2
346 qualified for such a post. In any event the climate of opinion now clearly favours an integrated Health Service organised under some kind of area health board. It seems almost certain that M.o.H.s will soon join with their medical administrative colleagues in regional hos- pital boards in a unified service. Whatever they are called, all these doctors will be specialists in community, or social, medicine and they will expect to be treated like all other specialists. The criteria for specialist status and the planning of postgraduate training should be in their own hands. The initiative can no longer be left to the G.M.C. or to Government departments. It is time for those professionally concerned to come together so that their views can be expressed. Having looked back to the origins of the medical officer of health and then looked forward to the estab- lishment of a unified Health Service with its specialised group of community physicians, is it too fanciful to wonder whether it might eventually be necessary to re- invent the M.o.H. ? The original concept of a " redresser of wrongs " was overtaken by the M.o.H.’s responsibility for running a large organisation for his local authority, thus making it difficult for him to act as a " public accuser ". When community physicians are all busily engaged in working as part of a huge organisation pro- viding comprehensive health services, might there be a place for a kind of medical Ombudsman outside the organisation and therefore in a position to criticise its working freely? Perhaps each region should have an Ombudsman, reporting his findings directly to the public as well as to the service. A suitable remit for this doctor might run as follows: " He shall inform himself, as far as practicable, respecting all influences affecting or threatening to affect injuriously the public health within the District. He shall inquire into and ascertain by such means as are at his disposal the causes, origin, and distri- bution of diseases within the District, and ascertain to what extent the same have depended on conditions cap- able of removal or mitigation ". In fact, these words come direct from an Order describing the duties of the medical officer of health in 1910.6 6 DUBOS pointed out that the leaders of the 19th- century sanitary movement were concerned not only with the treatment of disease but also with the creation of a healthier, happier world. He remarked that they " approached the problems of health with much prac- tical skill, but it must never be forgotten that a philo- sophical and humanitarian doctrine was the inspiration of their pragmatic genius. A similar ideal might again inspire a new pioneering venture to attack the health problems of the present day. It is not impossible that in future, as in the past, effective steps in the prevention of disease will be motivated by an emotional revolt against some of the inadequacies of the modem world and will result from the search for a formula of life more akin to the natural propensities of man ". Perhaps our fantasy of a re-created office of medical officer of health could attract the kind of doctors who would lead such a revolt. 6. Sanitary Officers (outside London) Order, 1910. Article XIX. 7. Dubos, R. Mirage of Health; p. 174. London, 1959. Replication of Genetic Material in a Test-tube ONE of the cornerstones of molecular biology is the belief that the genetic material of most organisms, deoxyribonucleic acid (D.N.A.), is faithfully copied by enzymes. Almost all our information about D.N.A. replication comes from work by the school of ARTHUR KORNBERG, now of Stanford University. The study was initiated by his discovery of the enzyme, D.N.A. polymerase, more than a decade ago. For this contri- bution, and detailed characterisation of the D.N.A. replication process, KORNBERG was awarded a Nobel prize in 1959. In the reaction catalysed by D.N.A. polymerase, a D.N.A. molecule isolated from a living organism is used as a template: four deoxyribonucleotides (adenylic, guanylic, cytidilic, and thymidilic acids) are the units which are polymerised. The reaction is such that the sequence of the four units in the newly synthesised chain is complementary to that in the template, adenine pairing with thymine, and guanine with cytosine (and vice versa). This complementarity is ensured by hydro- gen bonding of base pairs as the reaction proceeds. A second cycle of polymerisation on the newly synthesised strand should produce a polymer with the same sequence as in the original template. Available chemical techniques are not sufficiently precise to prove the exact fidelity of the complementary copying of a long D.N.A. chain. One way to establish such fidelity is to synthesise in a test-tube D.N.A. with biological activity dependent on a complete and unaltered chain. This is what has now been achieved by GOULIAN, KORNBERG, and SINSHEIMER.l The D.N.A. used in these experiments was obtained from the bacterial virus 0X174. ROBERT SINSHEIMER, of the California Institute of Technology, had shown that this D.N.A. is single-stranded, unlike the double- stranded D.N.A. of most organisms. Moreover, 0X174 D.N.A. is in the form of a circle, which can be broken open at one point by nuclease to give a linear mole- cule. Specially prepared forms (protoplasts) of the host bacterium, Escherichia coli, can be infected with the circular D.N.A. and reproduce the whole virus; the linear D.N.A. -is not infectious. In the experiments now described the template used was circular D.N.A. from 0X174 (called for convenience the z strand). With purified polymerase a comple- mentary (-) strand was synthesised in a test-tube; the ends of the newly synthesised D.N.A. chains were united by a recently discovered polynucleotide-joining enzyme 2 to form a circle. In the synthesising medium the naturally occurring monomer, thymidylic acid, was replaced by an unnatural but biologically active ana- logue, 5-bromodeoxyuridylic acid. The higher density of the newly synthesised bromine-containing D.N.A. allowed it to be separated from the template D.N.A. by density-gradient centrifugation. To prove that the isolated circles were free of contamination by the original circles, the former were labelled with 1. Goulian, M., Kornberg, A., Sinsheimer, R. L. Proc. natn. Acad. Sci. U.S.A. 1967, 58, 2321. 2. Gellert, M. ibid. 1967, 57, 148.

