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Authors: Matthew Cobb
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bacteria. Sharples was therefore placed in a specially constructed containment device that could be sterilised before opening. 9 Even so, using the equipment safely was no easy matter. After centrifugation, the cake of bacteria that had accumulated at the bottom of the tube had to be removed – this was impossible to do cleanly and ‘one would see small flecks of white material fly in one direction or another’, recalled a lab member. 10 All of the cake was handled with towels soaked in germicide and then heated at 65°C before it was studied further, in an attempt to reduce the risk to lab members. 11 This messy and dangerous procedure so distressed the fastidious Avery that he would leave the lab when the Sharples was in action.
The group soon found that adding calcium chloride to the liquid transforming principle produced a white precipitate that contained most, if not all, of the transforming activity: adding white precipitate from smooth bacteria to a rough colony would transform it into a smooth colony. This white substance was very powerful – even at 1/1,000 dilution it could still transform a rough colony. At the beginning of 1941, MacLeod noted that the white precipitate contained both the polysaccharides typical of the smooth capsule and nucleic acids – DNA and its close relative, ribonucleic acid or RNA. When MacLeod added an enzyme that was known to destroy RNA, this had no effect on the transforming activity of the extract, strongly suggesting that RNA played no role in producing the power of the white material. In April 1941, in his six-monthly report to the Rockefeller Institute, Avery described the progress he and MacLeod had made and hinted at the potential implications:
This study is being continued with the hope that knowledge of this important cellular mechanism may lead to a better understanding of the principles involved in certain induced variations of living cells, not only of the pneumococcus, but also those of other biological systems.
12
*
In the summer, MacLeod left the Institute and another young physician, Maclyn McCarty, joined the Avery group. By the end of November 1941, McCarty had shown that if he used an enzyme to remove the polysaccharide, the extract nevertheless retained its transforming activity, showing that – as expected – the polysaccharide was not involved. That apparently left just two possibilities: proteins or DNA.
In December 1941 the Japanese attacked Pearl Harbor and the US entered the war. The Avery group shifted its work towards more practical aspects of pneumonia as the disease began to appear among US troops. Nevertheless, McCarty continued with his research, and in January 1942 he found that if alcohol was added to the transforming principle, a stringy white material appeared that contained 99.9 per cent of the transforming activity. It soon became apparent that this stringy stuff also contained most of the DNA that was present in the sample.
Two floors above Avery’s office was the laboratory of Alfred E. Mirsky, one of the world’s leading experts on nucleic acids. Mirsky gave the Avery lab some mammalian DNA extracted from the thymus gland, the traditional source of DNA, and they compared it with the white stringy stuff produced by alcohol precipitation of the transforming principle. The two substances seemed to be very similar. McCarty took an extract of transforming principle that had been treated with enzymes to remove both proteins and polysaccharides, and placed it in an ultracentrifuge. After spinning the sample for a few hours at 30,000 r.p.m., a gelatinous ‘pellet’ appeared at the bottom of the tube, containing the heaviest components of the extract. This contained all the transforming activity of the original solution and was apparently composed entirely of DNA.
In the summer of 1942, the suggestion that the transforming principle was made of DNA became stronger when McCarty and Avery showed that enzymes that destroyed transforming
The group soon found that adding calcium chloride to the liquid transforming principle produced a white precipitate that contained most, if not all, of the transforming activity: adding white precipitate from smooth bacteria to a rough colony would transform it into a smooth colony. This white substance was very powerful – even at 1/1,000 dilution it could still transform a rough colony. At the beginning of 1941, MacLeod noted that the white precipitate contained both the polysaccharides typical of the smooth capsule and nucleic acids – DNA and its close relative, ribonucleic acid or RNA. When MacLeod added an enzyme that was known to destroy RNA, this had no effect on the transforming activity of the extract, strongly suggesting that RNA played no role in producing the power of the white material. In April 1941, in his six-monthly report to the Rockefeller Institute, Avery described the progress he and MacLeod had made and hinted at the potential implications:
This study is being continued with the hope that knowledge of this important cellular mechanism may lead to a better understanding of the principles involved in certain induced variations of living cells, not only of the pneumococcus, but also those of other biological systems.
12
*
In the summer, MacLeod left the Institute and another young physician, Maclyn McCarty, joined the Avery group. By the end of November 1941, McCarty had shown that if he used an enzyme to remove the polysaccharide, the extract nevertheless retained its transforming activity, showing that – as expected – the polysaccharide was not involved. That apparently left just two possibilities: proteins or DNA.
In December 1941 the Japanese attacked Pearl Harbor and the US entered the war. The Avery group shifted its work towards more practical aspects of pneumonia as the disease began to appear among US troops. Nevertheless, McCarty continued with his research, and in January 1942 he found that if alcohol was added to the transforming principle, a stringy white material appeared that contained 99.9 per cent of the transforming activity. It soon became apparent that this stringy stuff also contained most of the DNA that was present in the sample.
Two floors above Avery’s office was the laboratory of Alfred E. Mirsky, one of the world’s leading experts on nucleic acids. Mirsky gave the Avery lab some mammalian DNA extracted from the thymus gland, the traditional source of DNA, and they compared it with the white stringy stuff produced by alcohol precipitation of the transforming principle. The two substances seemed to be very similar. McCarty took an extract of transforming principle that had been treated with enzymes to remove both proteins and polysaccharides, and placed it in an ultracentrifuge. After spinning the sample for a few hours at 30,000 r.p.m., a gelatinous ‘pellet’ appeared at the bottom of the tube, containing the heaviest components of the extract. This contained all the transforming activity of the original solution and was apparently composed entirely of DNA.
In the summer of 1942, the suggestion that the transforming principle was made of DNA became stronger when McCarty and Avery showed that enzymes that destroyed transforming
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