[Recommended]genetics biology

genetic biology 671 Appearances of regeneration from naked masses of protoplasm have sometimes been noted, but never in examples in which it had been established…

genetic biology
671
Appearances of regeneration from naked masses of protoplasm have sometimes been noted, but never in examples in which it had been established that there were no small, fully formed cells from which the growth might have arisen.
It is, of course, possible that cultural conditions will be found under which masses of naked protoplasm will revert to cellular growth, such as has been reported by Ne&as4 in Saccharomyces cerevisiae and by Pease”‘ in Proteus vulgaris.
1 C. Weibull, Symposium Soc. Gen. Microbiol., 6, 111, 1956. 2J. Lederberg, these PROCEEDINGS, 42, 574, 1956; K. McQuillen, Biochim. et Biophys. Acta, 27,
410,1958.
3 K. McQuillen, Symposium Soc. Gen. Microbiol, 6, 127, 1956; S. Spiegelman, in The Chemical Basis of Heredity, ed. W. D. McElroy and B. Glass (Baltimore: Johns Hopkins Press, 1957), pp. 232-267; J. Spizizen, these PROCEEDINGS, 43, 694, 1957; D. Fraser, H. R. Mahler, A. L. Shug, and C. A. Thomas, Jr., these PROCEEDINGS, 43, 939, 1957; E. Chargaff, H. M. Schulman, and H. S. Shapiro, Nature, 180, 151, 1957.
40. Necas, Biol. Zentr., 75, 268, 1956; A. A. Eddy and D. H. Williamson, Nature, 179, 1252, 1957.
5 Mary R. Emerson, in manuscript. Our mutant strain, Em 11200, has been shown to be allelic to a number of others of independent origin: B 135 and M 16 obtained from Dr. D. D. Perkins, Stanford University; “wooly,” from Dr. H. L. K. Whitehouse, Cambridge University; and “ginger” and an unnamed mutant from Dr. J. R. S. Fincham, University of Leicester. From published descriptions it is likely that these are also allelic to the “cut” mutant of Dr. H. Kuwana
(Cytologia, 18,235, 1953).
6 Mary B. Mitchell and H. K. Mitchell, these PROCEEDINGS, 38, 442, 1952.
Mary B. Mitchell, H. K. Mitchell, and A. Tissieres, these PROCEEDINGS, 39, 606, 1953. 8 G. W. Beadle and E. L. Tatum, these PROCEEDINGS, 27, 499, 1941. 9 The hemicellulase preparation was obtained from the Nutritional Biochemical Corporation, Cleveland 28, Ohio, who inform us that it is a concentrate of microbiological origin, containing cellulase, gumase, and maltase in addition to hemicellulase. A single test in our laboratory indi cated strong chitinase activity.
1OJ. Lederberg and Jacqueline St. Clair, J. Bacteriol., 75, 143, 1958. 11 Phyllis Pease, J. Gen. Microbiol., 17, 64, 1957.
THE REPLICATION OF DNA IN ESCHERICHIA COLI* BY MATTHEW MESELSON AND FRANKLIN W. STAHL
GATES AND CRELLIN LABORATORIES OF CHEMISTRY, t AND NORMAN W. CHURCH LABORATORY OF CHEMICAL BIOLOGY, CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA
Communicated by Max Delbrick, May 14, 1958
Introduction.-Studies of bacterial transformation and bacteriaphage infection’-‘
strongly indicate that deoxyribonucleic acid (DNA) can carry and transmit heredi tary information and can direct its own replication.
Hypotheses for the mechanism of DNA replication differ in the predictions they make concerning the distribution among progeny molecules of atoms derived from parental molecules.6 Radioisotopic labels have been employed in experiments bearing on the distribu tion of parental atoms among progeny molecules in several organisms.6-9 We anticipated that a label which imparts to the DNA molecule an increased density might permit an analysis of this distribution by sedimentation techniques.
To this end, a method was developed for the detection of small density differences among
6;72
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bacterial Distance fuged stages FIG. at 1.-Ultraviolet in lysate 31,410 from the banding containing the rpm axis in absorption a of of approximately CsCl DNA rotation solution photographs from increases E. as 108 described coli. lysed showing toward cells An in was aliquot the successive the centri- right. text. of after The number reaching beside 31,410 rpm. each photograph gives the time elapsed
macromolecules.’0 By use of this method, we have observed the distribution of the heavy nitrogen isotope N15 among molecules of DNA following the transfer of
a uniformly N”5-labeled, exponentially growing bacterial population to a growth medium containing the ordinary nitrogen isotope N’4.
