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Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. Brand-new York: W H Freeman; 2002.

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Section 5.2A Pair the Nucleic acid Chains v Complementary order Can type aDouble-Helical Structure

The covalent structure of main point acids accounts for their capability to carryinformation in the form of a succession of bases follow me a nucleic acid chain. Otherfeatures of nucleic acid structure facilitate the procedure ofreplication—that is, the generation the two copies of a nucleicacid indigenous one. These functions depend top top the capacity of the bases discovered in nucleicacids to kind spe-cific basic pairs in such a way that a helicalstructure consisting of two strands is formed. The double-helical framework of DNAfacilitates the replication of the hereditary material (Section 5.2.2).

5.2.1. The dual Helix Is Stabilized through Hydrogen Bonds and also HydrophobicInteractions

The presence of particular base-pairing interactions was uncovered in the courseof researches directed at identify the three-dimensional structure of DNA.Maurice Wilkins and Rosalind Franklin derived x-ray diffraction photographs offibers the DNA (Figure 5.10). Thecharacteristics of these diffraction patterns suggested that DNA was formed oftwo chains that wound in a continual helical structure. Indigenous these and other data,James Watson and Francis Crick inferred a structural model for DNA thataccounted for the diffraction pattern and also was additionally the source of part remarkableinsights right into the practical properties of nucleic acids (Figure 5.11).


Figure 5.10

X-Ray Diffraction picture of a hydrated DNA Fiber. The central cross is diagnostic of a helical structure. The strongarcs on the meridian arise native the stack of nucleotide bases, whichare 3.4 Å apart.


Figure 5.11

Watson-Crick model of Double-Helical DNA. One polynucleotide chain is displayed in blue and the other in red. Thepurine and also pyrimidine bases are presented in lighter colors 보다 thesugar-phosphate backbone. (A) Axial view. The framework repeatsalong the helical (more...)

The functions of the Watson-Crick version of DNA deduced indigenous the diffractionpatterns are:


Two helical polynucleotide chains room coiled around a common axis. Thechains operation in the contrary directions.


The sugar-phosphate backbones space on the external and, therefore, thepurine and pyrimidine bases lie on the within of the helix.


The bases are nearly perpendicular to the helix axis, and surrounding basesare separated by 3.4 Å. The helical framework repeats every 34 Å, sothere are 10 bases (= 34 Å every repeat/3.4 Å every base) per revolve of helix.There is a rotation the 36 levels per base (360 levels per complete turn/10bases every turn).


The diameter that the helix is 20 Å.

How is such a constant structure able to accommodate an arbitrary sequence ofbases, offered the different sizes and also shapes of the purines and also pyrimidines? Inattempting to answer this question, Watson and Crick discovered that guanine canbe paired with cytosine and adenine v thymine to type base pairs that haveessentially the exact same shape (Figure 5.12).These base pairs are organized together by particular hydrogen bonds. This base-pairingscheme was sustained by earlier studies of the basic composition that DNA fromdifferent species. In 1950, Erwin Chargaff reported the the ratios the adenineto thymine and also of guanine come cytosine were virtually the exact same in every speciesstudied. Note in Table 5.1 that all theadenine:thymine and guanine:cytosine ratios room close come 1, vice versa, theadenine-to-guanine ratio varies considerably. The an interpretation of these equivalenceswas not apparent until the Watson-Crick model was proposed, as soon as it came to be clearthat castle represent an essential facet that DNA structure.


Figure 5.12

Structures the the basic Pairs suggest by Watson and Crick.


The spacing of about 3.4 Å between practically parallel basic pairs is readilyapparent in the DNA diffraction pattern (see number 5.10). The stacking that bases one on height of anothercontributes come the stability of the double helix in two means (Figure 5.13). First, surrounding base pairsattract one one more through van der Waals forces (Section 1.3.1). Energies connected with van der Waalsinteractions are quite small, together that common interactions add from 0.5to 1.0 kcal mol-1 per atom pair. In the twin helix, however, alarge variety of atoms are in van der Waals contact, and the network effect, summedover these atom pairs, is substantial. In addition, the double helix isstabilized through the hydrophobic impact (Section1.3.4): base stacking, or hydrophobic interactions between the bases,results in the exposure that the much more polar surface to the surrounding water.This arrangement is memory of protein folding, where hydrophobic aminoacids are internal in the protein and also hydrophilic are exterior (Section 3.4). Basic stacking in DNA is alsofavored by the conformations of the relatively rigid five-membered ring of thebackbone sugars. The street rigidity influence both the single-stranded and thedouble-helical forms.

