Abstract:
A method estimating a property or parameter of a nucleic acid material, which property or parameter is one to which the electrical conductivity of the nucleic acid material is related, comprises measuring the electrical conductivity of the nucleic acid material, and estimating from the measurement the property or parameter of the material by reference to a predetermined relationship between electrical conductivity and said property or parameter. This is based on the discovery that there are certain important properties of nucleic acids, the quantitative determination of which is frequently desirable, which can be assessed by measurement of the conductivity of a solution of the nucleic acid or acids. Changes in such properties may be reflected in corresponding changes of electrical conductivity. The concentration of nucleic acid in solution and the molecular weight of a species of nucleic acid are examples of important property which may be determined.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the estimation of nucleic acid and more particularly to the estimation of a property or parameter of a nucleic acid. The invention is especially directed to the determination of concentration (quantification) of nucleic acids in solution and to the determination of molecular weight (sizing) of nucleic acid molecules. 
     DESCRIPTION OF THE PRIOR ART 
     Conventionally, DNA concentration is determined spectrophotometrically, a method which relies on the characteristic absorption of ultra-violet light (ca. 260 nm) by the nucleotide ring structure of DNA molecules. To determine molecular weight, DNA molecules are normally size-separated by agarose gel electrophoresis and then visualised using the dye ethidium bromide. DNA can also be quantified in agarose gels by comparing with known standards by measuring the fluorescence emitted following excitation by ultra-violet light (ca. 300 nm). Both methods have inherent disadvantages. UV spectrophotometers are expensive pieces of equipment requiring the use of costly quartz curvettes and rely on the `desstructive` processing of relatively large sample volumes. The use of ethidium bromide for DNA visualisation and quantification, although a cheaper alternative, is also undesirable to its extremely toxic and carcinogenic nature. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a method for the estimation of a property or parameter of a nucleic acid material, said property or parameter being one to which the electrical conductivity of the nucleic acid material is related, which method comprises measuring the electrical conductivity of the nucleic acid material, and estimating from said measurement the property or parameter of the material by reference to a predetermined relationship between electrical conductivity and said property or parameter. 
     The present invention is based on the discovery that there are certain important properties of nucleic acids, the quantitative determination of which is frequently desirable, which can be assessed by measurement of the conductivity of a solution of the nucleic acid or acids. Changes in such properties may be reflected in corresponding changes of electrical conductivity. The concentration of nucleic acid in solution and the molecular weight of a species of nucleic acid are examples of important property which may be determined in accordance with the present invention. 
     For the purposes of this invention electrical conductivity may be conveniently measured as the electrical current flowing through a solution of the nucleic acid material using electrodes of fixed surface area. 
     The present invention is of primary interest for the determination of properties of single species of nucleic acid. However, absolute purity of the material is not always necessary and the invention is applicable to nucleic acids containing minor amounts or other materials, including other species of nucleic acid. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a graph of current/conductivity against DNA concentration; 
     FIG. 2 is a graph of conductivity against supply frequency; 
     FIG. 3 is a calibration curve of log molecular weight against gradient; 
     FIG. 4 is a graph of % response of DNA molecules against frequency; and 
     FIG. 5 is a graph of molecular weight against % response gradient. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The concentration of DNA may be determined by measuring the current/conductivity t a known alternating current frequency. The current recorded at any one fixed frequency has been found to be proportional to the DNA concentration. For example FIG. 1 shows how the current/conductivity recorded at 2 KHz varies with DNA concentration. This relationship applies not only to a single size of nucleic acid but is true for a range of different sized molecules. In designing apparatus for carrying out the method of this invention a conductivity meter may be readily adapted and calibrated in accordance with the predetermined relationship between current flow and concentration of nucleic acid. In practice it will usually be desirable to calibrate the instrument to deal with homogeneous DNA species but for a range of molecular weights. Thus the sample will normally first be &#34;sized&#34; following which the appropriate nucleic setting for concentration determination will be shown. 
     The molecular weight of DNA species is determined by the response of these molecules to varying frequencies applied across the electrodes. Plotting the current/conductivity recorded (Table 1) as percentage of the maximum current/conductivity response (% response), versus the frequency of the a.c. signal applied between the two electrodes gives characteristic curves which differ for the molecular weights of the molecules concerned (FIG. 2). 
     