Density-Gradient Centrifugation.- A small amount of DNA in a concentrated solution of cesium chloride is centrifuged until equilibrium is closely approached.
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673
The opposing processes of sedimentation and diffusion have then produced a stable concentration gradient of the cesium chloride. The concentration and pressure gradients result in a continuous increase of density along the direction
of centrifugal force.
The macromolecules of DNA present in this density gradient are driven by the centrifugal field into the region where the solution density is equal to their own buoyant density.’
This concentrating tendency is opposed by diffusion, with the result that at equilibrium a single species of DNA is distributed over a band whose width is inversely related to the molecular weight of that species
(Fig. 1).
If several different density species of DNA are present, each will form a band at the position where the density of the CsCl solution is equal to the buoyant density In this way DNA labeled with heavy nitrogen (N15) may be of that species.
a
N’14 N5
b
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A two at CsCl mixture FIG. 44,770 bands solution 2-a: rpm. of of N’4 Fig. as The b: gin. described and 2a. A resolution cm. N”1 microdensitometer The bacterial — in separation the of text. N14 lysates, DNA between The tracing each photograph from containing the showing N”1 peaks DNA was the about corresponds taken DNA by 10′ density-gradtient after distribution lysed to 24 cells, a hounrs difference was in of the centrifugation. centrifugation centrifuged region in buoyant of the in density of 0.014
Figure 2 shows the two bands formed as a result of centrifuging a mixture of approximately equal amounts of N14 and N1I- Escherichia coli DNA.
resolved from unlabeled DNA.
In this paper reference will be made to the apparent molecular weight of DNA
samples determined by means of density-gradient centrifugation. A discussion has been given”‘ of the considerations upon which such determinations are based,
as well as of several possible sources of error.’2
Experimental.-Escherichia coli B was grown at 360 C. with aeration in a glucose salts medium containing ammonium chloride as the sole nitrogen source.” The growth of the bacteria) population was followed by microscopic cell counts and by colony assays (Fig. 3).
Bacteria uniformly labeled with N”5 were prepared by growing washed cells for
S.674 BIOLOGY: MESELSON AND STAHL PROC. N. A.
14 generations (to a titer of 2 X 108/ml) in medium containing 100 ,ug/ml of N’5H4Cl An abrupt change to N14 medium was then ac of 96.5 per cent isotopic purity. complished by adding to the growing culture a tenfold excess of N14H4Cl, along with ribosides of adenine and uracil in experiment 1 and ribosides of adenine. guanine, uracil, and cytosine in experiment 2, to give a concentration of 10lg/ml During subsequent growth the bacterial titer was kept between of each riboside.
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-14 -12 -10 -8 -6 -4 -2 0 TIME (hours)
2 4 6 8
medium. withdrawals tion, Thereafter, centrifugation, on FIG. the the ordinates 3.-Growth generation The during and values the give additions. of the actual time bacterial the on period the was actual titer During ordinates 0.81 populations when was titers hours the kept samples of during period in the first between Experiment cultures were this in of N sampling later 15 being 1 and and up period 1 then and withdrawn to 2 for the X in 0.85 density-gradient have N14 108 time hours been medium. by for of additions in corrected density-gradient addition Experiment The centrifuga- of of for values fresh N’4. the 2.
1 and 2 X 108/ml by appropriate additions of fresh N 14 medium containing ribosides. Samples containing about 4 X 109 bacteria were withdrawn from the culture just before the addition of N14 and afterward at intervals for several generations. Each sample was immediately chilled and centrifuged in the cold for 5 minutes at After resuspension in 0.40 ml. of a cold solution 0.01 M in NaCl and 1,800 X g.
0.01 M in ethylenediaminetetra-acetate (EDTA) at pH 6, the cells were lysed by the
addition of 0.10 ml. of 15 per cent sodium dodecyl sulfate and stored in the cold.
