydroxyl.Nucleoside monophosphates have a solitary esterifiedphosphate (see number 4-1a),diphosphates contain a prophosphate group


and triphosphates have actually a third phosphate. Table 4-1 lists the surname of the nucleosides andnucleotides in main point acids and the various develops of nucleoside phosphates. Aswe will check out later, the nucleoside triphosphates are supplied in the synthetic ofnucleic acids. However, these compounds also serve numerous other functions in thecell: ATP, for example, is the most widely used power carrier in the cabinet (seeFigure 2-25), and also GTP dram crucialroles in intracellular signaling and acts together an power reservoir, particularlyin protein synthesis.


When nucleotides polymerize to form nucleic acids, the hydroxyl team attached tothe 3′ carbon of a street of one nucleotide develops an ester bond to thephosphate of another nucleotide, removed a molecule the water:

This condensation reaction is comparable to the in i m sorry a peptide shortcut is formedbetween two amino acids (Chapter3). Therefore a single nucleic mountain strand is a phosphate-pentose polymer (apolyester) v purine and pyrimidine bases as side groups. The links betweenthe nucleotides are called phosphodiesterbonds. Choose a polypeptide, a nucleic acid strand has actually an end-to-endchemical orientation: the 5′ finish hasa cost-free hydroxyl or phosphate group on the 5′ carbon the its terminalsugar; the 3′ end has a freehydroxyl team on the 3′ carbon of its terminal street (Figure 4-3). This directionality, to add thefact the synthesis proceeds 5′ to 3′, has offered rise come theconvention the polynucleotide sequences are written and also read in the5′ → 3′ direction (from left come right); forexample, the succession AUG is presume to be (5′)AUG(3′).(Although, strict speaking, the letter A, G, C, T, and U stand for bases,they are additionally often supplied in diagrams to represent the totality nucleotidescontaining this bases.) The 5′ → 3′directionality of a nucleic acid strand is an extremely important property ofthe molecule.

Figure 4-3

Alternative means of representing nucleic acid chains, in thiscase a solitary strand of DNA containing only three bases: cytosine(C), adenin (A), and also guanine (G). (a) Chemical framework of the trinucleotide CAG. Keep in mind the freehydroxyl team at the 3′ (more...)

The direct sequence of nucleotides linked by phosphodiester bonds constitutes theprimary structure of main point acids. As we comment on in the next section,polynucleotides have the right to twist and also fold right into three-dimensional conformationsstabilized by noncovalent bonds; in this respect, castle are comparable topolypeptides. Return the major structures that DNA and also RNA room generallysimilar, their conformations are quite different. Unequal RNA, i m sorry commonlyexists as a solitary polynucleotide chain, or strand, DNA has two intertwinedpolynucleotide strands. This structural difference is crucial to the differentfunctions the the two varieties of main point acids.

Native DNA Is a twin Helix of security Antiparallel Chains

The modern era of molecule biology began in 1953 once James D. Watson andFrancis H. C. Crick proposed correctly the double-helical framework of DNA,based top top the analysis of x-ray diffraction fads coupled with cautious modelbuilding. A closer look at the “thread the life,” together the DNAmolecule is sometimes called, mirrors why the discovery of its basic structuresuggests that is function.

DNA consists of two linked polynucleotide strands the windtogether through space to form a structure regularly describedas a twin helix. The 2 sugar-phosphate backbones room on theoutside of the twin helix, and the bases project into the interior. Theadjoining bases in every strand stack on top of one one more in parallel planes(Figure 4-4a). The orientation of thetwo strands is antiparallel; that is, their 5′ →3′ directions are opposite. The strands are held in an accurate registerby a consistent base-pairing in between the two strands: A is paired v T throughtwo hydrogen bonds; G is paired through C through three hydrogen binding (Figure 4-4b). This base-paircomplementarity is a repercussion of the size, shape, and also chemicalcomposition the the bases. The visibility of thousands of such hydrogen binding in aDNA molecule contributes greatly to the stability of the double helix.Hydrophobic and also van der Waals interactions between the stacked adjacent basepairs additionally contribute to the stability of the DNA structure.

Figure 4-4

Two depictions of contacts within the DNA doublehelix. (a) Space-filling design of B DNA, the most common kind of DNA incells. The sugar and also phosphate residues (gray) in every strand formthe backbone, i m sorry is traced by a red line, reflecting the helicaltwist (more...)