Replication of Genetic Material in a Test-tube

Embed Size (px)

Citation preview

346

qualified for such a post. In any event the climate of

opinion now clearly favours an integrated Health Serviceorganised under some kind of area health board. Itseems almost certain that M.o.H.s will soon join withtheir medical administrative colleagues in regional hos-pital boards in a unified service. Whatever they arecalled, all these doctors will be specialists in community,or social, medicine and they will expect to be treated likeall other specialists. The criteria for specialist statusand the planning of postgraduate training should be intheir own hands. The initiative can no longer be left tothe G.M.C. or to Government departments. It is timefor those professionally concerned to come together sothat their views can be expressed.Having looked back to the origins of the medical

officer of health and then looked forward to the estab-lishment of a unified Health Service with its specialisedgroup of community physicians, is it too fanciful towonder whether it might eventually be necessary to re-invent the M.o.H. ? The original concept of a

" redresserof wrongs " was overtaken by the M.o.H.’s responsibilityfor running a large organisation for his local authority,thus making it difficult for him to act as a " publicaccuser ". When community physicians are all busilyengaged in working as part of a huge organisation pro-viding comprehensive health services, might there be aplace for a kind of medical Ombudsman outside theorganisation and therefore in a position to criticise itsworking freely? Perhaps each region should have anOmbudsman, reporting his findings directly to the

public as well as to the service. A suitable remit for thisdoctor might run as follows:

" He shall inform himself,as far as practicable, respecting all influences affecting orthreatening to affect injuriously the public health withinthe District. He shall inquire into and ascertain by suchmeans as are at his disposal the causes, origin, and distri-bution of diseases within the District, and ascertain towhat extent the same have depended on conditions cap-able of removal or mitigation ". In fact, these wordscome direct from an Order describing the duties of themedical officer of health in 1910.6 6

DUBOS pointed out that the leaders of the 19th-

century sanitary movement were concerned not onlywith the treatment of disease but also with the creationof a healthier, happier world. He remarked that they" approached the problems of health with much prac-tical skill, but it must never be forgotten that a philo-sophical and humanitarian doctrine was the inspirationof their pragmatic genius. A similar ideal might againinspire a new pioneering venture to attack the healthproblems of the present day. It is not impossible that infuture, as in the past, effective steps in the prevention ofdisease will be motivated by an emotional revolt againstsome of the inadequacies of the modem world and willresult from the search for a formula of life more akin tothe natural propensities of man ". Perhaps our fantasyof a re-created office of medical officer of health couldattract the kind of doctors who would lead such a revolt.