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2
2
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gradient of cess on CsCl FIG. centrifugation each of solution Ni4 4-a: centrifugation substrates increases Ultraviolet at 44,770 to to of a growing the lysates absorption rpm under of Ni”labeled bacteria photographs the conditions sampled culture. showing at described Each various DNA photograph times in bands the after text. was resulting the taken The addition density from after of 20 density- position Ni4 of an hours the ex- in units the the and DNA species measurements base unlabeled of bands of the photograph. line DNA generation shown is DNA directly corresponds of in shown bacterial the The time. proportional adjacent in time to the right. The growth the of lowermost photographs. relative generation sampling Regions to presented the position frame, concentration is of times measured equal in The which Fig. of for density its microdensitometer Experiments 3. density serves band When from of 6 DNA. occupy between Microdensitometer the as allowance is just a time density 1 the The half-labeled and the of same pen is degree 2 the bands made reference. were displacement horizontal addition of estimated of for tracings is labeling provided fully the A of relative labeled test of above from of the by of a the conclusion that the DNA in the band of intermediate the amounts at50 ±[ frame 2 per of showing cent DNA of in the the the mixture distance three peaks, of between generations the the peak N” 0 of and and intermediate 1.9. NI,’ peaks. density is found to be centered
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BIOLOGY: MESELSON AND STAHL
PROC. NT, A. S.
For density-gradient centrifugation, 0.010 mnl. of the dodecyl sulfate lysate was added to 0.70 ml. of CsCl solution buffered at pH 8.5 with 0.01 M tris(hydroxy methyl)aminomethane. The density of the resulting solution was 1.71 gm. cm.-3 This was centrifuged at 140,000X g. (44,770 rpm) in a Spinco model E ultracentri fuge at 250 for 20 hours, at which time the DNA had essentially attained sedimenta tion equilibrium. Bands of DNA were then found in the region of density 1.71 gm. cm.-3, well isolated from all other macromolecular components of the bacterial lysate. Ultraviolet absorption photographs taken during the course of each cen trifugation were scanned with a recording microdensitometer (Fig. 4). The buoyant density of a DNA molecule may be expected to vary directly with The density gradient is constant in the the fraction of N15 label it contains. region between fully labeled and unlabeled DNA bands. Therefore, the degree of labeling of a partially labeled species of DNA may be determined directly from the relative position of its band between the band of fully labeled DNA and the The error in this procedure for the determination of band of unlabeled DNA. the degree of labeling is estimated to be about 2 per cent. Results.-Figure 4 shows the results of density-gradient centrifugation of lysates of bacteria sampled at various times after the addition of an excess of N’4-containing substrates to a growing N1″-labeled culture. It may be seen in Figure 4 that, until one generation time has elapsed, half labeled molecules accumulate, while fully labeled DNA is depleted. One generation time after the addition of N 14, these half-labeled or “hybrid” molecules alone are observed. Subsequently, only half-labeled DNA and completely unlabeled DNA
When two generation times have elapsed after the addition of N’4, half-labeled and unlabeled DNA are present in equal amounts.
are found.
Discussion.-These results permit the following conclusions to be drawn regard ing DNA replication under the conditions of the present experiment.
1. The nitrogen of a DNA molecule is divided equally between two subunits which remain intact through many generations. The observation that parental nitrogen is found only in half-labeled molecules at all times after the passage of one generation time demonstrates the existence in each DNA molecule of two subunits containing equal amounts of nitrogen. The finding that at the second generation half-labeled and unlabeled molecules are found in equal amounts shows that the number of surviving parental subunits is twice the number of parent molecules initially present. That is, the subunits are conserved. 2. Followinq replication, each daughter molecule has received one parental subunit. The finding that all DNA molecules are half-labeled one generation time after the addition of N’4 shows that each daughter molecule receives one parental sub unit.’4 If the parental subunits had segregated in any other way among the daughter molecules, there would have been found at the first generation some fully labeled and some unlabeled DNA molecules, representing those daughters which received two or no parental subunits, respectively. 3. The replicative act results in a molecular doubling. This statement is a corollary of conclusions 1 and 2 above, according to which each parent molecule passes on two subunits to progeny molecules and each progeny
BIOLOGY: MESELSON AND STAHL
molecule receives just one parental subunit.
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It follows that each single molecular reproductive act results in a doubling of the number of molecules entering into that act.