To maintain the geometry that the double-helical structure presented in number 4-4a, a bigger purine (A or G) mustpair through a smaller sized pyrimidine (C or T). In organic DNA, A practically alwayshydrogen bonds v T and also G with C, forming A·T and G·Cbase pairs often called Watson-Crick basic pairs. Twopolynucleotide strands, or areas thereof, in which all the nucleotides formsuch base pairs are stated to it is in complementary. However, in theory and also in artificial DNAs otherinteractions have the right to occur. Because that example, a guanine (a purine) might theoreticallyform hydrogen bonds v a thymine (a pyrimidine), resulting in only a minordistortion in the helix. The space available in the helix also would allowpairing between the two pyrimidines cytosine and also thymine. Although thenonstandard G·T and also C·T basic pairs are usually not foundin DNA, G·U base pairs room quite common in double-helical regionsthat type within otherwise single-stranded RNA.

Two polynucleotide strands can, in principle, kind either a right-handed or aleft-handed helix (Figure 4-5). Becausethe geometry of the sugar-phosphate backbone is an ext compatible through the former,natural DNA is a right-handed helix. The x-ray diffraction sample of DNAindicates the the stack bases are on regular basis spaced 0.34 nm apart follow me thehelix axis. The helix provides a finish turn every 3.4 nm; therefore there are about10 pairs every turn. This is referred to as the B kind of DNA,the normal form present in most DNA stretches in cell (Figure 4-6a). ~ above the external of B-form DNA, the spacesbetween the intertwined strands form two helical grooves of different widthsdescribed together the significant groove and the minorgroove (see figure 4-4a). Consequently,part of each base is available from exterior the helix come both small and largemolecules that bind to the DNA by contacting chemical groups within the grooves.These two binding surface of the DNA molecule are provided by different classes ofDNA-binding proteins.

Figure 4-5

Two possible helical forms of DNA room mirror images of eachother. The geometry that the sugar-phosphate backbone the DNA causes naturalDNA to be right-handed. (Right-handed andleft-handed are defined by convention.)

Figure 4-6

Models of miscellaneous DNA structures that are known to exist. The sugar-phosphate backbone of each chain is on the exterior in allstructures (one red and also one blue) with the bases (silver) orientedinward. Side views are presented at the top, and views along the (more...)

In enhancement to the major B form of DNA, three added structures have actually beendescribed. In really low humidity, the crystallographic framework of B DNA changesto the A form; RNA-DNA and also RNA-RNA helices also exist in thisform. The A kind is an ext compact 보다 the B form, having 11 bases every turn, andthe stacked bases room tilted (Figure4-6b). Short DNA molecules created of alternating purine-pyrimidinenucleotides (especially Gs and Cs) embrace an different left-handedconfiguration rather of the regular right-handed helix. This framework is calledZ DNA due to the fact that the bases seem to zigzag when viewed from theside (Figure 4-6c). The is entirelypossible the both A-form and Z-form stretches of DNA exist in cells.

Finally, a triple-stranded DNA structure can also exist at least in the testtube, and also possibly throughout recombination and DNA repair. For example, whensynthetic polymers the poly(A) and polydeoxy(U) room mixed, a three-strandedstructure is developed (Figure 4-6d).Further, lengthy homopolymeric follow me of DNA created of C and also T residues in onestrand and also A and also G residues in the other have the right to be target by brief matchinglengths that poly(C+T). The man-made oligonucleotide deserve to insert together athird strand, binding in a sequence-specific way by so-calledHoogsteen basic pairs. Particular cleavage of the DNA in ~ thesite where the triple helix ends can be achieved by attaching a chemicalcleaving agent (e.g., Fe2+-EDTA) come the shortoligodeoxynucleotide that makes up the 3rd strand. Together reactions may beuseful in researching site-specific DNA damages in cells.

By much the many important modifications in typical B-form DNA come around as aresult that protein binding to specific DNA sequences. Back the multitude ofhydrogen and also hydrophobic bonds in between the polynucleotide strands providestability come DNA, the double helix is somewhat flexible about its long axis.Unlike the α helix in proteins (see number 3-6), there are no hydrogen bonds in between successive residuesin a DNA strand. This prop- erty allows DNA to bend once complexed with aDNA-binding protein. Crystallographic analyses of proteins bound come particularregions of DNA have conclusively prove departures from the conventional B-DNAstructure in protein-DNA complexes. Two examples of DNA deformed by contact withproteins are shown in number 4-7. Thespecific DNA-protein contacts that happen in these tightly tied complexes havethe capacity both come untwist the DNA and also to bending the axis of the helix. AlthoughDNA in cells most likely exists in the B type most that the time, details regionsbound to protein plainly depart from the standard conformation.

Figure 4-7

Bending that OF DNA result from protein binding. (a) A linear DNA (left) is presented binding a repressorprotein encoded by bacteriophage 434 (center); theresulting bend in the DNA (right) is easily seen bycomparison with the linear molecule. Binding (more...)