Figure 5.13

Axial view of DNA. Base pairs are stacked nearly one on optimal of an additional in the doublehelix.

5.2.2. The dual Helix Facilitates the exact Transmission of HereditaryInformation

The double-helical version of DNA and also the presence of specific base pairsimmediately argued how the genetic material could replicate. The sequence ofbases of one strand that the double helix exactly determines the succession of theother strand; a guanine base on one strand is constantly paired through a cytosine baseon the various other strand, and so on. Thus, separation the a dual helix into its twocomponent chains would certainly yield two single-stranded templates top top which new doublehelices could be constructed, every of i beg your pardon would have the very same sequence ofbases together the parent dual helix. Consequently, as DNA is replicated, among thechains of each daughter DNA molecule would certainly be recently synthesized, whereas theother would certainly be passed unmodified from the parental DNA molecule. This distributionof parental atoms is achieved by semiconservativereplication..

Matthew Meselson and Franklin Stahl brought out a critical test that thishypothesis in 1958. They labeling the parent DNA v 15N, a heavyisotope that nitrogen, to make it denser than ordinary DNA. The labeling DNA wasgenerated by growing E. Coli for countless generations in a mediumthat included 15NH4Cl as the sole nitrogen source. Afterthe organization of hefty nitrogen to be complete, the bacteria to be abruptlytransferred come a medium that included 14N, the ordinary isotope ofnitrogen. The concern asked was: What is the circulation of 14N and15N in the DNA molecules after succeeding rounds ofreplication?

The distribution of 14N and also 15N was revealed by thetechnique that density-gradient equilibrium sedimentation. Asmall lot of DNA was liquified in a concentrated solution the cesium chloridehaving a density close to the of the DNA (1.7 g cm-3). This solutionwas centrifuged until it was almost at equilibrium. The opposing procedures ofsedimentation and diffusion created a gradient in the concentration the cesiumchloride across the centrifuge cell. The result was a stable density gradient,ranging indigenous 1.66 come 1.76 g cm-3. The DNA molecules in this densitygradient were propelled by centrifugal pressure into the region where the solution"sdensity was same to your own. The genomic DNA gave in a narrow band the wasdetected by its absorption of ultraviolet light. A mixture the 14N DNAand 15N DNA molecules gave clearly separate bands due to the fact that they differin density by about 1% (Figure 5.14).

Figure 5.14

Resolution of 14N DNA and also 15 N DNA by density-gradientcentrifugation. (A) Ultraviolet absorption picture of a centrifuge cell showingthe two distinctive bands that DNA. (B) Densitometric tracing the theabsorption photograph.

DNA to be extracted indigenous the bacteria at assorted times ~ they to be transferredfrom a 15N to a 14N medium and also centrifuged. Analysis ofthese samples verified that there was a single band the DNA ~ one generation.The thickness of this tape was exactly halfway between the densities that the14N DNA and 15N DNA bands (Figure 5.15). The absence of 15N DNA indicatedthat parental DNA to be not preserved as an undamaged unit after ~ replication. Theabsence that 14N DNA suggested that every the daughter DNA derived someof their atoms indigenous the parent DNA. This proportion had actually to be half because thedensity that the hybrid DNA tape was halfway in between the densities of the14N DNA and also 15N DNA bands.

Figure 5.15

Detection the Semiconservative Replication that E.coli DNA by density-gradient centrifugation. The place of a tape of DNA relies on its contents of14N and also 15N. After ~ 1.0 generation, all ofthe DNA molecules were hybrids include equal quantities of (more...)

After two generations, there were equal amounts of 2 bands the DNA. One washybrid DNA, and also the various other was 14N DNA. Meselson and also Stahl concludedfrom this incisive experiment “that the nitrogen in a DNA molecule is dividedequally between two physically continuous subunits; that adhering to duplication,each daughter molecule receives among these; and also that the subunits areconserved through many duplications.” Their outcomes agreed perfectly v theWatson-Crick design for DNA replication (Figure5.16).

Figure 5.16

Diagram the Semiconservative Replication. Parental DNA is shown in blue and also newly synthesized DNA in red.

