What are the steps of DNA replication? There are three steps of DNA replication: initiation, elongation, and termination. They are enzyme-catalyzed and well-coordinated steps.
DNA replication is a biological process that produces two identical replicas of DNA from one original DNA molecule. It occurs in all living cells. In addition, it is an essential process of biological inheritance. Therefore, it is important for cell division during growth and repairing damaged tissues. Cells show a distinctive process of cell division, making cell division essential.
Key Areas Covered
1. What is DNA Replication
– Definition, Features, Importance
2. What are the Steps of DNA Replication
– Initiation
– Elongation
– Termination
Key Terms
DNA Replication, DNA Replication Steps
What is DNA Replication
DNA replication refers to the process of producing an identical copy of a double-stranded DNA molecule. Each strand of the double-stranded DNA molecule serves as a template for the synthesis of the new DNA molecule. Therefore, the double-stranded DNA molecule should be unwounded prior to undergoing the replication. DNA replication occurs in a semi-conservative manner – each newly-synthesized, double-stranded DNA molecule contains an old DNA molecule.
Furthermore, several enzymes, such as helicase, RNA primase, and DNA polymerase, are involved in DNA replication. DNA is unwound by helicase, producing the replication fork. RNA primase adds primers to the DNA template. Since DNA polymerase adds complementary base pairs to the template strand, the recruitment of DNA polymerase through the DNA template produces a complementary DNA strand. The newly synthesized strand is complementarily base-paired with the old strand.
What are the Steps of DNA Replication
There are three steps of DNA replication. They include initiation, elongation, and termination.
Initiation
During the initiation process, DNA replication forms a pre-replication complex. This occurs in the early G1 phase of the late mitosis. In general, a large complex of initiator proteins binds together to form a pre-replication complex. This complex binds to a particular point of DNA called the origins. However, in E. coli, this is the DnaA, while in yeast, this is the origin recognition complex.
Figure 1: Initiation of DNA Replication
The initiator proteins bind to the AT-rich regions in the genome with two hydrogen bonds. The activation of S-Cdk and Cdc7 occurs during the early S phase, forming a massive preinitiation complex at the origin. This inactivates and dissembles the pre-replication complex. Moreover, loading of the preinitiation complex activates Mcm helicase in order to unwind the double helix. In addition to that, this complex also loads α-primase and other DNA polymerases onto the DNA. However, the main function of α-primase is to synthesize first primers at the primer-template junction. The components of the preinitiation complex associate with the replication fork as the DNA replication proceeds.
Elongation
Elongation is the second step of DNA replication in which DNA polymerase uses its 5′–3′ activity to synthesize DNA nucleotides that are a complement to the template strand. It initiates with the presence of 3’ hydroxyl groups. Apart from that, organisms such as viruses, phages, and plasmids use primase to synthesize small RNA primers. Retroviruses use tRNA to prime DNA replication. On the other hand, adenoviruses and some bacteriophages use side chains of amino acids as 3’ OH. However, single-stranded RNA viruses use a rolling circle replication mechanism.
Figure 2: DNA Replication Elongation
However, the leading strand receives the RNA primer, while the lagging strand receives many RNA primers. Therefore, the leading strand can extend continuously while the lagging strand extends discontinuously, forming Okazaki fragments. During elongation, RNase removes RNA primers and DNA polymerases with low processivity fill the gap. DNA ligase is the enzyme responsible for filling nicks. In the meanwhile, the replication fork is the structure that occurs during the replication of long helical DNA molecules created by helicase by breaking the hydrogen bonds.
Termination
During termination, the replication fork stops. It can also occur in two methods: at a termination site of DNA replication or a protein that binds to the DNA to physically stop the DNA replication. Bacterial species use Ter protein or terminus-site binding protein for the termination of DNA replication. Therefore, the main function of the termination species is to terminate the DNA replication. In eukaryotes, long linear chromosomes contain replication forks at multiple locations, and they meet and terminate the DNA replication. However, the very end of the chromosome may not undergo replication. Therefore, the ends of the chromosomes can lose their information. Hence, the ends of chromosomes contain telomeres containing repetitive DNA, preventing the loss of genes.
Conclusion
In brief, DNA replication is a biological process that is enzyme-catalyzed and well-coordinated. It is responsible for the formation of two DNA molecules from an original DNA molecule. More importantly, DNA replication occurs in cell division to double the DNA content of the cell. Apart from that, there are three steps of DNA replication. They are the initiation, elongation, and termination. During the initiation step, the origins of DNA bind with the prereplication complex during the G1 phase of cell division. Then, it is replaced by the preinitiation complex. Meanwhile, DNA helicase is responsible for unwinding DNA, while α-primase synthesizes RNA primers. During the second step of DNA replication, DNA polymerase synthesizes DNA strands forming the replication fork. During the termination, the binding of proteins into the DNA strands or meeting the termination sites stops DNA replication.
References:
- DNA replication. Genome.gov. Retrieved May 7, 2023
Image Courtesy:
- “Figure Role of initiators for initiation of DNA replication” By Emply shell China dry – Own work (CC BY-SA 3.0) via Commons Wikimedia
- “Eukaryotic DNA replication” By LadyofHats – Own Work (CC0) via Commons Wikimedia
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