6. Sanitary Officers (outside London) Order, 1910. Article XIX.7. Dubos, R. Mirage of Health; p. 174. London, 1959.

Replication of Genetic Material in a Test-tubeONE of the cornerstones of molecular biology is the

belief that the genetic material of most organisms,deoxyribonucleic acid (D.N.A.), is faithfully copied byenzymes. Almost all our information about D.N.A.

replication comes from work by the school of ARTHURKORNBERG, now of Stanford University. The studywas initiated by his discovery of the enzyme, D.N.A.polymerase, more than a decade ago. For this contri-bution, and detailed characterisation of the D.N.A.

replication process, KORNBERG was awarded a Nobelprize in 1959.

In the reaction catalysed by D.N.A. polymerase, a

D.N.A. molecule isolated from a living organism is usedas a template: four deoxyribonucleotides (adenylic,guanylic, cytidilic, and thymidilic acids) are the unitswhich are polymerised. The reaction is such that thesequence of the four units in the newly synthesisedchain is complementary to that in the template, adeninepairing with thymine, and guanine with cytosine (andvice versa). This complementarity is ensured by hydro-gen bonding of base pairs as the reaction proceeds. Asecond cycle of polymerisation on the newly synthesisedstrand should produce a polymer with the same

sequence as in the original template. Available chemicaltechniques are not sufficiently precise to prove theexact fidelity of the complementary copying of a longD.N.A. chain. One way to establish such fidelity is tosynthesise in a test-tube D.N.A. with biological activitydependent on a complete and unaltered chain. This iswhat has now been achieved by GOULIAN, KORNBERG,and SINSHEIMER.lThe D.N.A. used in these experiments was obtained

from the bacterial virus 0X174. ROBERT SINSHEIMER,of the California Institute of Technology, had shownthat this D.N.A. is single-stranded, unlike the double-stranded D.N.A. of most organisms. Moreover, 0X174D.N.A. is in the form of a circle, which can be brokenopen at one point by nuclease to give a linear mole-cule. Specially prepared forms (protoplasts) of the hostbacterium, Escherichia coli, can be infected with thecircular D.N.A. and reproduce the whole virus; thelinear D.N.A. -is not infectious.

In the experiments now described the template usedwas circular D.N.A. from 0X174 (called for conveniencethe z strand). With purified polymerase a comple-mentary (-) strand was synthesised in a test-tube; theends of the newly synthesised D.N.A. chains were

united by a recently discovered polynucleotide-joiningenzyme 2 to form a circle. In the synthesising mediumthe naturally occurring monomer, thymidylic acid, wasreplaced by an unnatural but biologically active ana-logue, 5-bromodeoxyuridylic acid. The higher densityof the newly synthesised bromine-containing D.N.A.

allowed it to be separated from the template D.N.A. bydensity-gradient centrifugation. To prove that theisolated circles were free of contamination by theoriginal circles, the former were labelled with1. Goulian, M., Kornberg, A., Sinsheimer, R. L. Proc. natn. Acad. Sci.

U.S.A. 1967, 58, 2321.2. Gellert, M. ibid. 1967, 57, 148.

347

radioactive phosphorus, the latter with the readilydistinguishable tritium. The synthetic (-) circles wereused to infect protoplasts, producing a new generationof intact, normal virus. Moreover, using the synthetic(-) strands as template, similar procedures allowedin-vitro synthesis of new (+) strands, which wereagain separated and shown to be infectious.These results reflect a high degree of technical

ingenuity and skill. Nevertheless, they are a logicalextension of the work carried out during the past decaderather than a spectacular breakthrough. They followKORNBERG’S own previous work and the synthesis ofinfectious ribonucleic acid (R.N.A.) in test-tubes bySPIEGELMAN and his colleagues 3 The recent findingsare of greater general interest because R.N.A. genomesare restricted to some viruses, whereas most livingorganisms have D.N.A. genomes. The main technicalachievement of the recent work has been the purifica-tion of the D.N.A. polymerase and joining enzymes tosuch a degree that there was no contamination withnucleases. The main result has been the demonstrationthat KORNBERG’s enzyme can copy perfectly a biologi-cally active D.N.A. strand 5500 nucleotide residues long.There is still, however, doubt whether the normalfunction of the polymerase in cells is the replicationof D.N.A. or the repair of damaged D.N.A. The recentresults make a role for the polymerase and joiningenzymes in chromosomal replication much more

