Genetic structure and function / P.F. Smith Keary.

  • Smith-Keary, P. F.
Date:
1975
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetic structure and function / P.F. Smith Keary. Source: Wellcome Collection.

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    THE MATERIAL BASIS OF HEREDITY 27 tubes and the amount of DNA present can be determined from uv absorption photographs of the centrifuge tubes. The Meselson and Stahl experiment Meselson and Stahl grew the bacterium Escherichia coli (so called after its discoverer Theodor Escherich) in a culture medium in which the only nitrogen source was the heavy isotope nitrogen-15 or ^ ^ N, having a mass greater than that of the normal After many generations of growth all the nitrogen in the bacterial DNA will be heavy ^^N, and this DNA will be more dense than the normal ^'^N DNA. Bacteria with fully labelled ^^N DNA were then transferred to normal culture medium containing only ^'^N as the nitrogen source and the culture was sampled after various times to determine the amounts of DNA of different densities that were present. Meselson and Stahl found that after growth in ^ ^ N medium all the DNA extracted from the E. coli cells concentrated in a single 'heavy' band. After a time corresponding to one generation cycle in '^N medium, all the DNA again concentrated in a single band, but this band formed at a position intermediate to the bands formed by pure ^ ^ N and pure ' '^N DNA. If replication is semi-conservative this is expected, since each daughter molecule will contain one ^^N 'heavy' and one ^'^N 'light' strand and so be of intermediate density (figures 2.10 and 2.11). After a further generation in ^'^N (a) ol)servation generation 0 all 'heavy' dna all 'intermediate' dna VAVWV dna -'light' 'N 'intermediate' 14M/15M heavy' Уг 'light' DNA У2'intermediate' dna % 'light' dna Уй ' intermediate' dna (b) interpretation -h -h Figure2.11 Observations and interpretation of the Meselson and Stahl experiment. The simplest explanation of the observed results is that each molecule of DNA replicates semi-conservatively, so that after one generation in ' '*N medium all the daughter molecules have one 'heavy' ' and one 'light' ^ N strand. After a further generation in 'N medium each of these hybrid molecules replicates to produce one ' ^N/' ^ N hybrid molecule and one »4N/t4N 'light' molecule, and so on. The labelled strands of each DNA duplex are represented by the thick lines and the ' ^ N unlabelled strands by the thin lines.