DNA deserve to Undergo Reversible Strand Separation

In DNA replication and also in the copying of RNA native DNA, the strands of the helixmust different at the very least temporarily. As we comment on later, throughout DNA synthesistwo new strands space made (one copied from every of the original strands),resulting in two double helices the same with the original one. In the case ofcopying the DNA layout to do RNA, the RNA is released and also the 2 DNAstrands reassociate through each other.

The unwinding and also separation that DNA strands, referred to as denaturation, or“melting,” can be induced experimentally. For example, if asolution of DNA is heated, the thermal energy increases molecular motion,eventually break the hydrogen bonds and other forces that stabilize thedouble helix, and the strands separate (Figure4-8). This melt of DNA transforms its absorption of ultraviolet (UV)light (in the 260-nm range), i beg your pardon is routinely supplied to measure up DNAconcentration since of the high absorbance the UV irradiate by nucleic acid bases.Native double-stranded DNA absorbs around one-half as lot light at 260 nm asdoes the tantamount amount that single-stranded DNA (Figure 4-9a). Thus, as DNA denatures, its absorption of UVlight increases. Close to the denaturation temperature, a tiny increase intemperature causes an abrupt, near simultaneous, loss of the multiple, weak,cooperative interactions holding the 2 strands together, so the denaturationrapidly wake up throughout the whole length that the DNA.

Figure 4-9

Light absorption and also temperature in DNA denaturation. (a) melting of doubled-stranded DNA can be monitored through theabsorption that ultraviolet irradiate at 260 nm. As regions ofdouble-stranded DNA unpair, the absorb of light by those regionsincreases nearly (more...)

The melt temperature, Tm, in ~ which the strands ofDNA will certainly separate counts on numerous factors. Molecules that contain a greaterproportion that G·C bag require higher temperatures come denaturebecause the three hydrogen bonds in G·C pairs do them an ext stablethan A·T pairs with two hydrogen binding (see number 4-4b). Indeed, the percentage of G·Cbase pairs in a DNA sample have the right to be approximated from itsTm (Figure4-9b). In enhancement to heat, services of low ion concentrationdestabilize the twin helix, resulting in it to melt at lower temperatures. DNA isalso denatured by exposure to various other agents the destabilize hydrogen bonds, suchas alkaline solutions and also concentrated services of formamide or urea:

The single-stranded DNA molecule that result from denaturation form random coilswithout a continual structure. Lowering the temperature or boosting the ionconcentration causes the two complementary strands to reassociate right into a perfectdouble helix (see number 4-8). The extentof together renaturation is dependence on time, the DNAconcentration, and also the ionic contents of the solution. Two DNA strands notrelated in sequence will remain as arbitrarily coils and also will not renature and, mostimportant, will certainly not considerably inhibit complementary DNA companion strands fromfinding every other. Denaturation and renaturation the DNA are the basis ofnucleic mountain hybridization, apowerful an approach used to research the relatedness of 2 DNA samples and also todetect and isolate particular DNA molecules in a mixture containing numerousdifferent DNA order (Chapter7).

Many DNA Molecules room Circular

All prokaryotes genomic DNAs and also many viral DNAs space circular molecules. CircularDNA molecules additionally occur in mitochondria, i m sorry are existing in almost alleukaryotic cells, and in chloroplasts, i beg your pardon are present in plants and someunicellular eukaryotes.

Each of the two strands in a one DNA molecule creates a close up door structurewithout free ends. Simply as is the situation for linear DNA, elevated temperature oralkaline pH destroy the hydrogen bonds and also other interactions that stabilizedouble-helical circular DNA molecules. Unlike direct DNA, however, the twostrands of one DNA can not unwind and also separate; attempts come melt such DNAresult in an interlocked, tangled massive of single-stranded DNA (Figure 4-10a).

Figure 4-10

Denaturation of one DNA. (a) If both strands room closed circles, denaturation disrupts thedouble helix, but the two solitary strands become tangled around eachother and also cannot separate. (b) If one or both strands room nicked,however, the two strands (more...)

Only if a native circular DNA is nicked (i.e., one of thestrands is cut), will the two strands unwind and also separate when the molecule isdenatured. In this case, among the be separate strands is circular, and the otheris direct (Figure 4-10b). Nicking ofcircular DNA occurs naturally during DNA replication and also can be inducedexperimentally with a short concentration the deoxyribonuclease (a DNA-degradingenzyme), therefore that just a solitary phosphodiester bond in the molecule is cleaved.The research of one DNA molecules lacking free ends an initial uncovered thecomplicated geometric shape alters that the double-stranded DNA molecule mustundergo as soon as the strands are not cost-free to separate.