probable.Will these experiments have any therapeutic use-

fulness ? KORNBERG is hopeful: " We expect that itshould be feasible now to synthesise modified forms ofthe polyoma viral D.N.A. and determine how it altersthe cancer-producing germs ... It may be possiblethen to attach a particular gene to a harmless viralD.N.A. and use this virus as a vehicle for delivering thisgene to the cells of a patient. In this way, a person maybe cured of a hereditary defect." This may be visionary,but there is no doubt that the synthesis, in vitro, ofbiologically active D.N.A. opens up interesting possi-bilities for the analysis of genetic functions.

When to Leave School ?

THE Government’s decision to postpone the raisingof the school-leaving age from 1971 to 1973 has reviveddiscussion on the merits of extending compulsory educa-tion to 16. The doubters ask some questions which areworth considering. Might it not be better to spend themoney on primary schools instead ? If there is to be anextra year, might it not be better at the beginning-nursery schools for enthusiastic 4-year-olds insteadof another year at the end of the secondary school forbored adolescents ? Might it not be better to concentrateon compulsory day-release, and enable all young peopleunder 18 to attend a technical college one day a week ?Among those who have not been disappointed by

3. Spiegelman, S., Haruna, I., Holland, G., Beaudreau, G., Mills, D.ibid. 1965, 54, 919.

postponement, two groups with separate views havebecome fused, or even confused. The first believes thatmany 15-year-olds are fed up with school and readyto leave. In some people’s minds this is linked withevidence of earlier physical maturity. This groupwould have preferred cancellation to postponement.The second group-notably members of the NationalAssociation of Schoolmasters, the only teachers’ pro-fessional association which has welcomed the post-ponement-takes a different position. It maintains thatschools are not yet ready to raise the age because oflack of adequate staff, buildings, and teaching materials.But in principle, it still favours a school-leaving age of16. A two-year postponement goes some way to meetits views; a delay of four or five years might have satisfiedit altogether, provided that improvement of buildingsand staffing was accelerated.

The pros and cons-mainly the pros-of raising theschool-leaving age were argued at length in the Crowtherreport of 1959. But the recommendation that the ageshould be 16 goes back a good way farther. The Spensreport in 1938 noted that " the adoption of a minimumleaving age of 16 years ... may not be immediatelypracticable, but in our judgment must even now beenvisaged as inevitable ". The 1944 Education Act

recognised this, and, in raising the age to 15, it providedthat " as soon as the minister is satisfied that it hasbecome practicable to raise [it] to 16, he shall lay beforeParliament the draft of an order in Council ". Even so,when and whether to do this was still a central questionwhen the Crowther Committee was set up. A leavingage of 16 and compulsory part-time education to 18 wereboth in the 1944 Act, and both still waiting to be carriedout. The balance of public opinion then seemed to be infavour of giving priority to compulsory part-timeeducation. But the Crowther Committee came down

solidly in favour of going straight for a 16-year leavingage, on the grounds that the country was a long wayfrom tapping all its talent; that this waste was unlikelyto be remedied within a reasonable period withoutcompulsion; that the part-time route, even with dayrelease, was not an efficient substitute for longer full-time education; that, though the number staying on atschool voluntarily was increasing, the trend was pre-carious and dependent on the continuation of generalprosperity; while the demand for more educatedworkers and for more deeply educated workers wasgrowing. But the committee gave greatest weight to theneed for education to be extended to 16 so as to coverthe difficult and important period of adolescence for allchildren. To paraphrase JOHN DEWEY, " what the bestand wisest parent wants for his own child, the com-munity must want for all its children ". Four yearslater the Newsom Committee went over the same

ground and reached the same conclusion. As its briefwas concerned primarily with " average and belowaverage children ", it saw the raising of the leaving ageas a necessary part in raising standards for working-classchildren and counteracting the social disadvantagesunder which these children labour.