The above conclusions are represented schematically in Figure 5. The Watson-Crick Model.-A molecular structure for DNA has been proposed by Watson and Crick.15 It has undergone preliminary refinement’6 without alteration of its main features and is supported by physical and chemical studies.”7 The structure consists of two polynucleotide chains wound helically about a common axis. The nitrogen base (adenine, guanine, thymine, or cytosine) at each level
ORIGINAL 7
PARENT MOLECULE
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tions. equally receives the FIG. data 5.-Schematic between one presented of these. two in subunits. representation Fig. The 4. subunits The Following nitrogen of are the conserved duplication, conclusions of each through DNA each drawn molecule successive daughter in the is text molecule duplica- divided from
on one chain is hydrogen-bonded to the base at the same level on the other chain. Structural requirements allow the occurrence of only the hydrogen-bonded base pairs adenine-thymine and guanine-cytosine, resulting in a detailed complemen tariness between the two chains.
This suggested to Watson and Crick”8 a definite and structurally plausible hypothesis for the duplication of the DNA molecule. According to this idea, the two chains separate, exposing the hydrogen-bonding sites of the bases.
Then, in accord with the base-pairing restrictions, each chain serves as a template for the synthesis of its complement. Accordingly, each daughter molecule contains one of the parental chains paired with a newly synthesized chain (Fig. 6).
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The results of the present experiment are in exact accord with the expectations of the Watson-Crick model for DNA duplication. However, it must be emphasized that it has not been shown that the molecular subunits found in the present ex periment are single polynucleotide chains or even that the DNA molecules studied here correspond to single DNA molecules possessing the structure proposed by Watson and Crick. However, some information has been obtained about the molecules and their subunits; it is summarized below.
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parent proposed two chain tains FIG. molecules one (white). chains 6.-Illustration by of Watson the remain each Upon parental with and intact, continued of one Crick. the chains parental so mechanism that Each duplication, (black) there chain. daughter paired will of DNA the always with molecule two duplication be one original found con- new
The DNA molecules derived from E. coli by detergent-induced lysis have a buoyant density in CsCl of 1.71 gm. cm.-3, in the region of densities found for T2 and T4 bacteriophage DNA, and for purified calf-thymus and salmon-sperm DNA. A highly viscous and elastic solution of N14 DNA was prepared from a dodecyl sulfate lysate of E. coli by the method of Simmons19 followed by deproteinization with chloroform. Further purification was accomplished by two cycles of preparative density-gradient centrifugation in CsCl solution. This purified bacterial DNA was found to have the same buoyant density and apparent molecular weight, 7 X 106, as the DNA of the whole bacterial lysates (Figs. 7, 8).
BIOLOGY: MESELSON AND STAHL
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Heat Denaturation.-It has been found that DNA from E. coli differs importantly from purified salmon-sperm DNA in its behavior upon heat denaturation. Exposure to elevated temperatures is known to bring about an abrupt collapse of the relatively rigid and extended native DNA molecule and to make available for acid-base titration a large fraction of the functional groups presumed to be blocked by hydrogen-bond formation in the native structure. 19, 20, 21, 22
Rice and Doty22 have reported that this collapse is not accompanied by a reduction in molecu
These findings are corroborated by lar weight as determined from light-scattering.
density-gradient centrifugation of salmon-sperm DNA.23 When this material is
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relative of FIG. Fig. 8.-The 7 concentration plotted square against of of the DNA. the width logarithm The of the divisions of band the persity the band. the to along any DNA the absence point the located weight of Linearity abscissa of the of such at density banded average the of set a corresponding plot this off heterogeneity, DNA. molecular intervals plot is directly indicates The of position weight value proportional 1 the mm.2. monodis- slope of of in the the the at In
FIG. 7.-Microdensitometer tracing of an ultraviolet optical density absorption in the region photograph of a band showing of N14 the E. coli DNA at equilibrium. About 2 fg. of DNA purified at 31,410 as rpm described at 250 in in 7.75 the molal text was CsCl centrifuged atpH 8.4. The over the density region gradient of the maximum band is and essentially is indicates 0.057 gm./cm.4. constant a the The ant optical density position density of of 1.71 the the above concentration gm. cm.-‘ the base In of line this DNA is directly tracing buoy- in the proportional rotating DNA at the centrifuge maximum to cell. is about The 50 concentration ,g./ml. of the weight responding sodium slope for corresponds the to salt. a Cs-DNA molecular to an salt weight apparent of 9.4 of 7.1 X molecular X 10., 10. cor- for
kept at 1000 for 30 minutes either under the conditions employed by Rice and Doty or in the CsCl centrifuging medium, there results a density increase of 0.014 gm. cm.r3 with no change in apparent molecular weight. The same results are ob tained if the salmon-sperm DNA is pre-treated at pH 6 with EDTA and sodium
Along with the density increase, heating brings about a sharp dodecyl sulfate.
reduction in the time required for band formation in the CsCl gradient.