Local Unwinding of DNA cause Supercoiling

So much we have explained DNA together a long consistent helical framework that have the right to havelocal perturbations, especially as result of protein binding. In addition, when thetwo ends of a DNA molecule room fixed, the molecule exhibits a superstructureunder certain conditions. This occurs as soon as the base pairing is interrupted and also alocal an ar unwinds. The stress and anxiety induced by unwinding is relieved through twisting ofthe twin helix ~ above itself, creating supercoils (Figure 4-11). Unwinding and also subsequentsupercoiling occurs throughout replication, transcription, and also binding of manyproteins come circular DNAs or to lengthy DNA loops whose ends are fixed withineukaryotic chromosomes. Supercoiling is recognized and also regulated through enzymescalled topoisomerases. As discussedin later chapters, these enzymes have an essential role in both DNA replicationand the warrior of DNA right into RNA.

Figure 4-11

Supercoiling in electron micrographs that DNA isolated indigenous theSV40 virus. Once isolated SV40 DNA is separated native its associated protein, theDNA duplex is underwound and also assumes the supercoiled configuration(form I). If one strand is nicked, the strands (more...)

RNA molecule Exhibit varied Conformations and Functions

As noted earlier, the main structure of RNA is generally comparable to that ofDNA; however, the street component (ribose) the RNA has an additional hydroxylgroup in ~ the 2′ position (see Figure4-1b), and thymine in DNA is changed by uracil in RNA (see number 4-2). The hydroxyl group onC2 of ribose makes RNA an ext chemically labile 보다 DNA andprovides a chemically reactive group that takes component in RNA-mediated enzymaticevents. Together a an outcome of this lability, RNA is cleaved into mononucleotides byalkaline solution, vice versa, DNA is not. Prefer DNA, RNA is a long polynucleotidethat deserve to be double-stranded or single-stranded, straight or circular. It have the right to alsoparticipate in a hybrid helix created of one RNA strand and one DNA strand;this hybrid has a slightly different conformation than the common B kind ofDNA.

Unlike DNA, which exists generally in a single, an extremely long three-dimensionalstructure, the dual helix, the various species of RNA exhibit differentconformations. Differences in the sizes and also conformations of the assorted typesof RNA permit them to carry out particular functions in a cell. The simplestsecondary frameworks in single-stranded RNAs are formed by pairing ofcomplementary bases. “Hairpins” are developed by pairing ofbases within ≈5 – 10 nucleotides of eachother, and also “stem-loops” through pairing the bases the areseparated by ≈50 to several hundred nucleotides (Figure 4-12a). These straightforward folds cancooperate to form more facility tertiary structures, one of which is termed a“pseudoknot” (Figure4-12b).

Figure 4-12

RNA secondary and tertiary structures. (a) Stem-loops, hairpins, and also other second structures can type bybase pairing in between distant complementary segment of an RNAmolecule. In stem-loops, the single-stranded loop (dark red) betweenthe base-paired (more...)

As disputed in information later, tRNA molecules adopt a well-definedthree-dimensional style in solution that is an important in protein synthesis.Larger rRNA molecules likewise have locally well identified three-dimensionalstructures, with much more flexible links in between. Secondary and tertiarystructures additionally have been recognized in mRNA, an especially near the ends ofmolecules. This recently found structures are under active study. Clearly,then, RNA molecules are prefer proteins in the they have actually structured domainsconnected by less structured, versatile stretches.

The folded domains of RNA molecules not just are structurally analogous to theα helices and also β strands uncovered in proteins, yet in part casesalso have actually catalytic capacities. Together catalytic RNAs, called ribozymes, can reduced RNA chains. SomeRNA domains additionally can catalyze RNA splicing, a remarkableprocess in which an inner RNA sequence, one intron, is cut and removed and the 2 resulting chains,the exons, are sealed together.This procedure occurs throughout formation that the majority of sensible mRNAmolecules in eukaryotic cells, and additionally occurs in bacteria and also archaea.Remarkably, some RNAs carry out self-splicing, with thecatalytic task residing in the intron sequence. The instrument of splicingand self-splicing are discussed in detail in thing 11. As listed later in this chapter, rRNA isthought come play a catalytic duty in the development of peptide bonds duringprotein synthesis.

In this chapter, we emphasis on the functions of mRNA, tRNA, and rRNA in geneexpression — the process of obtaining theinformation in DNA converted into proteins. In later on chapters we will encounterother RNAs, often linked with proteins, that get involved in various other cellfunctions.

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