Deoxyribonucleic acid, commonly known as DNA, serves as the fundamental blueprint for all known life forms. This remarkable molecule carries the genetic instructions necessary for an organism’s development, functioning, growth, and reproduction. For life to continue and for cells to divide, this genetic information must be accurately copied and passed on to new cells or offspring. Understanding precisely how DNA copies itself is a central question in biology.
Understanding DNA’s Blueprint
DNA’s structure is often described as a double helix, resembling a twisted ladder. Each side of this ladder is a strand composed of a sugar-phosphate backbone, and the rungs are formed by pairs of nitrogenous bases. There are four types of these bases: adenine (A), thymine (T), guanine (G), and cytosine (C). A crucial feature of DNA is its complementary base pairing, where adenine always pairs with thymine, and guanine always pairs with cytosine. This precise pairing rule means that the sequence of bases on one strand dictates the sequence on the other, providing a built-in mechanism for information replication.
The Semi-Conservative Concept
The concept of semi-conservative replication proposes a specific mechanism for how DNA duplicates itself. In this model, each new DNA molecule produced consists of one original strand from the parent DNA and one newly synthesized strand. This means that the original DNA molecule does not remain entirely intact, nor does it completely break down into fragments that reassemble randomly. Instead, it unwinds, and each original strand acts as a template for a new partner strand. This idea stood in contrast to other hypothetical models, such as conservative replication, where the original DNA would remain wholly intact and a completely new DNA molecule would be formed, or dispersive replication, where both new DNA molecules would be a mosaic of old and new DNA fragments.
The Replication Process
DNA replication begins with the unwinding of the double helix, a process catalyzed by an enzyme called DNA helicase. This enzyme moves along the DNA, breaking the hydrogen bonds between the complementary base pairs, effectively unzipping the two strands. Once separated, each single strand serves as a template for the synthesis of a new complementary strand.
DNA polymerase then moves along each template strand, adding new nucleotides according to the base-pairing rules (A with T, C with G). DNA polymerase also has proofreading capabilities, ensuring accuracy. As new nucleotides are added, they form a sugar-phosphate backbone, linking together to create a continuous new strand. This process results in two identical DNA molecules, each containing one original strand and one newly synthesized strand.
The Definitive Experiment
Matthew Meselson and Franklin Stahl’s 1958 experiment provided definitive proof for semi-conservative DNA replication. They used nitrogen isotopes to distinguish old and new DNA strands. They grew E. coli bacteria in a medium containing a heavy isotope of nitrogen ($^{15}$N) for several generations, making all bacterial DNA “heavy”. Heavy DNA was distinguished by density using density gradient centrifugation.
They then transferred bacteria to a medium containing the lighter isotope of nitrogen ($^{14}$N). After one round of replication in $^{14}$N medium, all DNA had an intermediate density. This ruled out the conservative model, which predicted two distinct bands: one heavy ($^{15}$N-$^{15}$N) and one light ($^{14}$N-$^{14}$N).
The intermediate band was consistent with both semi-conservative and dispersive models. To differentiate, Meselson and Stahl allowed a second round of replication in the $^{14}$N medium. After this second generation, two distinct bands appeared: one at intermediate density and another at lighter density (entirely $^{14}$N DNA). This supported the semi-conservative model, as the dispersive model predicted a single band of increasingly lighter density with each generation. The presence of both hybrid and light DNA confirmed that each new DNA molecule contained one original strand and one newly synthesized strand.