                       TABLE 1______________________________________    current (μA)frequency  25 mer       puc 18  lambda______________________________________2.00E + 03  6           62       934.00E + 03  6           63       966.00E + 03  6           64       988.00E + 03  6           64       991.00E + 04  7           64       992.00E + 04  8           65      1004.00E + 04 11           65      1026.00E + 04 15           66      1028.00E + 04 19           67      1021.00E + 05 24           68      1042.00E + 05 45           78      1094.00E + 05 80           99      1206.00E + 05 98           109     122______________________________________ 
    
     For a single DNA species the molecular weight can be determined by firstly calculating the gradient of the response versus frequency curve (over the frequency range 0-5×10 5  Hz). The gradient value can then be compared with a calibration curve (FIG. 3) of log molecular weight plotted against gradient. For all DNA molecules tested, the gradient varies with molecular weight such that the larger the gradient value, the lower the molecular weight. The basis of this relationship is presumed to be that the mobility of the DNA molecules changes as the frequency changes; as the frequency increases, large molecules are less responsive to changes compared with smaller molecules. 
     Methods of DNA quantification and molecular weight determination in accordance with the invention circumvent the problem associated with the known methodologies and offer a number of distinct advantages over those conventional methods. Thus, the invention allows rapid and accurate determination of both molecular weight and concentration; it is more sensitive and accurate, is safer for the operator and it requires small sample volumes. 
     The characterisation of DNA outlined above has been achieved using an a.c. signal, of modulating frequency, between two thin wire platinum electrodes (alternative metals such as copper, stainless steel would also be adequate) through a solution containing DNA. Generally a fixed a.c. signal of between the two thin wire conductive electrodes. A range of frequencies of this a.c. signal, between O and 1 MHz, is applied across these electrodes and the corresponding conductivity recorded at each frequency in turn as the current passing through the solution. For example the sample DNA is dissolved in water or TE buffer in a volume of at least 10 μl. For convenience a standard 500 μl plastic tubs can be used. Two platinum thin wire electrodes are placed in the solution and connected to a function generator operating at 1 V a.c. A range of frequencies from 0-1 MHz are passed through the solution and the resulting current measured as mA using an ammeter. The DNA concentration is calculated by comparing the current passing through the solution at a frequency of 2 KHz with a standard curve (relating DNA concentration to current at a fixed frequency). Alternative frequencies can also be used. 
     Specific Example 
     Confirmation that the molecular weight of DNA molecules in solution can be determined by conductivity methods was achieved by preparing four different DNA solutions in both milli Q water and Tris EDTA (TE) buffer (25 base oligonucleotide, 700 base pair fragment, pUC 18 (=2,690 base pair) and lambda (=50,000 base pair) and a fifth solution, containing DNA of unknown molecular weight (UN). The conductivity of these solutions was measured in uA over the range of frequencies from 2×10 4  Hz to 10 5  Hz. There was no significant difference between milli Q and TE buffer indicating that DNA molecules can be accurately sized in the most common buffer used to store DNA. 
     To confirm the relationship between molecular weight and conductivity, solutions were prepared in triplicate and tested. Table 2 provides this confirmation and shows that interrreplicate variation is slight. Table 3 shows the main conductivity in uA and also as a % of maximum conductivity. The main reason for expressing data in terms of % response is that although within experiment variation is low (as shown in table 1) the fragility of the current probe system causes significant variation between experiments in terms of uA readings. However the trends of conductivity changes (as expressed in % response) is consistent between experiments. 
     FIG. 4 shows the mean % response data from table 3 plotted as a graph. The most obvious difference between the DNA solutions is the gradient of the slopes. When this gradient is calculated from the conductivity at 2×10 4  and 4×10 5  a near linear relationship is observed between gradient and log molecular weight (FIG. 5). Thus gradient can be used to calculate molecular weight. 
     The molecular weight of sample UN found from FIG. 5 is about 1047 bps, which corresponds well with estimates made from agarose gel sizing of the same fragment. 
     As mentioned above, the fragility of the current probe system causes significant variation between experiments. To overcoming these problems the following designs are suggested. 
     Production Description 
     The technology outlined herein is intended to form the basis of a laboratory instrument used to size and quantify nucleic acids. The instrument is based on the pipette, a tool used by all molecular biologists to accurately transfer small volumes of solutions (e.g. nucleic acids). Solutions are drawn up into disposable tips using a vacuum generated in the body of the pipette unit. The solution can then be dispensed. The DNA-Pipette has sensing electronics housed in the body of the pipette. The sensor is separate from the pipette unit and is implanted into the disposable pipette tips which may or may not be sterile. The user draws up a sample into the pipette tip. Once a suitable volume has been drawn up into the tip, the pipette can display molecular weight and concentration on the display (e.g. LCD). 
     The preferred sensor consists of a working electrode and a counter electrode manufactured as an interdigitated array on a suitable substrate such as silicon, glass or polycarbonate. Reference electrodes may be used but are not necessary. The electrodes may be of any suitable material. Inert metals such as platinum, gold and silver, carbon, graphite, carbon-pastes and platinum inks, modified electrodes where electron transfer is mediated by electron-accepting or electron-donating compounds may also be used. Electrode geometry may include any convenient symmetry. Spherical hemispherical, disk-shaped, ring-shaped and linear electrodes which form single thin wire electrodes, interdigitated or multiple arrays of sensing units may be used. Electrodes may be of macro, micro or ultra-micro dimensions. Contact between the tip and pipette body is via an interface/contact on the barrel of the pipette. 
     The product described is a portable instrument. Some applications may require a stand alone instrument (e.g. where multiple samples require rapid processing). This embodiment can also be applied to other instruments which have high sample a through put. In particular microtitre plate technology allows a large number of samples to be screened using a standard plate format. This would have application in screening and diagnostics laboratories. 
     