5.2.3. The dual Helix have the right to Be Reversibly Melted

During DNA replication and other processes, the 2 strands the the double helixmust be separated indigenous one another, at the very least in a local region. In thelaboratory, the dual helix have the right to be disrupted by heating a systems of DNA. Theheating disrupts the hydrogen bonds in between base pairs and thereby reasons thestrands come separate. The dissociation the the twin helix is often calledmelting due to the fact that it occurs fairly abruptly in ~ a certaintemperature. The melt temperature(Tm) is identified as the temperature at which halfthe helical structure is lost. Strands may likewise be be separate by including acid oralkali to ionize the nucleotide bases and disrupt basic pairing.

Stacked bases in nucleic acids absorb less ultraviolet irradiate than execute unstackedbases, one effect called hypochromism. Thus, the melting ofnucleic acids is conveniently followed by surveillance their absorption of light, whichpeaks at a wavelength that 260 nm (Figure5.17).

Figure 5.17

Hypochromism. (A) Single-stranded DNA absorbs light much more effectively than doesdouble-helical DNA. (B) The absorbance the a DNA systems at awavelength the 260 nm boosts when the twin helix is melted intosingle strands.

Separated complementary strands of main point acids spontaneously reassociate toform a double helix as soon as the temperature is lower belowTm. This renaturation procedure is sometimescalled annealing. The facility v which double helices can bemelted and also then reassociated is crucial for the biological functions that nucleicacids. The course, within cells, the double helix is no melted by the additionof heat. Instead, proteins referred to as helicases usage chemical energy(from ATP) to disrupt the structure of double-stranded main point acidmolecules.

The ability to reversibility melt and reanneal DNA in the laboratory gives apowerful device for investigating succession similarity as well as gene structureand expression. Because that instance, DNA molecules from two different organisms can bemelted and permitted to reanneal or hybridize in the visibility ofeach other. If the sequences room similar, hybrid DNA duplexes, v DNA fromeach organism contributing a strand of the twin helix, deserve to form. Indeed, thedegree that hybridization is an indication that the relatedness that the genomes andhence the organisms. Similar hybridization experiments v RNA and DNA canlocate genes in a cell"s DNA that correspond come a specific RNA. We will certainly returnto this important technique in Chapter6.

5.2.4. Part DNA Molecules are Circular and also Supercoiled

The DNA molecule in human chromosomes space linear. However, electron microscopicand various other studies have displayed that undamaged DNA molecules from some various other organismsare one (Figure 5.18A). The termcircular describes the continuous of the DNA chains, not totheir geometrical form. DNA molecules within cells necessarily have a verycompact shape. Keep in mind that the E. Coli chromosome, fullyextended, would be around 1000 times as lengthy as the biggest diameter the thebacterium.

Figure 5.18

Electron Micrographs of circular DNA native Mitochondria. (A) tranquil form. (B) Supercoiled form.

A brand-new property shows up in the counter of a straight DNA molecule into a closedcircular molecule. The axis of the double helix have the right to itself it is in twisted right into asuperhelix (Figure5.18B). A one DNA molecule without any kind of superhelical transforms isknown together a calm molecule. Supercoiling is biologicallyimportant for 2 reasons. First, a supercoiled DNA molecule has actually a morecompact form than go its tranquil counterpart. Second,supercoiling might hinder or donate the volume of the twin helix tounwind and also thereby affect the interactions between DNA and othermolecules. This topological attributes of DNA will certainly be consideredfurther in section 27.3.

5.2.5. Single-Stranded main point Acids Can embrace Elaborate Structures

Single-stranded nucleic acids regularly fold back on themselves to kind well-definedstructures. Early in evolutionary history, main point acids, specifically RNA, mayhave adopted complex and diverse structures both to store hereditary informationand to catalyze its transmission (Section2.2.2). Such frameworks are also important in all modern-day organisms inentities such together the ribosome, a large complex of RNAs and proteins top top whichproteins room synthesized.

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The simplest and most usual structural motif created is astem-loop, produced when 2 complementary order withina solitary strand come together to form double-helical frameworks (Figure 5.19). In countless cases, these doublehelices are comprised entirely that Watson-Crick base pairs. In various other cases,however, the structures incorporate mismatched or unparalleled (bulged) bases. Suchmismatches destabilize the regional structure yet introduce deviations indigenous thestandard double-helical structure that can be essential for higher-order foldingand for role (Figure 5.20).

Figure 5.19

Stem-Loop Structures. Stem-loop structures might be formed from single-stranded DNA and also RNAmolecules.