Читать книгу Cell Biology - Stephen R. Bolsover - Страница 96

In 1958 Matthew Meselson and Franklin Stahl designed an ingenious experiment to test whether each strand of the double helix does indeed act as a template for the synthesis of a new strand. They grew the bacterium E. coli in a medium containing the heavy isotope ¹⁵N that could be incorporated into new DNA molecules. After several cell divisions they transferred the bacteria, now containing “heavy” DNA, to a medium containing only the lighter, normal, isotope ¹⁴N. Any newly synthesized DNA molecules would therefore be lighter than the original parent DNA molecules containing ¹⁵N. The difference in density between the heavy and light DNAs allows their separation using very high‐speed centrifugation. The results of this experiment are illustrated in the figure. DNA isolated from cells grown in the ¹⁵N medium had the highest density and migrated the furthest during centrifugation. The lightest DNA was found in cells grown in the ¹⁴N medium for two generations, whereas DNA from bacteria grown for only one generation in the lighter ¹⁴N medium had a density halfway between these two. This is exactly the pattern expected if each strand of the double helix acts as a template for the synthesis of a new strand. The two heavy parental strands separated during replication, with each acting as a template for a newly synthesized light strand, which remained bound to the heavy strand in a double helix. The resulting DNA was therefore of intermediate density. Only in the second round of DNA replication, when the light strands created during the first round of replication were allowed to act as templates for the construction of complementary light strands, did DNA double helices composed entirely of ¹⁴N‐containing building blocks appear.