                       TABLE 2______________________________________Conductivity (μA) of DNA solutions over arange of frequenciesreplicates1     2       3       mean (μA)                         frequency                                  size______________________________________20    21      22      21      2.00E + 04                                  25 b21    22      22      21      4.00E + 0422    23      22      22      6.00E + 0435    35      36      36      8.00E + 0454    56      56      55      1.00E + 0580    86      82      82      2.00E + 05111   115     116     114     4.00E + 0532    35      33      33      2.00E + 04                                  700 bp35    37      38      36      4.00E + 0440    40      43      41      6.00E + 0452    54      55      53      8.00E + 0463    69      69      67      1.00E + 0390    96      93      93      2.00E + 05115   118     119     117     4.00E + 0551    53      53      52      2.00E + 04                                  un55    58      55      56      4.00E + 0460    64      62      62      6.00E + 0465    71      68      68      8.00E + 0469    75      75      73      1.00E + 0594    100     100     98      2.00E + 05120   130     125     125     4.00E + 0568    68      72      70      2.00E + 04                                  2,690 bp69    75      70      71      4.00E + 0475    80      79      78      6.00E + 0476    77      80      77      8.00E + 0478    81      82      80      1.00E + 05102   104     111     106     2.00E + 05135   145     139     139     4.00E + 05134   137     138     136     2.00E + 04                                  50,000 bp138   142     152     144     4.00E + 04139   147     150     145     6.00E + 04142   143     153     146     8.00E + 04145   154     146     148     1.00E + 03151   154     163     156     2.00E + 03162   169     169     166     4.00E + 05______________________________________ 
    
     
                       TABLE 3______________________________________Conductivity (as μA and % response) of DNA solutionsover a range of frequencies.                      50,00025 b 700 bp  un     2,690 bp                      bp______________________________________21   33      52     70     136   2.00E + 04                                    μA21   36      56     71     144   4.00E + 0422   41      62     78     145   6.00E + 0436   53      68     77     146   8.00E + 0455   67      73     80     148   1.00E + 0582   93      98     106    156   2.00E + 05114  117     125    139    166   4.00E + 05114  117     125    139    166   100%18   28      42     50     82    2.00E + 04                                    % response18   31      45     51     87    4.00E + 0419   35      50     56     87    6.00E + 0432   45      54     55     88    8.00E + 0448   57      58     58     89    1.00E + 0572   79      78     76     94    2.00E + 05100  100     100    100    100   4.00E + 05______________________________________ 
    
     It is to be understood that various modifications may be made to the described embodiments within the ability of the skilled person. The scope of the invention should therefore only be determined by the claims.