DNA Replication

Section 3: DNA Replication

The Hypothesis of DNA Replication

Following their groundbreaking work, Watson and Crick published a second paper in which they proposed a hypothesis about the self-replication of genetic material: During DNA replication, the DNA double helix unwinds, and the hydrogen bonds between the complementary bases break. Each of the separated single strands then serves as a template for replication. Free deoxyribonucleotides, following the principle of complementary base pairing, bind to the template single strands by forming hydrogen bonds. Since each newly synthesized DNA molecule retains one of the original strands, this method of replication is referred to as semiconservative replication (Figure 3-9, left).

After this hypothesis was proposed, other theories emerged, including the conservative replication hypothesis, which suggested that DNA replication uses the entire double helix as a template, resulting in both strands of the daughter DNA being newly synthesized (Figure 3-9, right). But which hypothesis was correct?

Experimental Evidence for Semiconservative DNA Replication

To prove that DNA replication is semiconservative, it was necessary to experimentally distinguish between parental and daughter DNA.

In 1958, American biologists Matthew Meselson (M. Meselson, 1930-) and Franklin Stahl (F. Stahl, 1929-) conducted an ingenious experiment using Escherichia coli (E. coli) as the experimental material and employing isotope labeling techniques.

The results of their experiment were as follows: After centrifuging the DNA of the parental E. coli, only one DNA band appeared near the bottom of the test tube, indicating that it was the heaviest and represented the parental double-stranded DNA labeled with “N (N/N-DNA). After centrifuging the DNA from the first generation of bacteria after the transfer, only one band appeared in the middle of the tube, indicating an intermediate density and representing the daughter double-stranded DNA with one strand labeled with "N (N/N-DNA). After centrifuging the DNA from the second generation of bacteria, two bands appeared: one in the middle, representing N/N-DNA, and another higher up, indicating the lightest density and representing daughter double-stranded DNA with both strands unlabeled (N/N-DNA). The experimental results confirmed that DNA replication occurs in a semiconservative manner.

The Process of DNA Replication

DNA replication refers to the process of synthesizing daughter DNA using parental DNA as a template. In eukaryotic organisms, this process occurs during the interphase before cell division and is completed alongside chromosome replication.

At the beginning of replication, driven by energy provided by the cell, helicase unwinds the two strands of the DNA double helix, a process known as unwinding. Then, DNA polymerase and other enzymes use each unwound parental strand as a template to synthesize a complementary daughter strand using the four free deoxyribonucleotides in the cell as raw materials, following the principle of complementary base pairing. As the template strand continues to unwind, the newly synthesized daughter strand also extends. At the same time, each new strand coils with its corresponding template strand to form a double helix structure (Figure 3-10). Thus, after replication, one DNA molecule has formed two identical DNA molecules. The two newly replicated daughter DNA molecules are then distributed to daughter cells through cell division.

DNA replication is a process that occurs simultaneously with unwinding, requiring basic conditions such as templates, raw materials, energy, and enzymes. The unique double helix structure of DNA provides an accurate template, ensuring that replication can proceed with precision through complementary base pairing.

Through replication, DNA transmits genetic information from parental cells to daughter cells, thereby maintaining the continuity of genetic information.