7/31/2019 Articulo DNA Ingles

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These bands appear in new positions, because denatured DNA has a higher density than native DNA. Insemiconservative replication, the single strands of denatured DNA should have half the molecular weight of the nativehybrid molecule. The experimental results favored again the semiconservative model, but the experimental accuracywas not sufficiently high to make the results entirely clear-cut. However, put together, the evidence in favor of semiconservative replication is sufficient to allow us to accept this model.

In this connection (see also later) we should mention that the denatured DNA can reform to the native from in spite of the fact that in denatured DNA the two strands have come apart.

We note in passing that there is relatively little DNA between three band positions indicating that the extracted DNA hasalmost always the densities of pure N15-N15, pure N14-N15, or pure N14-N14 DNA. this observation implies that thereplication process replicates a complete length of DNA before going on to the next molecule. However, this dramaticconclusion applies only to the relatively short length of DNA which were prepared for analysis by the methods availableat the time of the Messelson-Stahl experiment. Had the molecules survived intact, then such unsynchronized cellpopulations would have produced some DNA of intermediate buoyant densities, and this would have obscured theresultant picture.

PROPERTIES OF DNA

in the isolation of DNA a major difficulty is to keep such a long rigid molecule physically intact. it has been shown thatthe shear forces set up when a solution of DNA is vigorously stirred are sufficient to break the long rigid DNA molecule.The molecules are first broken near the middle, then into quarters, and so on until a short-enough size is reached thatthe molecule will be relatively insensitive to breakage by shearing.

It is difficult to extrapolate the molecular weight of DNA found in solution to the actual molecular weight that exists invivo because of the great ease of degradation by shearing during extraction or by the inadvertent action of hydrolyticenzymes. perhaps the best estimates are derived from radioautographic work in wich the DNA is labeled with tritium.for example, in E coli, cairns was able to detect DNA molecules up to 1.1 mm in length. this corresponds to a molecularweight of 2.8x109, similarly the DNA from T2 bacteriophage has an average length of 49 micras, wich would correspondto a molecular weight of 108. a rather smaller bacteriophage, called lambda, has a DNA 23 micras in length. but even inthe most carefully prepared samples of extracted DNA there is always 0.1 to 0.2 percent of protein attached to thematerial. therefore the possibiliy existed that a DNA molecule is actually made up shorter lengths of DNA held together

by protein peptide-like materials. however, the successful synthesis in vitro by known reactions of the circular double-stranded replicative form of 174 DNA, capable of infecting host cell, makes it likely that such DNA molecules arecomposed only of nucleotides.