DNA replication is similar to transcription in its most general idea: a polymerase enzyme reads a strand of DNA one nucleotide at a time, it takes a random nucleotide from the nucleoplasm, and if it is complementary to the nucleotide in the DNA, the polymerase adds it to the new strand it is creating. Of course, there are significant differences between replication and transcription too, not the least of which is that both strands of DNA are being read simultaneously in order to create two new complementary strands that will eventually result in a complete and nearly perfect copy of an entire organismal genome. One of the most important concepts of DNA replication is that it is a semi-conservative process (Figure \(\PageIndex{7}\)). This means that every double helix in the new generation of an organism consists of one complete “old” strand and one complete “new” strand wrapped around each other. This is in contrast to the two other possible models of DNA replication, the conservative model, and the dispersive model. A conservative mechanism of replication proposes that the old DNA is used as a template only and is not incorporated into the new double-helix. Thus the new cell has one completely new double-helix and one completely old double-helix. The dispersive model of replication posits a final product in which each double helix of DNA is a mixture of fragments of old and new DNA. In light of current knowledge, it is difficult to imagine a dispersive mechanism, but at the time, there were no mechanistic models at all. The Meselson-Stahl experiments (1958) clearly demonstrated that the mechanism must be semi-conservative, and this was confirmed once the key enzymes were discovered and their mechanisms elucidated.
In the Meselson-Stahl experiments, E. coli were first incubated with 15N, a heavy isotope of nitrogen. Although it is only a difference in mass of one neutron per atom, there is a great enough difference in mass between heavy nitrogen-containing DNA (in the purine and pyrimidine bases) and light/normal nitrogen-containing DNA that they can be separated from one another by ultracentrifugation through a CsCl concentration gradient (Figure \(\PageIndex{7}\)). Over 14 generations, this led to a population of E. coli that had heavy nitrogen incorporated into all of the DNA (shown in blue). Then, the bacteria are grown for one or two divisions in “light” nitrogen, 14N. When the DNA from the bacterial populations was examined by centrifugation, it was found that instead of light DNA and heavy DNA, as would be expected if DNA replications was conservative, there was a single band in and intermediate position on the gradient. This supports a semi-conservative model in which each strand of original DNA not only acts as a template for making new DNA, it is itself incorporated into the new double-helix.
DNA is a double-stranded molecule. One of the two strands of DNA is an original strand while the other is the newly-synthesized strand. Since one of the two strands of DNA is always conserved, DNA replication is considered as a semiconservative process. DNA replication is the process of synthesizing new DNA. It synthesizes a new DNA strand by adding complementary nucleotides to the template strand. DNA polymerase is the enzyme responsible for the synthesis of new DNA strand. Key Areas Covered1. What is DNA Replication Key Terms: DNA Replication, Elongation, Initiation, Lagging Strand, Leading Strand, Original Strand, Termination  What is DNA ReplicationDNA replication is the cellular process by which an exact replica of a particular DNA molecule is produced using the original DNA strands as templates. The three steps of DNA replication are initiation, elongation, and termination. The initiation of DNA replication occurs at the origin of replication of the chromosome. Once DNA polymerase binds to the origin of replication, it starts adding nucleotides to the 3′ end of the RNA primer. RNA primer is synthesized by RNA primase. The growth/elongation of the new DNA strand occurs in the 3′ to 5′ direction. Both strands of DNA serve as templates. The unwound DNA strands form a replication fork. DNA replication at the replication fork is shown in figure 1. Figure 1: DNA Replication The main function of DNA polymerase in DNA replication is to add complementary nucleotides to the growing chain. Sugar-phosphate backbone is formed by the phosphodiester bond formation between the proximal phosphate group and the 3′ OH of the pentose ring of incoming nucleotide. The termination of DNA replication occurs in the telomeric regions of the chromosome. Since DNA is a double-stranded molecule, both strands serve as templates in DNA replication. Hence, DNA replication occurs bi-directionally at the replication fork. However, one template strand runs in the 5′ to 3′ direction while the other template strand runs in the 3′ to 5′ direction. The strand with 5′ to 3′ directionality is called the leading strand as a continuous DNA replication occurs on that strand. The other strand with 3′ to 5′ directionality is called the lagging strand. It is synthesized as pieces called Okazaki fragments. Bi-directional DNA replication is shown in figure 2. Figure 2: Bi-Directional DNA Replication As both strands serve as templates in DNA replication, a particular double-stranded DNA is composed of an old strand and a newly synthesized strand. The old strand serves as the template for replication, forming the new strand, which is complementary to the template strand. Therefore, each DNA molecule is composed of an original DNA strand along with a newly synthesized DNA strand. Since one of the two strands of DNA is not changed or conserved, DNA replication is considered as a semiconservative process. Semiconservative DNA replication is shown in figure 3. Figure 3: Semiconservative DNA Replication The semiconservative DNA replication allows DNA repair mechanisms to work on the newly-synthesized DNA strand. ConclusionDNA replication is a semiconservative process as one of the two strands of the double-stranded DNA is an original DNA strand, which served as the template for the synthesis of the new strand. Since one of the two strands is always conserved, DNA replication is considered as a semiconservative process. Reference:1. “Semi-Conservative DNA Replication: Meselson and Stahl.” Nature News, Nature Publishing Group, Available here. Image Courtesy:1. “DNA Replication” (Publci Domain) via Public Domain Files |
Why is DNA replication described as a semi-conservative process?
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