In the absence of an increase in molecular weight, the decrease in banding time must be ascribed10 to an increase in the diffusion coefficient, indicating an extensive col lapse of the native structure.
680
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hours the denaturation. unheated densitometry for density FIG. position comparison. of 9.-The has centrifugation N1″ been indicated. -3 bacterial of Each and dissociation removed an Heating a ultraviolet smooth reduction lysates. in Unheated by CsCi has of subtraction. curve the brought of absorption solution Heated about lysate subunits connects lysate about half at was A: photograph 44,770 of points in added A E. alone the a mixture coli density rpm. apparent obtained to gives DNA this taken of The one increase experiment heated upon molecular by after baseline band for micro- heat and one 20 in of weight generation brid The 0.016 in Fig. density DNA gm. 4. of cm. C: the contained in difference A N14 DNA. mixture growth in is B: this of 0.015 medium. Heated heated lysate gm. N14 lysate cm. forms Before and -‘ of only heated N16 heat one bacteria denaturation, N” band, bacterial as grown may lysates. the be seen hy
The decrease in banding time and a density increase close to that found upon heating salmon-sperm DNA are observed (Fig. 9, A) when a bacterial lysate containing uniformly labeled N”5 or N14 E. coli DNA is kept at 100° C. for 30 minutes in the CsCl centrifuging medium; but the apparent molecular weight of
681
the heated bacterial DNA is reduced to approximately half that of the unheated material. Half-labeled DNA contained in a detergent lysate of N’5 E. coli cells grown for one generation in N14 medium was heated at 1000 C. for 30 minutes in the CsCl centri This treatment results in the loss of the original half-labeled fuging medium. material and in the appearance in equal amounts of two new density species, each with approximately half the initial apparent molecular weight (Fig. 9, B). The density difference between the two species is 0.015 gm. cm.-, close to the increment produced by the N’6 labeling of the unheated DNA. This behavior suggests that heating the hybrid molecule brings about the dis sociation of the NI5-containing subunit from the N’4 subunit. This possibility was tested by a density-gradient examination of a mixture of heated N’5 DNA and heated N14 DNA (Fig. 9, C). The close resemblance between the products of heating hybrid DNA (Fig. 9 B) and the mixture of-products obtained from heating N14 and N’5 DNA separately (Fig. 9, C) leads to the conclusion that the two molecular subunits have indeed dissociated upon heating. Since the apparent molecular weight of the subunits so obtained is found to be close to half that of the intact molecule, it may be further concluded that the subunits of the DNA molecule which are conserved at duplication are single, continuous structures. The scheme for DNA duplication proposed by Delbrfick24 is thereby ruled out. To recapitulate, both salmon-sperm and E. coli DNA heated under similar conditions collapse and undergo a similar density increase, but the salmon DNA retains its initial molecular weight, while the bacterial DNA dissociates into the two subunits which are conserved during duplication. These findings allow two On the one hand, if we assume that salmon DNA con different interpretations. tains subunits analogous to those found in E. coli DNA, then we must suppose that the subunits of salmon DNA are bound together more tightly than those of the On the other hand, if we assume that the molecules of salmon DNA bacterial DNA. do not contain these subunits, then we must concede that the bacterial DNA molecule is a more complex structure than is the molecule of salmon DNA. The latter interpretation challenges the sufficiency of the Watson-Crick DNA model to explain the observed distribution of parental nitrogen atoms among progeny molecules. Conclusion.-The structure for DNA proposed by Watson and Crick brought forth a number of proposals as to how such a molecule might replicate. proposals6 make specific predictions concerning the distribution of parental atoms The results presented here give a detailed answer to among progeny molecules. the question of this distribution and simultaneously direct our attention to other problems whose solution must be the next step in progress toward a complete understanding of the molecular basis of DNA duplication. What are the molecular structures of the subunits of E. coli DNA which are passed on intact to each daughter molecule? What is the relationship of these subunits to each other in a DNA molecule? What is the mechanism of the synthesis and dissociation of the sub units in vivo? Summary.-By means of density-gradient centrifugation, we have observed the distribution of N’5 among molecules of bacterial DNA following the transfer of a uniformly N’5-substituted exponentially growing E. coli population to N’4 medium.
