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 Covered
1. What is DNA Replication
– Definition, Steps, Function
2. Why is DNA Replication Described as Semiconservative
– Synthesis of a New DNA Strand
Key Terms: DNA Replication, Elongation, Initiation, Lagging Strand, Leading Strand, Original Strand, Termination
What is DNA Replication
DNA 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.
Why is DNA Replication Described as Semiconservative
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.
Conclusion
DNA 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
2. “DNA replication split” By I, Madprime (CC BY-SA 3.0) via Commons Wikimedia
3. “Semiconservative replication” By Lizanne Koch – lgkoch – own work with chemdraw (Public Domain) via Commons Wikimedia
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