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By the end of this section, you will be able to:
  • Explain how the structure of DNA reveals the replication process
  • Describe the Meselson and Stahl experiments

The elucidation of the structure of the double helix provided a hint as to how DNA divides and makes copies of itself. This model suggests that the two strands of the double helix separate during replication, and each strand serves as a template from which the new complementary strand is copied. What was not clear was how the replication took place. There were three models suggested ( [link] ): conservative, semi-conservative, and dispersive.

Illustration shows the conservative, semi-conservative, and dispersive models of DNA synthesis. In the conservative model, when DNA is replicated and both newly synthesized strands are paired together. In the semi-conservative model, each newly synthesized strand pairs with a parent strand. In the dispersive model, newly synthesized DNA is interspersed with parent DNA within both DNA strands.
The three suggested models of DNA replication. Grey indicates the original DNA strands, and blue indicates newly synthesized DNA.

In conservative replication, the parental DNA remains together, and the newly formed daughter strands are together. The semi-conservative method suggests that each of the two parental DNA strands act as a template for new DNA to be synthesized; after replication, each double-stranded DNA includes one parental or “old” strand and one “new” strand. In the dispersive model, both copies of DNA have double-stranded segments of parental DNA and newly synthesized DNA interspersed.

Meselson and Stahl were interested in understanding how DNA replicates. They grew E. coli for several generations in a medium containing a “heavy” isotope of nitrogen ( 15 N) that gets incorporated into nitrogenous bases, and eventually into the DNA ( [link] ).

Illustration shows an experiment in which E. coli was grown initially in media containing ^{15}N nucleotides. When the DNA was extracted and run in an ultracentrifuge, a band of DNA appeared low in the tube. The culture was next placed in ^{14}N medium. After one generation, all of the DNA appeared in the middle of the tube, indicating that the DNA was a mixture of half ^{14}N and half ^{15}N DNA. After two generations, half of the DNA appeared in the middle of the tube, and half appeared higher up, indicating that half the DNA contained 50% ^{15}N, and half contained ^{14}N only. In subsequent generations, more and more of the DNA appeared in the upper, ^{14}N band.
Meselson and Stahl experimented with E. coli grown first in heavy nitrogen ( 15 N) then in 14 N. DNA grown in 15 N (red band) is heavier than DNA grown in 14 N (orange band), and sediments to a lower level in cesium chloride solution in an ultracentrifuge. When DNA grown in 15 N is switched to media containing 14 N, after one round of cell division the DNA sediments halfway between the 15 N and 14 N levels, indicating that it now contains fifty percent 14 N. In subsequent cell divisions, an increasing amount of DNA contains 14 N only. This data supports the semi-conservative replication model. (credit: modification of work by Mariana Ruiz Villareal)

The E. coli culture was then shifted into medium containing 14 N and allowed to grow for one generation. The cells were harvested and the DNA was isolated. The DNA was centrifuged at high speeds in an ultracentrifuge. Some cells were allowed to grow for one more life cycle in 14 N and spun again. During the density gradient centrifugation, the DNA is loaded into a gradient (typically a salt such as cesium chloride or sucrose) and spun at high speeds of 50,000 to 60,000 rpm. Under these circumstances, the DNA will form a band according to its density in the gradient. DNA grown in 15 N will band at a higher density position than that grown in 14 N. Meselson and Stahl noted that after one generation of growth in 14 N after they had been shifted from 15 N, the single band observed was intermediate in position in between DNA of cells grown exclusively in 15 N and 14 N. This suggested either a semi-conservative or dispersive mode of replication. The DNA harvested from cells grown for two generations in 14 N formed two bands: one DNA band was at the intermediate position between 15 N and 14 N, and the other corresponded to the band of 14 N DNA. These results could only be explained if DNA replicates in a semi-conservative manner. Therefore, the other two modes were ruled out.

During DNA replication, each of the two strands that make up the double helix serves as a template from which new strands are copied. The new strand will be complementary to the parental or “old” strand. When two daughter DNA copies are formed, they have the same sequence and are divided equally into the two daughter cells.

Click through this tutorial on DNA replication.

Section summary

The model for DNA replication suggests that the two strands of the double helix separate during replication, and each strand serves as a template from which the new complementary strand is copied. In conservative replication, the parental DNA is conserved, and the daughter DNA is newly synthesized. The semi-conservative method suggests that each of the two parental DNA strands acts as template for new DNA to be synthesized; after replication, each double-stranded DNA includes one parental or “old” strand and one “new” strand. The dispersive mode suggested that the two copies of the DNA would have segments of parental DNA and newly synthesized DNA.

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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