These
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PROC. N. A. S.
We find that the nitrogen of a DNA molecule is divided equally between two physi cally continuous subunits; that, following duplication, each daughter molecule receives one of these; and that the subunits are conserved through many duplica
tions.
* Aided by grants from the National Foundation for Infantile Paralysis and the National In stitutes of Health.
t Contribution No. 2344.
R. D. Hotchkiss, in The Nucleic Acids, ed. E. Chargaff and J. N. Davidson (New York:
· Academic Press, 1955), p. 435; and in Enzymes: Units of Biological Structure and Function, ed.
· H. Gaebler (New York: Academic Press, 1956), p. 119.
H. Goodgal and R. M. Herriott, in The Chemical Basis of Heredity, ed. W. D. McElroy and
· Glass (Baltimore: Johns Hopkins Press, 1957), p. 336. 3 S. Zamenhof, in The Chemical Basis of Heredity, ed. W. D. McElroy and B. Glass (Baltimore: Johns Hopkins Press, 1957), p. 351. 4A. D. Hershey and M. Chase, J. Gen. Physiol., 36, 39, 1952. 5 A. M. D. Delbruck Hershey, and Virology, G. S. Stent, 1, 108, in 1955; The Chemical 4, 237, 1957. Basis of Heredity, ed. W. D. McElroy and
· Glass (Baltimore: Johns Hopkins Press, 1957), p. 699. 7C. Levinthal, these PROCEEDINGS, 42, 394, 1956.
· 2 S.
J. H. Taylor, P. S. Woods, and W. L. Huges, these PROCEEDINGS, 43, 122, 1957.
9 R. B. Painter, F. Forro,
10 M. S. Meselson, F. W.
Jr., and W. L. Hughes, Nature, 181, 328, 1958.
Stahl, and J. Vinograd, these PROCEEDINGS, 43, 581, 1957.
11 The buoyant density of a molecule is the density of the solution at the position in the centri fuge cell where the sum of the forces acting on the molecule is zero. 12 Our attention has been called by Professor H. K. Schachman to a source of error in apparent molecular weights determined by density-gradient centrifugation which was not discussed by In evaluating the dependence of the free energy of the DNA Meselson, Stahl, and Vinograd. component upon the concentration of CsCl, the effect of solvation was neglected. It can be shown that is bound solvation preferentially. may introduce A method an error for into estimating the apparent the error molecular due to weight such selective if either CsCl solvation or water will be presented elsewhere. 13 In addition to NH4Cl, this medium consists of 0.049 M Na2HPO4, 0.022 M KH2PO4, 0.05 M NaCl, 0.01 M glucose, 10-3 M MgSO4, and 3 X 10-6 M FeCl3. 14 This result also shows that the generation time is very nearly the same for all DNA mole This raises the questions of whether in any one nucleus all DNA mole cules in the population. cules are controlled by the same clock and, if so, whether this clock regulates nuclear and cellular division as well.
15 F. H. C. Crick and J. D. Watson, Proc. Roy. Soc. London, A, 223, 80, 1954.
16 R. Langridge, W. E. Seeds, H. R. Wilson, C. W. Hooper, M. H. F. Wilkins, and L. D. Hamil
ton, J. Biophys. and Biochem. Cytol., 3, 767, 1957.
ed. 18 W. J. D. D. Watson McElroy and and F. B. H. Glass C. Crick, (Baltimore: Nature, 171, Johns 964, Hopkins 1953. Press, 1957), p. 532. 19 C. E. Hall and M. Litt, J. Biophys. and Biochem. Cytol., 4, 1, 1958.
17 For reviews see D. 0. Jordan, in The Nucleic Acids, ed. E. Chargaff and J. D. Davidson (New York: Academic Press, 1955), 1, 447; and F. H. C. Crick, in The Chemical Basis of Heredity,
20 21 R. P. D. Thomas, Lawley, Biochim. Biochim. et Biophys. et Biophys. Acta, Acta, 14, 21, 231, 481, 1954. 1956. 22 23 S. Kindly A. Rice supplied and P. by Doty, Dr. J. Michael Am. Chem. Litt. Soc., The 79, preparation 3937, 1957. of this DNA is described by Hall
and 24 M. Litt Delbruck, (J. Biophys. these and PROCEEDINGS, Biochem. Cytol., 40, 783, 4, 1, 1955. 1958).

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